Georgia Institute of Technology

Declaring Variables – Mod 4

Barb EricsonGeorgia Institute of Technology

August 2005

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Learning Goals

• Understand at a conceptual level– Declaring variables: what is allowed and what

isn’t– When variables can be garbage collected– How to change the value of a variable– The difference between primitive and object

variables

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Working with Variables

• You must declare a variable before you can use it

• Declare a variable once and only once– You will get an error if

you declare it more than once

• You can reuse variables– That you have

declared

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Declaring Variables

• To declare a variable– Specify the type and a name

String name;World world1;Turtle turtle1;int x;

– You can also assign a value to the variable when you declare it

String fullName = “Susan Dorda”;World world1 = new World();Turtle turtle1 = new Turtle(world1);int x = 0;

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Object and Primitive Variables• Primitive variables

allocate space based on their size– The contents of the space

is set to the variable’s value

• Object type variables cause space allocation for a reference to an object– The contents of the space

is a way to calculate the address of the object

– Or null if it isn’t referring to an object yet.

int a = 3

String str =“Hi”;

3a

str

“Hi”

reference

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Memory Exercise

• Draw the memory used for the following:int x = 2;

int y = 7;

int z = x + y;

• Draw the memory used for the following:String fullName = null;

String firstName = “James”;

String lastName = “Clark”;

fullName = firstName + lastName;

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Variables are Temporary

• Hold values until– Reassigned

int x = 2;

x = 3; // read ‘=‘ as takes on the value of

– DrJava is exited– The interactions pane is reset

• You can force this by clicking the “Reset” button• Or by right click and choose “Reset Interactions”• Happens automatically when you compile

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Objects can be Garbage Collected

• When objects that have been created have no reference to them– They can be garbage collected

• This means the memory they are using can be freed and reused

– String name1 = “Bill”;– String name1 = new String(“Mary”);– The string object created by “Bill” can be

garbage collected

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Variable Practice

• Given the following code– What is the value of z at each step?

int z = 0;

int x = 3;

int y = 2;

z = x * y;

z = z + x;

z = z – y;

– How many variables where declared?– How much space do they take?

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Variable Practice

• What is the value of str1 at each step?– String str1 = “Hello World”;– str1.toLowerCase();– str1 = str1.toUpperCase();– str1 = “Goodbye World”;

• How many variables where declared?

• How many objects were created?– How many can be garbage collected?

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Variable Practice• What is the value of string1 after the following

code has executed?– String string1 = “HELLO”;– System.out.println(string1.toLowerCase());

• What type of variable is string1 (primitive or object)?– How can you tell?

• What is the value of string2 and string3 after the following code has executed?– String string2 = “Mary”;– String string3 = “Clark”;– string2 = string2 + string3;

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Variable Practice

• Declare variables for each of the following– The number of kids in a family– The cost of a pair of shoes– The name of a pet– The score of a soccer game– The hourly wage of a worker– The answer to do you want insurance on a purchase – The color of your eyes– Your weight – The distance you drive on a trip– Your test score– The number on a lottery ball

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Variable Practice

• Which turtle does turtle1 refer to after the following code?

World myWorld = new World();Turtle tommy = new Turtle(myWorld);Turtle mary = new Turtle(myWorld);Turtle turtle1 = tommy;turtle1.forward();turtle1 = mary;turtle1.turnLeft();

• Which turtle will go forward?• Which turtle will turn left?

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Variable Practice

• Which of these statements will cause an error in the interactions pane and why?

World1 world = new World();

World world2 = new World();

World world3 = newWorld();

World world1 = new World1();

World world5 = new World;

World world2 = new World();

World my World = new World();

World earth = new World();

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Allowed Types in Java

• The type for a variable declaration can be– A primitive type: int, double, char, or boolean– The name of a Class

• in the Java language– String, JFrame, BufferedImage, etc

• or the name of a class created by you or someone else

– Picture, Sound, FileChooser

• Class names start with an uppercase letter!

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Summary

• You can declare variables by specifying a type and a name– You can also assign a value to the variable

when you declare it

• Primitive variables reserve space based on the type– Object variables reserve space for a

reference to an object

• An object can be garbage collected if there is no reference to it