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Java Program Structure. Execution begins with first statement in main() Every Java program MUST have a static method called main( )! public static void main(String[] args ) { … }. Identifiers. Letters, digits, underscore, $ Must begin with letter, underscore, or $ Case sensitive

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java program structure
Java Program Structure
  • Execution begins with first statement in main()
  • Every Java program MUST have a static method called main( )!

public static void main(String[] args)

{

}

Java Elements

identifiers
Identifiers
  • Letters, digits, underscore, $
  • Must begin with letter, underscore, or $
  • Case sensitive
  • No spaces

Java Elements

identifiers1
Identifiers
  • Legal:
    • Hello numPeople person2
  • Illegal
    • Hello! One+two employee salary
  • Self documenting

Java Elements

data and data types
Data and Data Types
  • all data must have a data type
  • data type determines:
    • internal representation and storage size.
    • Range of values
    • processing/operations

Java Elements

variables
All Java identifiers must be declared before they are used

Declarations - create and labels storage

Memory location assigned

Declare one variable per line

type name;

int a;

inta,b;

int a; // preferable

int b;

Variables

Java Elements

primitive types
Primitive Types

Java Elements

integer data type
int

whole numbers and their negatives in the range allowed by your computer, no decimal point

5,-99,3456

examples:

int x;

int y;

int total;

int keys;

integer data type

Java Elements

boolean
boolean
  • true or false
  • Example:

boolean done;

done = true;

Java Elements

slide9
char
  • one character
    • a letter, a digit, or a special symbol
  • sample values: 
    • 'A',  'B',  'a',  'b',  '1',  '2', '+', '-', '$', '#', '?', '*', etc
  • Unicode
  • Each character is enclosed in single quotes.
  • The character '0' is different than the integer 0.

Java Elements

slide10
char
  • Example

char letter;

letter = 'A',

Java Elements

real numbers
Real numbers
  • Numbers with decimals
  • For very large numbers or very small fractions

3.67 * 1017  = 367000000000000000.0 = 3.67E17

5.89 * 10-6 = 0.00000589 = 5.89E-6

  • Scientific notation/floating point notation
    • e stands for exponent and means "multiply by 10 to the power that follows“
  • Examples:

5.274  .95  550. 9521e-3   -95e-1 95.213e2

Java Elements

slide12

float

    • 4 bytes
    • -3.4E+38 – 3.4E+38
    • 6- 7 significant digits
    • Single precision
    • Use if size is an issue
  • double
    • 8 bytes
    • -1.7E+308 – 1.7E+308
    • 15 significant digits
    • Double precision
    • Use if precision is an issue, i.e currency

Java Elements

double
double
  • Example:

double price;

double velocity;

price = 10.6;

velocity = 47.63555;

Java Elements

initialization
Initialization
  • give a variable a value to start with
  • can initialize in declaration

int a = 1;

int alpha = 32;

int stars = 15;

intcount = 0;

// the following example is legal in java, but

// violates security guidelines

int length, width = 5; // only width is initialized

// and now we see why

Java Elements

constants
Constants
  • cannot be changed
  • class constant – can be accessed anywhere in the class
  • Make programs easier to read
  • Makes value easier to change
  • Generally declare outside of method

Java Elements

declaring constants
Declaring constants
  • final type name = value; //local
  • public static final type name = value; //global

public static final double PI = 3.14159;

public static final char BLANK = ' ';

public static final double INT_RATE = 0.12;

  • use all caps and underscore

Java Elements

assignment
Assignment
  • variable = expression;
  • different than equality
    • How it works: First the expression on the right-hand side is evaluated and then the resulting value is stored in the variable (in memory) on the left-hand side of the assignment operator.
  • variable is the name of a physical location in computer memory
  • an expression may be
      • a constant, or
      • a variable
      • a formula to be evaluated

Java Elements

expressions
Expressions
  • Simple value or set of operations that produces a value
  • literal
    • 24 or -3.67
  • Evaluation – obtain value of expression
  • Operator
    • A symbol used to indicate an operation to be performed on one or more values

Java Elements

literals
Literals
  • int
  • 0 2 0372 0xDadaCafe 1996 0x00FF00FF
  • double

28.43 -.98 208. 2.3e4

  • char

'a' 'X' '?'

'\\' '\''

  • bool

true false

Java Elements

integer division
Integer division

=> Rounds towards 0

3 /4 => 0

19 / 5 => 3

5/3 => 1

  • Division by zero is illegal and an ArithmeticException is thrown
  • Security issue
    • If the dividend is the negative integer of largest possible magnitude for its type, and the divisor is -1, then integer overflow occurs and the result is equal to the dividend.
    • No exception is thrown in this case

Java Elements

modulus
Modulus
  • Remainder

28 % 7 => 0

19 % 5 => 4

25 % 2 => 1

  • Testing for even or odd
    • Number % 2 = 0 for evens
  • Finding individual digits of a number
    • Number % 10 is the final digit
  • Also works for floats

Java Elements

combined assignment operators
Combined Assignment Operators
  • number = number + 5;  number += 5;
  • number = number * 10;  number *= 10;
  • +=
  • -=
  • *=
  • /=
  • %=

Java Elements

increment and decrement
Increment and Decrement
  • Increment ++

x = x+ 1;

++x;

x++;

  • Decrement ‑‑

--x;

x--;

Java Elements

increment and decrement1
Increment and Decrement
  • Prefix: ++x => increment x before using it
    • Generally more efficient than postfix
  • Postfix: x++ => increment x after using value
  • Standalone, result is the same
    • ++x;
    • x++;
  • You see the behavior when used in expressions

x = 5;

y = 5;

System.out.println(++x + “ “ + y++);

//outputs 6 and 5

// final value of x and y is 6

Java Elements

precedence
Precedence
  • Orders of operation
  • "who goes first" when expressions have multiple operators
  • rules are similar to rules used in algebra
  • () parentheses will override the precedence rules
  • When operators have same precedence, operations are performed left to right

Java Elements

precedence1
Precedence
  • 3 + 5 + 6 / 2 => 11
  • (3 + 5 + 6)/2 => 14
  • Left to right
  • 40 – 25 - 9 =>

(40-25) – 9 => 15 – 9 => 6

EXAMPLE:

13 * 2 + 239 / 10 % 5 – 2 * 2 => 25

Java Elements

mixing types and casting
Mixing Types and Casting
  • explicit conversion of a value from one data type to another.
  • (type) variable
  • Example:
    • (int) 2.5 /.15 //converts 2.5 to int
    • (int) (2.5 /.15) //converts result to int

Java Elements

assignment notes
Assignment notes
  • Cannot switch variable and expression
  • left side cannot be constant or formula
  • Only one variable can be on the left-hand side
  • Note: Make sure you initialize variables before using them in right-hand side of an assignment statement. An uninitialized variable will have some "garbage value"

Java Elements

assignment examples
int stamp;

intanswer;

Intwidget;

char letter;

stamp = 14; // valid

14 = stamp; // invalid

answer = stamp;

widget = stamp * 3;

letter = 'a';

letter = "alpha"; // invalid

Assignment examples

Java Elements

intro to strings
Intro to Strings
  • String not built into Java
  • String is a class
  • Part of java.lang class
  • Automatically imported
  • Declare:
    • String object; // note caps
  • String name = "Tom Jones";
  • String s1 = "Hello”;
  • String s2 = "World!";

Java Elements

strings
Strings
  • Strings can be created implicitly by using a quoted string
    • String s1 = "Hello”;
    • String s2 = "World!";
  • or, by using + on two String objects to create a new one
    • String combo = s1 + " " + s2;

Java Elements

string methods
String methods
  • <variable>.<method name>(<expression>,<expression>,…,<expression>)
    • any String method requiring an index will throw an IndexOutOfBoundsException if 0 > index > length()-1
    • String s1 = "Hello”;
    • String s2 = "World!";
    • System.out.println("Length of s1 = " + s1.length());
    • Length of s1 = 5
    • Index – integer used to indicate location
      • Zero-based

Java Elements

console input
Console Input
  • Scanner is a class
  • Need to import java.util

import java.util.*; //allows use of Scanner class

Scanner console = new Scanner (System.in);

  • Creates object console (can use any identifier name)
  • Associates console with standard input device
  • System.in – standard input device

Java Elements

console input1
Console input
  • console.nextInt() // retrieves next item as an integer
  • console.nextDouble() //retrieves next item as double
  • console.next() //retrieves next item as string
  • console.nextLine() // retrieves next item as string up to newline
  • ch = console.next().charAt(0); // reads a single character
  • Inappropriate type -> exception

Java Elements

slide38

import java.util.*;

public class BMICalculator

{

public static void main(String [] args)

{

double height;

double weight;

double bmi;

Scanner console = new Scanner(System.in);

System.out.println("Enter height");

height = console.nextDouble();

System.out.println("Enter weight");

weight = console.nextDouble();

bmi = weight/(height * height) * 703;

System.out.println("Current BMI:");

System.out.println(bmi);

}

}

Java Elements

output
Output
  • System.out – standard output device
  • System.out.print(expression);
  • System.out.println(expression);//goes to next line
  • System.out.println();//blank line

Java Elements

packages classes methods import
Packages, Classes, Methods, import
  • Few operations defined in Java
  • Many methods & identifiers are defined in packages
  • Class – set of related operations, allows users to create own type
  • Method – set of instructions designed to accomplish a specific task
  • Package – collection of related classes
    • java.util, contains class Scanner and methods nextInt, etc.

import packageName.*;

Import java.util.*; // compiler determines relevant classes

import java.util.Scanner;

Java Elements

creating a java application program
Creating a Java Application program
  • Program consists of one or more classes
  • Declare variables inside method
  • Declare named constants and input stream objects outside of main

Java Elements

java application program
Java application program

import statements if any

publicclassClassName

{

declare names constants and/or stream objects

public static void main(String[] args)

{

variable declarations

executable statements

}

}

Java Elements

programming style and form
Programming Style and Form
  • Use of blanks
    • Separate numbers when data is input
    • Use blank lines to separate data and code
  • All Java statements must end with a semicolon
  • Use uppercase for constants
  • Begin variables with lowercase
  • For run-together-words, capitalize each new word

Java Elements

programming style and form cont d
Programming Style and Form, cont’d
  • Use clearly written prompt lines

System.out.println(“Please enter a number between 1 and 10 and “ + “press Enter”);

  • Use comments to document
  • Use proper indentation and formatting

Java Elements