1. Declaring Variables – Mod 4
Barb Ericson
Georgia 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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3. 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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5. 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 a 3
String str =“Hi”;
str
reference
See for a discussion of the sizes of primitives and the differences between primitive variables and object variables.
“Hi”
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Memory Exercise
Draw the memory used for the following:
int x = 2;
int y = 7;
int z = x + y;
String fullName = null;
String firstName = “James”;
String lastName = “Clark”;
fullName = firstName + lastName;
Each int type takes 4 bytes (32 bits) of space.
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7. 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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8. Objects can be Garbage Collected
When objects that have been created have no more references 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?
String methods do not change the current string. They return a new string object.
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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?
Notice that we don’t capitalize Tommy and Mary even though they are names. This is because we don’t capitalize variable names in Java.
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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 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!
Note that primitive types are all lowercase. Class names start with an upper case letter.
We created the classes Picture, Sound, and FileChooser. These are not part of the Java language.
The class JFrame is a main window in Swing.
The class BufferedImage is what really holds the picture data.
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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
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