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Java. From Python to Java. Key Points. At the end of the lecture y ou should understand Interpreted versus compiled languages Static Typing Declaration of and Assignment to variables in Java Operators and Expressions Selection Structure in Java The for loop structure.

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Java

From Python to Java

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Key points
Key Points

At the end of the lecture you should understand

  • Interpreted versus compiled languages

  • Static Typing

  • Declaration of and Assignment to variables in Java

  • Operators and Expressions

  • Selection Structure in Java

  • The for loop structure

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Why learning another language
Why learning another language?

  • The recruitment companies say employers, many of them in London, are looking for flexible IT staff that have knowledge of three or four different programming languages.

  • Research suggests that learning more than one programming language can increase your salary by between £3,000 and £10,000 per annum.

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Why learning another language1
Why learning another language?

  • Specific applications need specific languages.

  • To learn new programming paradigms.

  • To understand differences (sometime subtle) between languages. Knowing what to look for.

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Python java
Python & Java

  • Java syntax is more formal than Python syntax.

  • Java is an industrial strength language, Python is a scripting language.

  • Java is better for large project where several developers are working together.

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Python java1
Python & Java

  • There are many features that are common to both languages:

    • variables,

    • loops, (while and for)

    • conditional, if-elif-else

    • functions, methods

    • classes and inheritance.

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First program
First Program

  • A small program to print a string:

Python Code

Java Code : file Hello.java

defmain():

print “Hello World"

public class Hello {

public static void main(String[] args){

System.out.println(“hello world”);

}

}

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Execution
Execution

  • Java is not an interpreted language

  • Java is compiled into a Java bytecode

  • Java bytecode is interpreted by the Java Virtual Machine (JVM)

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Execution1
Execution

DOS prompt

Compile the file using:

javac filename.java

execute the file using:

java filename

Python Interpreter

>>> main()

“Hello World"

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First program1
First Program

  • Every java program MUST define a class

    • ALL code is inside a class

  • The name of the file MUST be the name of the class plus the extension .java

  • Every executable Java programs MUST have a function called

    public static void main(String[] args)

Java Code: Hello.java

public class Hello {

public static void main(String[] args){

System.out.println(“hello world”);

}

}

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Types
Types

  • Everything in Java must have a type

  • There is two classes of type:

    • primitives

    • Objects

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Variables
Variables

  • Python is dynamically typed

    • type checking at run-time

  • Java is statically typed

    • type checking at compile-time

      Consequences:

  • A variable must be declared with a specific type

  • A variable can only be assigned a value of the declared type

  • A variable cannot change its type

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Declaration and assignment
Declaration and Assignment

  • Declaration Syntax:

    Type variable_1, variable_2, … ;

  • Assignment Syntax:

    variable = expression ;

  • Combined Syntax:

    Type variable_1 = expr_1, variable_2, … ;

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Variables1
Variables

variable declaration

Expression

Assignment Statements

Java Code extract

public class EggBasket{

public static void main(String[] args){

intnumberOfBaskets, eggsPerBasket, totalEggs;

numberOfBaskets = 10;

eggsPerBasket = 6;

totalEggs = numberOfBaskets* eggsPerBasket;

System.out.println("If you Have:");

System.out.println(eggsPerBasket+ " eggs per basket and");

System.out.println(numberOfBaskets+ " baskets, then");

System.out.println("total number of eggs is: " + totalEggs);

}

}

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Variables2
Variables

Java Code extract

public class EggBasket{

public static void main(String[] args){

intnumberOfBaskets, eggsPerBasket, totalEggs;

String numberOfBaskets = "number";

eggsPerBasket = 6;

totalEggs = numberOfBaskets* eggsPerBasket;

}

}

changing the type of a variable

Compiled

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Variables3
Variables

Java Code extract

public class EggBasket{

public static void main(String[] args){

intnumberOfBaskets, eggsPerBasket, totalEggs;

numberOfBaskets = "number";

eggsPerBasket = 6;

totalEggs = numberOfBaskets* eggsPerBasket;

}

}

Assigning a different type to a variable

Compiled

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Selection structure
Selection Structure

Python Code

ifcondition: Statement A1

Statement An

else :

Statement B1

Statement Bk…

Java Code

if(condition){ Statement A1

Statement An

} else {

Statement B1

Statement Bk

}…

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Simple if statement
Simple if-statement

Python Code

ifcondition: Statement A1

Statement An

statement X

statement Y

Java Code

if(condition){ Statement A1

Statement An

}

statement X

statement Y

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If else if else statement
if-else if-elsestatement

Python Code

ifcond_1: Statement A

elifcond_2:

Statement B

elifcond_n:

Statement C

else :

Statement D

Java Code

if(cond_1){ Statement A

} else if (cond_2){

Statement B

} else if (cond_n) {

Statement C

} else {

Statement D

}

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Nested if else statements
Nested if-else statements

Python Code

ifcond_1: Statement A

else :

Statement B1

Statement B2

if cond_2:

Statement C

else:

Statement D

Java Code

if(cond_1){ Statement A

} else {

Statement B1

Statement B2

if (cond_2){

Statement C

} else{

Statement D

}

}

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Iteration for loops
Iteration: For loops

Python Code

sum_square = 0

forvalin [1,2,3]:

square_val = val * val

sum_square += square_val

print sum_square

Java Code

intsum_square= 0;

for(val=1; val<=3; val+=1){

intsquare_val= val * val;

sum_square+= square_val;

}

System.out.println(sum_square)

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Iteration for loop syntax
Iteration: For loop Syntax

for (initialisation; boolean_expression; update){

body //executed only if boolean_expression is true

}

Example:

for(int index = 10; index > 0; index--){

System.out.println(index)

}

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Summary
Summary

During the lecture we have covered:

  • Interpreted versus compiled languages

  • Static Typing

  • Declaration of and Assignment to variables in Java

  • Operators and Expressions

  • Selection Structure in Java

  • The for loop structure

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Exercises
Exercises

  • Rewrite in Java your answers to questions 1-4 of practical 2.

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Personal research more on selection
Personal Research: More on Selection

  • Investigate the use of the following statement:

    switch (expr){

    case : …

    case : …

    }

  • How does it differs from:

    if – else if - else

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