Lilian Blot 11 Spring 2014 TPOP 1. Lilian Blot 22 Spring 2014 TPOP 2.

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Chart Title

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FROM PYTHON TO JAVA

Java

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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?

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 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

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 & 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

A small program to print a string:

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def main(): print “Hello World"

Python Code

public class Hello { public static void main(String[] args){ System.out.println(“hello world”); }}

Java Code : file Hello.java

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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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Execution

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>>> main()“Hello World"

Python Interpreter

DOS prompt

Compile the file using: javac filename.java

execute the file using: java filename

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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 calledpublic static void main(String[] args)

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public class Hello { public static void main(String[] args){ System.out.println(“hello world”); }}

Java Code: Hello.java

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Types

Everything in Java must have a typeThere is two classes of type:

primitives Objects

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Primitive Object (wrapper)

int Integer

long Long

short Short

float Float

double Double

char Char

boolean Boolean

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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

typeA variable can only be assigned a value of the

declared typeA variable cannot change its type

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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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Variables

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public class EggBasket{

public static void main(String[] args){

int numberOfBaskets, 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); }}

Java Code extract

variable declaration

Assignment Statements

Expression

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Variables

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String numberOfBaskets = "number"; eggsPerBasket = 6;

totalEggs = numberOfBaskets * eggsPerBasket; }}

Java Code extract

Compiled

changing the type of a variable

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Variables

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numberOfBaskets = "number"; eggsPerBasket = 6;

totalEggs = numberOfBaskets * eggsPerBasket; }}

Java Code extract

Compiled

Assigning a different type to a variable

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Operator Precedence

Operator Python Operator Java Name

Precedence

+, - +, - Unary plus and minus (e.g. -10)

not !

*, /, //, % *, /, %Multiplication, division, integer division, and remainder

+, - +, - Binary plus and minus (e.g. 3-10)

<, <=, >= <, <=, >= Comparison

==, != ==, != Equality

and &&

or ||

=, +=, -=, *=, /=, //=, %=

=, +=, -=, *=, /=, %=

Assignment operators

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Selection Structure

Python Code

……if condition : Statement A1 … Statement Anelse : Statement B1 … Statement Bk…

Java Code

……if (condition) { Statement A1 … Statement An} else { Statement B1 … Statement Bk}…

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Simple if-statement

Python Code

……if condition : Statement A1 … Statement An

statement Xstatement Y…

Java Code

……if (condition) { Statement A1 … Statement An}statement Xstatement Y…

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if-else if-else statement

Python Code

…if cond_1 : Statement Aelif cond_2 : Statement B …elif cond_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

Python Code

……if cond_1 : Statement Aelse : 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

Python Code

sum_square = 0

for val in [1,2,3]: square_val = val * val sum_square += square_val

print sum_square

Java Code

int sum_square = 0;

for(val=1; val<=3; val+=1){ int square_val = val * val; sum_square += square_val;}

System.out.println(sum_square)

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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

During the lecture we have covered:

Interpreted versus compiled languagesStatic TypingDeclaration of and Assignment to variables in

JavaOperators and ExpressionsSelection Structure in JavaThe for loop structure

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Exercises

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

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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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Lilian Blot 11 Spring 2014 TPOP 1. Lilian Blot 22 Spring 2014 TPOP 2.

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