Defining Classes II - Java - page 286

PITFALL: Null Pointer Exception

If the compiler asks you to initialize a class variable, you can always initialize the vari-
able to null . However, null is not an object, so you cannot invoke a method using a
variable that is initialized to null . If you try, you will get an error message that says
“Null Pointer Exception.” For example, the following code would produce a “Null
Pointer Exception” if it were included in a program:

ToyClass2 aVariable = null ;
String representation = aVariable.toString();

The problem is that you are trying to invoke the method toString() using null as a
calling object. But null is not an object; it is just a placeholder. So null has no meth-
ods. Because you are using null incorrectly, the error message reads “Null Pointer
Exception.” You get this error message any time a class variable has not been assigned a
(reference to an) object, even if you have not assigned null to the variable. Any time
you get a “Null Pointer Exception,” look for an uninitialized class variable.
The way to correct the problem is to use new to create a class object, as follows:

ToyClass2 aVariable = new ToyClass2("Chiana", 3);
String representation = aVariable.toString(); ■

The new Operator and Anonymous Objects
Consider an expression such as the following, where ToyClass is defined in Display
5.11:

ToyClass variable1 = new ToyClass("Joe", 42);

As illustrated in Display 5.13, the portion new ToyClass("Joe", 42) is an invocation
of a constructor. You can think of the constructor as returning a reference to the loca-
tion in memory of the object created by the constructor. If you take this view, the
equal sign in this line of code is just an ordinary assignment operator.
There are times when you create an object using new and use the object as an argument
to a method, but then never again use the object. In such cases, you need not give the
object a variable name. You can instead use the expression with the new operator and
the constructor directly as the argument. For example, suppose you wanted to test to see
whether the object in variable1 (in the earlier line of code) is equal to an object with the
same number and with the name spelled in all uppercase letters. You can do so as follows:

if (variable1.equals( new ToyClass("JOE", 42)))
System.out.println("Equal");
else
System.out.println("Not equal");

This is equivalent to the following: