Declaring Variables – Mod 4 Barb Ericson Georgia Institute of Technology August 2005 Georgia Institute of Technology

Georgia Institute of Technology 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 Georgia Institute of Technology

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 Georgia Institute of Technology

Georgia Institute of Technology 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; Georgia Institute of Technology

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 http://www.javaranch.com/campfire/StoryCups.jsp for a discussion of the sizes of primitives and the differences between primitive variables and object variables. “Hi” Georgia Institute of Technology

Georgia Institute of Technology 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. Georgia Institute of Technology

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 Georgia Institute of Technology

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 Georgia Institute of Technology

Georgia Institute of Technology 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? Georgia Institute of Technology

Georgia Institute of Technology 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. Georgia Institute of Technology

Georgia Institute of Technology 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; Georgia Institute of Technology

Georgia Institute of Technology 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 Georgia Institute of Technology

Georgia Institute of Technology 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. Georgia Institute of Technology

Georgia Institute of Technology 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(); Georgia Institute of Technology

Georgia Institute of Technology 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. Georgia Institute of Technology

Georgia Institute of Technology 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 Georgia Institute of Technology