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Java gives us the option of initializing a declared variable or leaving it uninitialized. When we attempt to use the uninitialized variable, we can get different behavior depending on what type of variable or array we are dealing with (primitives or objects). The behavior also depends on the level (scope) at which we are declaring our variable. An instance variable is declared within the class but outside any method or constructor, whereas a local variable is declared within a method (or in the argument list of the method).
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Local variables are sometimes called stack, temporary, automatic, or method variables, but the rules for these variables are the same regardless of what you call them. Although you can leave a local variable uninitialized, the compiler complains if you try to use a local variable before initializing it with a value, as we shall see.
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Instance variables (also called member variables) are variables defined at the class level. That means the variable declaration is not made within a method, constructor, or any other initializer block. Instance variables are initialized to a default value each time a new instance is created. Table 1-3 lists the default values for primitive and object types.
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In the following example, the integer year is defined as a class member because it is within the initial curly braces of the class and not within a method s curly braces:
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public class BirthDate { int year; // Instance variable public static void main(String [] args) { BirthDate bd = new BirthDate(); bd.showYear(); } public void showYear() { System.out.println("The year is " + year); } }
When the program is started, it gives the variable year a value of zero, the default value for primitive number instance variables.
1: Language Fundamentals
TABLE 1-3
Variable Type Object reference byte, short, int, long float, double boolean char
Default Value null (not referencing any object) 0 0.0 false \u0000
Default Values for Primitive and Reference Types
It s a good idea to initialize all your variables, even if you re assigning them with the default value. Your code will be easier to read; programmers who have to maintain your code (after you win the lottery and move to Tahiti) will be grateful.
Object Reference Instance Variables
When compared with uninitialized primitive variables, Object references that aren t initialized are a completely different story. Let s look at the following code:
public class Book { private String title; public String getTitle() { return title; } public static void main(String [] args) { Book b = new Book(); System.out.println("The title is " + b.getTitle()); } }
This code will compile fine. When we run it, the output is
The title is null
The title variable has not been explicitly initialized with a String assignment, so the instance variable value is null. Remember that null is not the same as an empty String ( ). A null value means the reference variable is not referring to any object on the heap. Thus, the following modification to the Book code runs into trouble:
public class Book { private String title;
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public String getTitle() { return title; } public static void main(String [] args) { Book b = new Book(); String s = b.getTitle(); // Compiles and runs String t = s.toLowerCase(); // Runtime Exception! } }
When we try to run the Book class, the JVM will produce the following error:
%java Book Exception in thread "main" java.lang.NullPointerException at Book.main(Book.java:12
We get this error because the reference variable title does not point (refer) to an object. We can check to see whether an object has been instantiated by using the keyword null, as the following revised code shows:
public class Book { private String title; public String getTitle() { return title; } public static void main(String [] args) { Book b = new Book(); String s = b.getTitle(); // Compiles and runs if (s != null) { String t = s.toLowerCase(); } } }
The preceding code checks to make sure the object referenced by the variable s is not null before trying to use it. Watch out for scenarios on the exam where you might have to trace back through the code to find out whether an object reference will have a value of null. In the preceding code, for example, you look at the instance variable declaration for title, see that there s no explicit initialization, recognize that the title variable will be given the default value of null, and then realize that the variable s will also have a value of null. Remember, the value of s is a copy of the value of title (as returned by the getTitle() method), so if title is a null reference, s will be too.
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