OBJECTS AND CLASSES - Introduction to Java Programming: Comprehensive Version - page 332

System.out.println( "age? " + student.age);
System.out.println( "isScienceMajor? " + student.isScienceMajor);
System.out.println( "gender? " + student.gender);
}
}

The following code has a compile error, because the local variables x and y are not initialized:

class Test {
public static void main(String[] args) {
int x; // x has no default value
String y; // y has no default value
System.out.println( "x is " + x);
System.out.println( "y is " + y);
}
}

Caution
NullPointerException is a common runtime error. It occurs when you invoke
a method on a reference variable with a null value. Make sure you assign an object
reference to the variable before invoking the method through the reference variable (See
Checkpoint Question 9.11c).

NullPointerException

9.5.4 Differences between Variables of Primitive Types
and Reference Types
Every variable represents a memory location that holds a value. When you declare a variable,
you are telling the compiler what type of value the variable can hold. For a variable of a primi-
tive type, the value is of the primitive type. For a variable of a reference type, the value is a
reference to where an object is located. For example, as shown in Figure 9.7, the value of int
variable i is int value 1 , and the value of Circle object c holds a reference to where the
contents of the Circle object are stored in memory.
When you assign one variable to another, the other variable is set to the same value. For
a variable of a primitive type, the real value of one variable is assigned to the other variable.
For a variable of a reference type, the reference of one variable is assigned to the other vari-
able. As shown in Figure 9.8, the assignment statement i = j copies the contents of j into i

Created using new Circle()

Primitive type

int i = 1 i

1

Circle c c

c: Circle

Object type reference

radius = 1

F IGURE 9.7

A variable of a primitive type holds a value of the primitive type, and a variable
of a reference type holds a reference to where an object is stored in memory.

Primitive type assignment i = j

Before: After:

i i

1 2

j j

2 2

F IGURE 9.8

Primitive variable j is copied to variable i .