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GSAMS’ Un distinguished Lecture Series presents . . . Java for the Impatient. WHO. Russ Shackelford, B.A., M.A., M.S., Ed.S., Ph.D. David Dagon, B.S., J.D., etc. . @. [email protected] [email protected] WHAT. Covers essential Java syntax and debugging techniques.

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slide2

WHO

Russ Shackelford, B.A., M.A., M.S., Ed.S., Ph.D.

David Dagon, B.S., J.D., etc.

@

[email protected]

[email protected]

slide3

WHAT

Covers essential Java syntax

and debugging techniques

Presumes familiarity with computer, and an IDE or text editor

David Dagon:

Georgia Tech’s introductory computing course has the students use NTEmacs with JDE to edit their code. You can get a copy of this distribution at:

http://www.cc.gatech.edu/classes/cs1502

Look for the links on “Help with IDEs”

a note on noise
A Note on Noise
  • Some noise is good
  • We must share the medium, and should be aware of others.
  • However, please

feel free to

ask questions

what is java
What is Java?

"A simple, object-oriented, distributed, interpreted, robust, secure, architecture neutral, portable, high-performance, multithreaded, and dynamic language”

-- Sun

Sounds like marketing;

let’s take a closer look . . .

David Dagon:

FYI. The next set of slides give a brief explanation of each of these terms. Some of the slides introduce some complicated subjects; future sets of slides will cover them in greater detail.

object oriented
“… object oriented …”
  • Java is an “object oriented” (or “OO”) language.
  • It is possible (but seldom desirable) to write non-object oriented Java code (so-called “hybrid OO”).
  • In an OO language, classes are used to encapsulate data (state) and behavior.
  • Instances of a class are then used to manipulate data and drive the program.
  • Classes are arranged in an hierarchy, or package structure.
slide7

“… object oriented, …”

Example:

A “Stack” is a class found in the package

java.util. We create an instance of the class

Stack using the keyword new thus:

import java.util.*;

Stack c = new Stack();

c.push(“3”);

java.util.Stack x = new Stack();

David Dagon:

It’s difficult to come up with a simple example of an object. One might use java.lang.String; however, there are some exceptions to String creation built into Java. One could also use the java.lang.Integer class; however, this presumes some knowledge of wrapper classes.

Note also that I’ve called the Stack in this example “c”, instead of “myStack” or something similar. It’s very common to use the ‘my-’ prefix in object creation; however, those not used to OO language might find it confusing. To emphasize that the Stack instance is merely an object, we use the familiar variable ‘c’. Of course, we could have called the Stack “foo”.

This differs from procedural (e.g., C) and functional languages (e.g., Lisp) where other data structures would be used to model the Stack. More on classes in later slides . . .

distributed
“… distributed, …”

Java is also a distributed language.

It was built with networking in mind

Fully supports IPv4, with structures to support IPv6.

Includes support for “Applets”:

small programs embedded in

HTML documents.

See:

java.net package

RMI/CORBA

David Dagon: Java provides convenient access to the OSI Network layer. Other APIs allow one to access even lower layers.

interpreted
“… interpreted, …”

Java is an interpreted language, meaning that each instruction is translated into machine code at the time of execution, not during compilation.

This allows for platform neutrality:

“WORA”

This allows one to rewrite and

change a program while it

is running.

Penalty: speed

robust
“… robust, …”

Java is simple--no pointers/stack concerns (for the most part)

In-bound checking at runtime of array pointers--no memory corruption and cryptic error messages.

Exception handling: try/catch

/finally series allows for

simplified error recovery

Strongly typed language:

many errors caught during

compilation.

secure
“… secure, …”
  • Byte-code verification on loading (not just compilation).
  • Applet code runs in \'sandbox\', with significant restrictions
    • Security is enforced by the
    • SecurityManager class
    • Work-arounds for applet
    • security restrictions include
    • digitally signing code, and
    • Servlets
slide12

Evaluation of Java

  • Strengths of Java:
    • A real language, in demand in industry
    • Portability
    • Comparatively easy to learn
    • Difficult to destroy a machine with it ;-)
    • Advanced built-in GUI/Graphics features
  • Weaknesses of Java:
    • Slow: interpreted and OO
    • GUI/Graphics via “Least

Common Denominator”

approach (due to platform

independence)

    • Awkward/annoying syntax
structure of java programs
Structure of Java Programs
  • Applications (“normal” computer programs):
    • Each program consists of multiple files.
    • Each file contains a class.
    • First method (module) called is:

public static void main(String[ ] argv)

    • The main method controls

program flow (but OO

orientation means that it

starts a process that

features decentralized

control).

structure of java programs14
Structure of Java Programs
  • Applets (transportable over WWW):
    • Similar to applications, but...
    • First method is:

public void init( )

    • Remainder of applet is a series of handlers that respond to events (e.g., user actions).
    • Program is executed by

Java interpreter running

in Web browser or

applet viewer.

sample program in a file called helloworld java
Sample Program (in a file called “HelloWorld.java”)

public class HelloWorld

{

public static void main(String argv[])

{

System.out.println(“Hello World!”);

}

}

basic syntax
Basic Syntax

All java programs contain (within one or more* classes):

public static void main (String a[ ])

{ ... }

The Java interpreter runs until

the main() returns, a System

or Runtime exit is called,

a fatal error occurs, or until

the end of

main is found.

David Dagon: The following slide is the traditional HelloWorld program. Note that white space is irrelevant. Also note that the naming of local variables is arbitrary (compare a[ ] to arg[ ]).

goodbye world

import java.lang.Runtime; /* not necessary */

class GoodByeWorld {

public static void main (String arg[]) {

Runtime.getRuntime().traceInstructions(true);

Runtime.getRuntime().traceMethodCalls(true);

System.out.println (”Here’s some stats:");

System.out.println

(Runtime.getRuntime().totalMemory() +

" total memory\n” +

Runtime.getRuntime().freeMemory()

+ " available");

if (true) return; /* 1 */

System.exit(0);

/* 2 terminates */

Runtime.getRuntime().exit(0);

/* 3 terminates only if

the thread is running */

} /* 4 --end of main*/

}//test

GoodBye World
vocabulary
Vocabulary
  • Structured Programming:
    • A programming paradigm in which the actions (or verbs, or procedures) are emphasized.
  • OO Programming:
    • A programming paradigm in which the actors (or nouns, or objects) and their interaction is emphasized.
  • Byte Compiler:
    • A compiler which translates

human-readable source code

into byte code (transportable

to various virtual machines)

instead of object code written

for a specific kind of machine.

vocabulary cont d
Vocabulary (cont’d)
  • Byte Interpreter:
    • An interpreter which translates byte code into object code for a particular kind of machine and executes them on that machine.
  • Byte Code:
    • An instruction for a virtual machine.
  • Java Virtual Machine (JVM):
    • The virtual machine (software)

for which all Java programs

are compiled. A byte code

interpreter is required to

translate from the JVM byte

code instructions into to

instructions for a given

actual machine.

java s popularity
Java’s Popularity
  • Keys to Java’s Popularity:
    • An OO language that’s relatively simple.
    • Virtual Machine approach, allowing transportability to various different kinds of computers (operating systems).
    • Presence of JVM as

part of Web browsers,

encouraging movement

of Java programs

over the Web.

primitives

Java is not a purely OO language, and supports several primitive data types, including:

Primitives
  • Primitive Type Default Value
  • boolean false
  • char \'\u0000\' (null)
  • byte (byte) 0
  • short (short) 0
  • int 0
  • long 0L
  • float 0f
  • double 0d
  • void N/A

David Dagon: According to the Java Language Specification, void is a primitive. But introducing void to students might be confusing. We include it here to be complete; in teaching, you might omit this 9th primitive

variable declarations
Variable Declarations
  • Java:
    • <datatype> <identifier>;
  • or (optional initialization at declaration)
    • <data type> <identifier> = <init value>;

int x;

x = 3;

int y = 5;

naming conventions
Naming Conventions
  • Begin variable identifiers with abbreviation of their type:
    • i for int (integer)
    • f for float
    • b for boolean
    • ch for char
    • str for String
examples
Examples
  • int iCounter;
  • int iNumStudents = 583;
  • float fGPA;
  • float fBatAvg = .406;
  • char chGender;
  • char chGender = ‘f’;
  • boolean bSafe;
  • boolean bEmpty = true;
  • String strPersonName
  • = “Fred”;
  • String strStreetName;
operators
Operators
  • Assignment: =
  • Arithmetic: +, -, *, /, % (mod)

int iNumLect = 2;

int iNumStudents = 583;

int iStudentsPerLect;

iStudentsPerLect = iNumStudents / iNumLect;

// gives 291 due to integer division

int iNumQualPoints = 30;

int iNumCreditHours = 8;

float fGPA;

fGPA =

iNumQualPoints / iNumCreditHours;

// gives 3.0 due to integer division

iVar = iVar * flVar

// gives compile-time error

shorthand operators
Shorthand Operators
  • iCounter = iCounter + 1; OR iCounter++;
  • iCounter = iCounter - 1; OR iCounter--;
  • iCounter = iCounter + 2; OR iCounter+=2;
  • iCounter = iCounter * 5; OR iCounter*=5;
  • latter 2 examples:
    • it’s “op” then “equals”

(e.g., +=2), not “equals”

then “op” (e.g., isn’t =+2)

some comments on comments

worthless

Good for blocks

Never closed

Some Comments on Comments
  • 1. C-style comments with /* */; no nesting
  • 2. C++ style comments beginning //
  • 3. A unique "doc comment" starting with /** ...*/
  • Fun with comments:
    • /*/
    • /* // */
    • ///////////////////
    • /* ========= */
javadoc
Javadoc

/**

* <PRE>

* getName(int i) - returns the name at a

* specified array location

* </PRE>

* @param int i - the index of the array to

* be retrieved

* @return String strName - the name

* @see Employees#isEmployed() - called to

* verify employment

*/

public String getName (int i) {

if (myEmpl.isEmployed())

return myArray[i];

else return "Nada";

} // getName(int)

David Dagon: Javadoc can generate an API listing of your code

javadoc cont d

You may include HTML tags (but avoid structuring tags, like <H1>, etc.)

  • Javadoc comments should immediately preceed the declaration of the class, field or method. The first sentence should be a summary. Use the special javadoc [email protected] When \'@\' tags are used, the parsing continues until the doc compiler encounters the next \'@\' tag.
Javadoc (Cont’d)

@ see <class name>

@ see <full-class name>

@ see<full-class name#method.name>

@ version

David Dagon: The @deprecated tag interacts with the compiler to turn off warnings.

@author @param

@return @exception

@deprecated // jdk 1.1 @since // jdk 1.1

@serial // jdk 1.2

constants
Constants
  • Valid:
    • public final <type> <IDer> = <value>;
  • Preferred:
    • public final static <type> <IDer> = <value>;
    • public final static int
    • iMIN_PASSING = 60;
    • public final static

float fPI = 3.14159;

  • Details on “why this

syntax” to come soon...

printing to screen
Printing to Screen
  • Various techniques:
    • System.out.println(<arguments>);
    • System.out.println( ); // prints blank line
    • System.out.println(5); // prints 5
    • System.out.println(“Hello World”);

// prints Hello World

    • “println” vs. “print” in Java:
      • println includes “carriage

return” at end, next print or

println on new line

      • print causes next print or

println to begin at next

location on same line

printing cont d
Printing (cont’d)
  • When starting Java, there are at least three streams created for you:

System.in // for getting input

System.out // for output

System.err // for bad news output

  • These are InputStream and PrintStream objects
  • Note: For Win95/NT System.out is

"almost" identical to System.err -

the both display to the screen

(the one exception is when using

DOS redirect, >, where System.out

is redirected, while System.err is

still put to the screen )

slide33

Printing (cont’d)

System.out.println

("This line is printed out")

System.err.println

("This is the error stream\'s output");

These are both instances of the PrintStream class.

There are other methods you might find useful in these classes:

System.out.flush();

System.out.write(int);

System.out.write(byte[] buf,

int offset, int length);

decision statements
Pseudocode:

if (condition) then

statements

else

other statements

endif

Java:

if (condition)

single statement;

else

single statement;

or:

if (condition)

{

statements;

}

else

{

statements;

}

Decision Statements
conditional assignment
Conditional Assignment

<boolean> ? <true condition>

: <false condition>

boolean b;

int iCount;

b = checkCompletion();

iCount = (b) ? 3 : 1;

Must resolve

to boolean

If true . . .

. . . if false

examples36
Pseudocode:

test_grade isofftype Num

...

is_passing isoftype Boolean

is_passing <- TRUE

if (test_grade < 60) then

is_passing <- FALSE

endif

print (is_passing)

Java: what happens here?

boolean bPassing = true;

int iTestGrade;

...

if (iTestGrade < 70)

bPassing = false;

System.out.println

(bPassing);

Examples
boolean and relational operators
Boolean and Relational Operators
  • Boolean: Pseudocode: Java:

AND AND &&

OR OR ||

NOT NOT !

  • Relational

equal to = = =

not equal to <> !=

less than < <

less than or equal to <= <=

greater than > >

greater than

or equal to >= >=

errors
Errors
  • Compile-time errors:
    • Syntax errors: illegal language statements
    • Certain kinds of semantic errors.e.g., type mismatch (assign a Character value to Boolean var)
  • Run-time errors:
    • Certain kinds of semantic errors.e.g., try to access non-existent

dynamic data (dereference a

nil/null pointer)

    • Logic errors: legal program

that produces wrong behavior

debugging strategies
Debugging Strategies
  • Compile-time errors:
    • The compiler will yell at you about them.
  • BUT . . . industry research shows . . .
    • It is a time-waster to:
      • code sloppy
      • use the compiler to find your mistakes
  • It is faster to:
    • check the “annoying details”

of your code to catch them

BEFORE compilation.

  • Lesson: don’t become a

compiler junkie!

debugging strategies40
Debugging Strategies
  • Run-time errors:
    • Programming environment provides tools for tracing values as code executes.
    • But: the most basic tools are independent of the programming environment:
      • 1. Code tracing: use your eyes and mind!
      • 2. Use print statements: insert statments that tell you

what key values are at given

points in your program.

      • 3. Use DEBUG flags: build in print statements

in a way that lets you turn

them on and off.

the debug flag idea
The “DEBUG” Flag Idea
  • In Pseudocode:

// in the main

DEBUG is TRUE // or FALSE

// inside of procedure This_Proc . . .

if DEBUG then

print (“am entering proc This_Proc”) print (“this_param: “, this_param:)

print (“that_param: “, that_param:)

endif

// code goes here

if DEBUG then

print (“am leaving proc This_Proc”) print (“this_param: “, this_param:)

print (“that_param: “, that_param:)

endif

the debug flag idea42
The “DEBUG” Flag Idea
  • In Java:

public static final boolean DEBUG = true;

public void myMethod()

{

if (DEBUG)

System.out.println

(“Entering myMethod()”);

if (DEBUG)

System.out.println

(“Leaving myMethod()”);

}

java file names
Java File Names

Source code files must have the ".java" extension. The file name should match the class name. This naming convention is enforced by most reasonable compilers.

Thus, an improperly named java file, saved as "myTest.java":

class test { ... }

Compiled byte code has the

".class" extension.

java file structure
Java File Structure
  • Java Files:
    • 1. Consist of the optional package statement,
    • 2. followed by any necessary import statements
    • 3. followed by the class name,
    • 4. followed by any inheritance
    • and interface declarations.
    • 5. Note: if the file defines more than one
    • class or interface, only one can be declared
    • public, and the source file name must match
    • the public class name.
an average java file

Thus:

    • package edu.gatech.cc.dagon.gsams-java;
    • import java.util.*;
    • import edu.gatech.cc.dagon.gsams-java.hashtable.*;
    • import netscape.javascript.JSObject;
    • import netscape.javascript.JSException;
    • public class SplayTree implements TreeType, TreeConstants
    • { ...
    • }// SplayTree
  • Note the globally unique
  • package name. Without a
  • package specification,
  • the code becomes part of an
  • unnamed default package in
  • the current directory.
An Average Java File
import statements
Import Statements
  • Import statements Come in three flavors:
      • import package.class; // 1
      • import package.*; // 2
      • import java.lang.*; // 3 (Implicit)
  • What it does: provides the Java interpreter with a reference to other classes necessary
  • for the compilation of the
  • present .java file
  • What it does NOT: actually
  • "import" or ”#include" the
  • code. There’s no overhead
  • or object bloat.
why no include statements
Why No #include statements?
  • Java maps fully qualified class names to a directory path, and therefore does not need an #include, #ifdef, etc. (and no preprocessor as well)
  • Thus:
      • java.awt.TextField
  • is mapped to:
      • java/awt/TextField
  • and dynamically loaded,
  • as needed.

David Dagon: This explains the mystery behind the error message reported when one attempts to run a file with the “.class” extension passed into the java VM:

java FooBar.class

Exception in thread "main" java.lang.NoClassDefFoundError: FooBar/class

Here, the VM looks for a file “FooBar in a folder called “class”

Also, some might argue that javadoc and doclets are types of preprocessors.

slide49

Java Methods

  • There exists a single construct, the method, for both procedures and functions:
  • when a procedure is called for, specify the return type “void” before method name

public void printHelloWorld( )

{

System.out.println(“Hello World!”);

} // of printHelloWorld

  • Note: All methods must have parentheses for parameters . . . even if no parameters!
slide50

Java Methods

  • Single construct for both procedures and functions:
  • when a function is called for, specify the
  • appropriate return type before method name

public float average (float fNum1,

float fNum2, float fNum3)

{

float fReturnVal;

fReturnVal =

(fNum1 + fNum2 + fNum3)/ 3;

return (fReturnVal);

} // of average

slide51

Writing Methods: A Larger Look

A Java requirement:

--All methods belong to an object (or class).

--Name of object (or class) must be unambiguous when method called.

--To run a program, there must be a class (whose name is the name-of-the-program), containing a special method called main:

for command line parameters

visible to all

nothingreturned

public static void main (String[ ] argv)

a class method,not aninstancemethod

Method name

slide52

Multiple Selections via switch

  • Use if construct for one selection.
  • Use if/else construct for double selection.
  • Use switch construct for multiple selection. (e.g., situations appropriate for if-elseif-elseif-else)Note:
  • Usefulwhen making a selection
  • among multiple values of the
  • same variable.
  • Not useful when selecting
  • among values of different variables.
slide53

Multiple Selections via switch--Notes

  • The switch statement can only be used with
  • the following types:
  • int, char, short & byte
  • (You can cast floats, doubles, etc.)
  • The case values must all be of the same type.
  • The case values must all be
  • FINAL constants.
slide54

Multiple Selections via switch

same

switch (chGrade) {

case ‘A’: case ‘a’:

iCountOfAGrades++;

break;

case ‘B’: case ‘b’:

iCountOfBGrades++;

break;

case ‘C’: case ‘c’:

iCountOfCGrades++;

break;

case ‘D’: case ‘d’:

iCountOfDGrades++;

break;

case ‘F’: case ‘f’:

iCountOfFGrades++;

break;

default:

System.out.println(“Invalid grade”);

break;

}

If (chGrade==‘A’ || chGrade==‘a’)

iCountOfAGrades++;

else if (chGrade==‘B’ || chGrade==‘b’)

iCountOfBGrades++;

else if (chGrade==‘C’ || chGrade==‘c’)

iCountOfCGrades++;

else if (chGrade==‘D’ || chGrade==‘d’)

iCountOfDGrades++;

else if (chGrade==‘F’ || chGrade==‘f’)

iCountOfFGrades++;

else

System.out.println

(“Invalid grade”);

(assume these variables exist and have value)

slide55

Multiple Selections via switch

Note the “optional” default case at the end of the switch statement.

It is optional only in terms of syntax.

switch (iNumber) {

case 1:

System.out.println (“One”);

break;

case 2:

System.out.println (“Two”);

break;

case 3:

System.out.println (“Three”);

break;

default:

System.out.println(“Not 1, 2, or 3”);

} // switch

In practice you should always include a ‘default’

case statement.

E.g., 1989 AT&T phone system crash

This would

work without

the default, but

would be

poor

technique

slide56

Java Basics: Iteration Constructs

  • In Pseudocode, we had a single iteration construct, flexible enough to be used in all iteration contexts.
  • Java, like most programming languages does not provide a single flexible construct.
  • Instead, Java offers three special case loop constructs, each good for a particular context.
  • Do not get accustomed to only one
  • of them and try to use it for
  • all situations.
slide57

Java Iteration Constructs: “For Loops”

Java syntax:

for (<initialization>; <continue if>;<increment>)

Pseudocode:

i isoftype Num

i <- 0

loop

exitif (i =10)

<some

statements>

i <- i + 1

endloop

Java example:

int i;

for (i=0; i<10; i++)

{

<some

statements>

}

for loops
For Loops

int count;

for (count = 0; count < 10; count ++)

for ( ; count < 10; count ++)

for (; count++<10;)

for (; ++count<10;)

for (int count =10; count -- > 0;)

for ( ; ; ) // infinite

for (count = 0; count < 10 &&

bNotDoneYet; count ++, otherCount--)

for (count = 0; count < 10;

printCount(count++); )

java iteration constructs for loops
Java Iteration Constructs: “For Loops”

for (i=0; i<N; i++);

{ … }

Common Problems with For Loops include:

--Spins on ending semicolon; code in braces executed once!

for (int i=0; i<N; i++){…}

--variable declared inside for

loop signature; poor style

--the variable may be needed

outside the for loop structure

slide60

Java Iteration Constructs: “Do While Loops”

Java example:

do {

statement 1;

...

statement N;

} while (condition);

Pseudocode:

loop

statement 1

...

statement N

exitif

(NOT(condition))

endloop

slide61

Java Iteration Constructs: “While Loops”

Java example:

<get the first value>

while (condition) {

<process the

current value>

<get the next value>

}

Pseudocode:

loop

<get the next value>

exitif (NOT(condition))...

<process the value>

endloop

slide62

Java Iteration Constructs: “While Loops”

  • When repeating steps, people naturally want to follow the pattern:
    • get a value, then process that value
  • The while loop construct calls for the unnatural pattern:
    • obtain the first loop control value before entering the loop itself;
    • then, within the loop body,
      • first do the process steps,
      • then do the get next steps
slide63

ASK:

Is it simply a count of the number of iterations?

Is it a value that the loop itself must compute?

Is it a value that already exists somewhere, and the loop only obtains it from elsewhere?

Java Iteration Constructs: When to Use

--The term “control variable” refers to the variable whose value is tested to determine if the loop should continue for another iteration or halt.

--For example, variable thisVar, below:

while (thisVar < = SOME_CONSTANT)

--To determine which loop construct is appropriate for a given situation, ask yourself “where does the control variable’s value come from?”

slide64

Java Iteration Constructs: When to Use

The for loop: used when the control variable is a simple count of the number of iterations,

e.g.: “create a loop that reads and processes the next 100 numbers.”

The while loop: used when the control variable has a value that already exists and is simply obtained by the loop.

e.g.: “create a loop that reads in numbers and processes them until it reads in a 100.”

The do-while loop: used when the control

variable’s value must be calculated

by the loop itself.

e.g.: “create a loop that reads in numbers until their sum is greater than 100.”

slide65

Java Iteration Constructs: Review

Which loop construct would you use if...

You need to perform a series of steps exactly N times?

You need to traverse a linked list of unknown size, and stop when you find a certain value?

You need to perform a series of

steps at least once, and continue

performing these steps for an

unknown number of times

slide66

Debugging Tools/Strategies: The Assert Statement

CS1502 has created a general purpose utility class to assist you in programming. (See the class web page, and labs.) One useful method is ASSERT(), which can be used to validate assumptions and conditions.

Precondition: statement that must be true

before the method can begin execution.

Postcondition:statement that must be true

after the method has executed.

Usage:

util.ASSERT(iDenominator!=0,

“Can’t divide by zero!”);

iFraction =

iNumerator/iDenominator;

slide67

Writing Methods--Flawed Example

public char letterGrade (int iGrade) {

util.ASSERT(iGrade >= 0 && iGrade <=100,

“iGrade param has invalid value.”);

if (iGrade >= 90)

return (“A”);

if (iGrade >= 80 || iGrade < 90)

return (“B”);

if (iGrade >= 70 || iGrade < 80)

return (“C”);

if (iGrade >= 60 || iGrade < 70)

return (“D”);

if (iGrade <= 60)

return (“F”);

} // of letterGrade

What’s wrongwith this?

--Error: returns a String, not a char!

--Style: use if/else chain to avoid unintended execution of code

--Style: multiple returns are to be avoided, if possible

slide68

Writing Methods--Repaired Example

public char letterGrade (int iGrade)

{

char chReturnValue = ‘I’;

util.ASSERT(iGrade >= 0 && iGrade <=100,

“iGrade param has invalid value.”);

if (iGrade >= 90)

chReturnValue = ‘A’;

else if (iGrade >= 80 && iGrade < 90)

chReturnValue = ‘B’;

else if (iGrade >= 70 && iGrade < 80)

chReturnValue = ‘C’;

else if (iGrade >= 60 && iGrade < 70)

chReturnValue = ‘D’;

else // given Assertion, iGrade must be < 60

chReturnValue = ‘F’;

return (chReturnValue);

} // of letterGrade

Is this right?

slide69

Writing Methods--Flawed Example

/**

* Calculate the recurrence relation, r(n) = r(n-1) +

* r(n-2) - r(n-3), where: r(1)=1, r(2)=2, r(3)=3, n>=1.

* @param is the value of recurrence relation to calculate.

* @return the integer value of the recurrence relation at n.

*/

public int recurrence (int iN) {

int iReturnVal = 0;

util.ASSERT(iN >= 1, “int param is less than 1”);

if (iN == 1) iReturnVal = 1;

else if (iN == 2) iReturnVal = 2;

else if (iN == 3) iReturnVal = 3;

else

iReturnVal = recurrence (iN-1) +

recurrence(iN-2) - recurrence(iN-3);

return (iReturnVal);

} // of recurrence

What’s wrongwith it?

slide70

Writing Methods--Repaired Example

/**

* Calculate the recurrence relation, r(n) = r(n-1) +

* r(n-2) - r(n-3), where: r(1)=1, r(2)=2, r(3)=3, n>=1.

* @param is the value of recurrence relation to calculate.

* @return the integer value of the recurrence relation at n.

*/

public int recurrence (int iN)

{

util.ASSERT ( iN >= 1, “int param is less than 1”);

return (iN);

} // of recurrence

Lesson: while creating code that works, remember to think!

slide71

Classes and Objects

Class:

describes the form of an object,

a template or blueprint or mold

specifies data representation, behavior, and inheritance (via variables, methods and parents)

Object:

an instance of a class

--has unique copy of every non-static variable (i.e., the “instance variables” but not the class variables).

Difference between “a class and an object of that class” is analogous to the difference between “a type and a variable of that type”.

KEY CONCEPT

Naming Conventions:

Classes: Identifiers begin with cap letters for

each word in the Identifier,

e.g., NeuralNetwork.java

Objects: Identifiers begins with lower case letter,

then caps for other words in identifier,

e.g., myObjectIdentifier

slide72

Java Data Structures

In Java, any data structure that is not a primitive must be an object of some class.

Thus, to create what is logically a record, define a class (ala a type), specifying both:

the data fields of the record

all the methods that can act upon those data.

Because we can use a class to encapsulate

both the data and the methods associated

with the what would be a record, there is no

need for a record construct.

Java doesn’t need a record construct

Java doesn’t have a record construct.

Instead, one implements records

(with methods) as a class.

slide73

Evolution of an Object: An Example

Pro:

Straightforward, no special techniques

Con:

No data type; no reusability, cannot create multiples of it.

Poor abstraction (none, except for data identifiers)

Pseudocode:

length isoftype Num

read(length)

width isoftype Num

read(width)

height isoftype Num

read(height)

volume isoftype Num

volume <- length * width * height

slide74

Evolution of an Object (cont’d)

Box_Type definesa record

length isoftype Num

width isoftype Num

height isoftype Num

volume isoftype Num

endrecord

this_box isoftype Box_Type

read(this_box.length, this_box.width,

this_box.height)

this_box.volume <- this_box.length *

this_box.width*this_box.height

Pro:

Grouping of data, better data abstraction: box as data entity, not just a set of variables.

More reusable: can easily get multiple boxes

Con:

Direct manipulation of data

Low procedural abstraction: must think about “how to do it?” instead of “what I want to do?”

slide75

Evolution of an Object (cont’d)

How to model a box? (Java example)

Instance variables (because they’re not static)

class Box {

int iLength;

int iWidth;

int iHeight;

public void setLength (int iNewLength) {

util.ASSERT (iNewLength > 0, “iNewLength <= 0”);

iLength = iNewLength;

} // of setLength

public void setWidth (int iNewWidth) {

util.ASSERT (iNewWidth > 0,

“iNewWidth <= 0”);

iWidth = iNewWidth;

} // of setWidth

public int getLength ( ) {

return (iLength);

} // of getLength

Methods to change (‘modify’) instance variables

Method to get (‘access’) instance variable

slide76

Methods to get (‘access’) instance variables

Evolution of an Object (cont’d)

How to model a box? (Java example cont’d)

public void setHeight (int iNewHeight) {

util.ASSERT (iNewHeight > 0, “iNewHeight <= 0”);

iHeight = iNewHeight;

} // of setHeight

public int getWidth ( ) {

return (iWidth);

} // of getWidth

public int getHeight ( ) {

return (iHeight);

} // of getHeight

public int getVolume ( ) {

return ( getLength( ) *

getWidth( ) * getHeight( ) );

} // of getVolume

} // of class Box

slide77

Object Terminology

“Accessor” and “Modifier” Methods: “state objects.”

Accessor Methods:

The “get” methods . . .

( The identifier name prefix “get” not required, but is preferred practice. )

They return a value about

the state of an object.

They can return a variable value, and can return something that is calculated (e.g. getVolume; no need for an iVolume attribute).

Modifier Methods:

The “set” methods . . .

( The identifier name prefix ‘set” not required, but is preferred practice. )

Allows us to restrict access to variables and thereby control their values. (Otherwise, we would have to include util.ASSERT statements everywhere!)

slide78

Using Objects: Creating a Box

class BoxesExample {

public static void main (String[ ] argv) {

Box shoeBox;

shoeBox = new Box( );

shoeBox.setLength(35);

shoeBox.setWidth(19);

shoeBox.setHeight(13);

Box cdBox = new Box( );

cdBox.setLength(14);

cdBox.setWidth(9);

cdBox.setHeight(1);

int iTotalVolumeOfBoxes;

iTotalVolumeOfBoxes =

shoeBox.getVolume()

+ cdBox.getVolume();

System.out.println

(“The combined volume of the boxes”);

System.out.println

(“is: “, iTotalVolumeOfBoxes);

} // of main

} // of class BoxesExampleProgram

A second class is used

to create an instance

of our box class.

The BoxesExample

class has a main

method, and is run

as a program

slide79

Using Objects: Creating a Box

class BoxesExample {

public static void main (String[ ] argv) {

Box shoeBox;

shoeBox = new Box( );

shoeBox.setLength(35);

shoeBox.setWidth(19);

shoeBox.setHeight(13);

Box cdBox = new Box( );

cdBox.setLength(14);

cdBox.setWidth(9);

cdBox.setHeight(1);

int iTotalVolumeOfBoxes;

iTotalVolumeOfBoxes =

shoeBox.getVolume()

+ cdBox.getVolume();

System.out.println

(“The combined volume of the boxes”);

System.out.println

(“is: “, iTotalVolumeOfBoxes);

} // of main

} // of class BoxesExampleProgram

First, we declare

a variable

called ‘shoeBox.’

At this point, shoeBox

is merely a null

reference.

Second, we

instantiate shoeBox.

Values are assigned.

slide80

Using Objects: Creating a Box

class BoxesExample {

public static void main (String[ ] argv) {

Box shoeBox;

shoeBox = new Box( );

shoeBox.setLength(35);

shoeBox.setWidth(19);

shoeBox.setHeight(13);

Box cdBox = new Box( );

cdBox.setLength(14);

cdBox.setWidth(9);

cdBox.setHeight(1);

int iTotalVolumeOfBoxes;

iTotalVolumeOfBoxes =

shoeBox.getVolume()

+ cdBox.getVolume();

System.out.println

(“The combined volume of the boxes”);

System.out.println

(“is: “, iTotalVolumeOfBoxes);

} // of main

} // of class BoxesExampleProgram

These steps are

repeated for our

next variable,

cdBox. Note that the

1st two steps are here

collapsed into

one line:

Declaration &

Instantiation

slide81

Using Objects: Creating a Box

class BoxesExample {

public static void main (String[ ] argv) {

Box shoeBox;

shoeBox = new Box( );

shoeBox.setLength(35);

shoeBox.setWidth(19);

shoeBox.setHeight(13);

Box cdBox = new Box( );

cdBox.setLength(14);

cdBox.setWidth(9);

cdBox.setHeight(1);

int iTotalVolumeOfBoxes;

iTotalVolumeOfBoxes =

shoeBox.getVolume()

+ cdBox.getVolume();

System.out.println

(“The combined volume of the boxes”);

System.out.println

(“is: “, iTotalVolumeOfBoxes);

} // of main

} // of class BoxesExampleProgram

Another variable

is declared

and assigned a value.

NOTE: We obtain

information about

the box ONLY through

it’s accessor methods

slide82

Using Objects: Creating a Box

class BoxesExample {

public static void main (String[ ] argv) {

Box shoeBox;

shoeBox = new Box( );

shoeBox.setLength(35);

shoeBox.setWidth(19);

shoeBox.setHeight(13);

Box cdBox = new Box( );

cdBox.setLength(14);

cdBox.setWidth(9);

cdBox.setHeight(1);

int iTotalVolumeOfBoxes;

iTotalVolumeOfBoxes =

shoeBox.getVolume()

+ cdBox.getVolume();

System.out.println

(“The combined volume of the boxes”);

System.out.println

(“is: “, iTotalVolumeOfBoxes);

} // of main

} // of class BoxesExampleProgram

Comments:

1. Variable declaration and assignment

is mixed together. A more complex

program might require better organization

2. “Hybrid OO”: We have classes, but

everything occurs within a static method,

“main”, making things linear. This is fine

for small programs, but larger programs

would benefit from “Pure OO”. Keep this

in mind; it becomes very important later.

slide83

Review: Our Box Model

public void setHeight (int iNewHeight) {

util.ASSERT (iNewHeight > 0,

“iNewHeight <= 0”);

iHeight = iNewHeight;

} // of setHeight

public int getHeight ( ) {

return (iHeight);

} // of getHeight

public int getVolume ( ) {

return ( getLength( ) * getWidth( )

* getHeight( ) );

} // of getVolume

} // of class Box

class Box {

int iLength;

int iWidth;

int iHeight;

public void setLength (int iNewLength) {

util.ASSERT (iNewLength > 0,

“iNewLength <= 0”);

iLength = iNewLength;

} // of setLength

public int getLength ( ) {

return (iLength);

} // of getLength

public void setWidth (int iNewWidth) {

util.ASSERT (iNewWidth > 0,

“iNewWidth <= 0”);

iWidth = iNewWidth;

} // of setWidth

public int getWidth ( ) {

return (iWidth);

} // of getWidth

slide84

Declaring Objects

shoeBox

  • We found that to use this class, we had to ‘declare an object’. “Declaring an object” really means declaring a reference to an object.”
  • A reference is an implict (or automatic) pointer that can point to an object of the specified class.
  • Thus, the code:
  • Box shoeBox;
  • does not create an object of class Box.
  • does create a reference (or ptr) shoeBox that can point to an object of class Box.
  • gives us what amounts to a ptr to a Box
  • which is null:
slide85

Objects and References

So far, we have:

Box shoeBox;

shoeBox

  • To make the reference shoeBox be not null,
  • it is necessary to instantiate it, e.g.,
  • shoeBox = new Box( );

shoeBox

an instance of class Box

When Java encounters the keyword “new”, it allocates space in memory for an instance of that object. Now, shoeBox refers to an instance of class Box, i.e., an object.

Note that the instance (or object) “gets” everything defined in class Box. It has unique copies of all the variables. (The methods are shared between all instance of the class, but Java knows which instance you are referring to.)

slide86

Objects and References

The data fields (“attributes”):

int iLength;

int iWidth;

int iHeight;

What can be done to that data(“methods”):

public void setLength (int iNewLength)

public int getLength ( )

public void setWidth (int iNewWidth)

public int getWidth ( )

public void setHeight (int iNewHeight)

public int getHeight ( )

public int getVolume ( )

A closer look:

shoeBox

(a reference

to an object

of class Box)

slide87

Objects and References

Box shoeBox

= new Box();

Box cdBox

= new Box();

Box present

= new Box();

Each time we instantiate

a Box, therefore, we get a unique copy to work with. This is one of the most powerful aspect of Object-Oriented Programming!

shoeBox

The data fields (“attributes”):

int iLength;

int iWidth;

int iHeight;

What can be done to that data(“methods”):

public void setLength (int iNewLength)

public int getLength ( )

public void setWidth (int iNewWidth)

public int getWidth ( )

public void setHeight (int iNewHeight)

public int getHeight ( )

public int getVolume ( )

cdBox

The data fields (“attributes”):

int iLength;

int iWidth;

int iHeight;

What can be done to that data(“methods”):

public void setLength (int iNewLength)

public int getLength ( )

public void setWidth (int iNewWidth)

public int getWidth ( )

public void setHeight (int iNewHeight)

public int getHeight ( )

public int getVolume ( )

present

David Dagon:

(Cautionary Note: Here, we suggest that each object gets a unique copy of each method. Although each object is allocated unique memory space for variables, Java efficiently shares methods in common with all objects. For now, you might find it helpful to picture objects in the manner, even though it’s not technically what happens with the heap’s method space in the Java Virtual Machine.)

The data fields (“attributes”):

int iLength;

int iWidth;

int iHeight;

What can be done to that data(“methods”):

public void setLength (int iNewLength)

public int getLength ( )

public void setWidth (int iNewWidth)

public int getWidth ( )

. . .

slide88

Objects and References

What do we conclude from this?

1. all objects are dynamic data.

2. because all objects are dynamic, Java “knows” that, whenever we reference an object, it must “follow the pointer”.

For example:

shoeBox.setLength(35);

shoeBox.setWidth(19);

shoeBox.setHeight(13);

is equivalent to the pseudocode:

shoeBox^.setLength(35)

shoeBox^.setWidth(19)

shoeBox^.setHeight(13)

slide89

References vs. Pointers

Java is advertised as “having no pointers.”

In reality, Java is mostly pointers!

Every non-primitive datum must be an object. All objects are dynamic data, accessible via references. And references are really implicit pointers.

Java does not have explicit pointers:

There exists no way to explicitly manipulate pointers. There is no explicit dereferencing operator

So:

if you don’t know about pointers, then

references are “magic”

if you do understand pointers, then

you know what references really are!

slide90

Calling (or Invoking) Methods

  • Restrictions:
  • 1. Invocation must be unambiguous re: which object or class the method is to act upon.
  • 2. If the method call appears inside a class, then that class is presumed to contain the appropriate method:

class CompanyStock {

public double getAmountEarned ( ) {

double dOpen = getOpenValue();

/* calls method in this class */

double dClose = getCloseValue();

/* calls method in this class */

. . .//etc. etc.

} // of getAmountEarned

public double getOpenValue() {

return 1234.5d;

}

public double getCloseValue(){

return 2345.6d;

}

} // of CompanyStock

slide91

Calling (or Invoking) Methods

  • If the method is NOT inside the class where method declared, then the object must be specified.
  • class WallstreetJitters {
  • public void panicSell ( ) {
  • Greenspan fedChair
  • = new Greenspan();
  • if (fedChair.raisesInterest())
  • {
  • buyOnMargin(); }
  • else
  • {
  • sellAllStock();
  • }
  • } // of panicSell
  • } // of WallstreetJitters

Greenspan class must have this method!

These methods must appear in this class!

FORMAT:

<object reference>.<method name>

slide92

Java Constructors

Note: Constructor method has same identification as the class.

Can now do:

Box subwooferBox =

new Box (46, 46, 82);

Equivalent to:

Box subwooferBox = new Box; subwooferBox.setLength(46); subwooferBox.setWidth(46); subwooferBox.setHeight(82);

Motivation: We need a means of initializing the attributes of a new object (or “instance”) when it is created.

Means: “Constructor methods” that are invoked automatically upon “instantiation” (creation) of new object.

Example:

public Box (int iNewLength, int

iNewWidth, int iNewHeight) {

setLength (iNewLength);

setWidth (iNewWidth);

setHeight (iNewHeight);

} // of constructor

slide93

Java Constructors

Given:

public String ( )

public String (String value)

We can:

String strInput1 = new String ( );

String strInput2 = new String

(“A valid constructor.”);

A class may have more than one constructor.

If so, then each constructor must have unique formal parameter list.

Constructor calls must match one of the available constructors

Terminology: Creating multiple methods with same identifier is called “method overloading.”

Such that:

strInput1 will be an empty

String (with the value ““)

strInput2 will be a String containing

“A valid constructor.”

slide94

class person {

String strName;

int iAge;

public Person (String strNewName){

setName (strNewName);

} // of constructor

public Person (String strNewName, int iNewAge) {

setName (strNewName);

setAge (int iNewAge);

} // of constructor

public void setName (String strNewName){

strName = strnewName;

} // of setName

public void setAge (int iNewAge) {

iAge = iNewAge:

} // of setAge

} // of Person

Java Constructors:

Another Example

Note that the

constructors call

the modifiers

slide95

Java Constructors

Can now create a new Person via:

Person guitarist1 = new Person (“Clapton”);

Person guitarist2 = new Person (“Hendrix”, 27);

Determining which constructor to invoke requires unique signature.

Signature means “identifier and parameter list”

Thus, one cannot do:

public Person (String strNewFirstName) {

. . .

} // of constructor

public Person (String strNewLastName) {

. . .

} // of constructor

due to ambiguity. Can’t tell which one to invoke.

slide96

Java Constructors

Default constructors:

If you don’t define a constructor, a default constructor will be automatically invoked.

The default constructor expects no parameters.

The default constructor initializes instance variables to standard Java default values (0 for nums, false for booleans, null for references).

Default constructor equivalent to:

public Person ( )

{

;

} // of default constructor

You can override this by creating your own

default constructor (no params) that does

contains code.

slide97

Java Constructors

Constructors CANNOT have return values:

Do NOT do this:

public int Person ( )

{

. . . // whatever code

} // of constructor

public void Person ( )

{

. . . // whatever code

} // of constructor

Nope!

A return value (including void) means that the method is NOT a constructor, and it won’t be auto-invoked.

slide98

Creating Instances of Classes

Involves three things:

1. Creating the reference: Box thisBox ;

2. Instantiating the object: thisBox = new Box( );

OR do first two steps at once, e.g.,

Box thisBox = new Box( )

3. Having constructor(s) set initial values:

public Box (int iNewLength, int iNewWidth, int iNewHeight) {

setLength (iNewLength);

setWidth (iNewWidth);

setHeight (iNewHeight);

} // of constructor

With an appropriate constructor, we can do all three at once:

Box thisBox = new Box (10, 5, 25);

slide99

Objects and References

Distinguish between primitives and objects.

Assignment with Primitives:

Code: Memory:

int x;

x

x

y

int y;

x=5

y

iThis = 5;

x=5

y=5

iThat = iThis;

slide100

Objects and References

Assignment with References to Objects:

Code: Memory:

Box box1;

Box box2;

box1

box2

box1 box2

box1 = new Box(8, 5, 7);

L=8, W=5, H=7

box2 = box1;// note: two references// but only one object

box1box2

L=8, W=5, H=7

box1box2

L=3, W=9, H=2

box1 =

new Box(3, 9, 2);

L=8, W=5, H=7

box1 = box2;

// Old reference lost!

box1box2

L=3, W=9, H=2

L=8, W=5, H=7

slide101

Instance vs. Class Declarations

  • A distinction that applies to both:
    • variables
    • methods
  • An instance <variable or method> is one thatbelongs to each object of a class.
  • A class <variable or method> is one that belongs only to the class itself.
  • The keyword static:
  • indicates a class variable or class method.
  • absence of the keyword static indicates an instance variable or instance method.
slide102

Instance vs. Class Variables

  • Suppose we wanted to track the total number of objects created.
  • Consider:
  • class Human {
  • String strName;
  • int iPopulation = 0;
  • public Human (String strName) {
  • this.strName = strName;
  • iPopulation++; //WRONG!
  • } // of constructor
  • } // of Human

Declares a strName String for each instance .

Thus, each Human will have its own name.

But . . .

Also declares an iPopulation counter for each instance of Human.

Thus, each Human will have its own iPopulation variable, each having a value of one. This makes no sense!

slide103

Instance vs. Class Variables

class Human {

String strName;

static int iPopulation = 0;

public Human (String strName) {

this.strName = strName;

iPopulation++;

} // of constructor

} // of Human

one change

As we know, this declares a strName String for each instance.

Thus, each Human will have its own name.

NOTE:

Each Human does not get an iPopulation counter.

This declares a single iPopulation counter for the class Human itself. It is a class variable.

Thus, each Human will increment this single shared counter by 1.

slide104

Instance vs. Class Variables:

When to Use

  • Use instance variables whenever each object should have its own variable.
  • e.g.,
    • attributes of the particular object.
  • Use a class variable whenever the class itself should maintain a single copy of datum pertaining to all instances of the class.
  • e.g.,
    • population counts.
    • summary data.
    • assigning serial numbers.
    • shared resources.
slide105

Instancevs.Class Variables

Constants Revisited:

class ConstantExample

{

final int iMAXSIZE = 10;

} // of ConstantExample

Declares a different-but-identical constant for each instance of the class.

Wasteful with zero benefit.

class ConstantExample

{

static final int iMAXSIZE = 10;

} // of ConstantExample

Declares a single constant for use by all instances of the class.

slide106

Objects vs. References--An Example

class MyObject {

String name;

public MyObject (String n) {

name = n;

}

public String toString () {

return (name);

}

}

public class SwapTester {

public static void swapInt (int x, int y) {

int temp;

System.out.println ("Doing swapInt");

temp = x;

x = y;

y = temp;

}

slide107

/* in class Basics (cont’d) . . . */

public static void swapObject1 (MyObject x, MyObject y) {

MyObject temp;

System.out.println ("Doing swapObject1");

temp = x;

x = y;

y = temp;

}

public static void swapObject2 (MyObject x, MyObject y) {

MyObject temp = new MyObject ("temp");

System.out.println ("Doing swapObject2");

temp.name = x.name;

x.name = y.name;

y.name = temp.name;

}// swapObject2

slide108

/* in class SwapTester (cont’d) . . . */

public static void main (String argv[]) {

int x, y, z; x = 5; y = 10; z = y; y = 5;

System.out.println ("x=" + x + " y=" + y + " z=" + z);

System.out.println ("x==5 is " + (x==5));

System.out.println ("x==y is " + (x==y));

System.out.println ("y==z is " + (y==z));

swapInt (x, z);

System.out.println ("x=" + x + " y=" + y + " z=" + z);

System.out.println ();

Output for this portion of the program:

x=5 y=5 z=10

x==5 is true

x==y is true

y==z is false

Doing swapInt

x=5 y=5 z=10

slide109

/* in class SwapTester’s main method (cont’d) . . . */

String a, b, c;

a = "hello";

b = new String ("hello");

c = b;

System.out.println ("a=" + a + " b=" + b + " c=" + c);

System.out.println ("a==hello is " + (a=="hello"));

System.out.println ("b==hello is " + (b=="hello"));

System.out.println ("a==b is " + (a==b));

System.out.println ("a.equals(b) is " + (a.equals(b)));

System.out.println ("b==c is " + (b==c));

System.out.println ("b.equals(c) is " + (b.equals(c)));

System.out.println ();

a=hello b=hello c=hello

a==hello is true

b==hello is false

a==b is false

a.equals(b) is true

b==c is true

b.equals(c) is true

Output

slide110

/* in class SwapTester’s main method (cont’d) . . . */

MyObject p, q, r, s;

p = new MyObject ("hello");

q = new MyObject ("hello"); r = q;

s = new MyObject ("world");

System.out.println ("p=" + p + " q=" + q + " r=" + r + " s=" + s);

System.out.println ("p==q is " + (p==q));

System.out.println ("q==r is " + (q==r));

swapObject1 (p, s);

System.out.println ("p=" + p + " q=" + q + " r=" + r + " s=" + s);

swapObject2 (p, s);

System.out.println ("p=" + p + " q=" + q + " r=" + r + " s=" + s);

}// end of main

}// class SwapTester

p=hello q=hello r=hello s=world

p==q is false

q==r is true

Doing swapObject1

p=hello q=hello r=hello s=world

Doing swapObject2

p=world q=hello r=hello s=hello

Output:

slide111

Objects vs. References: Summary

Key Concepts:

= and == with objects and references.

Parameter passing, call by value, call by constant reference.

Summary:

Java has no pointers, only references.

Null is a special value that means "no object" or "absenceof reference”.

All objects and arrays (i.e., everyting except primitivetypes) are handled by reference

= assigns references to objects

(use clone() to copy theobject itself).

== and != test references with references

(use equals() totest the objects themselves).

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