7.13 Ap0130: self-assessment, comparing objects, packages, import  (Page 5/9)

An IndexOutOfBounds exception

As a result, the program throws an IndexOutOfBounds exception. The error produced under JDK 1.3 looks something like the following:

Attempting to access an element with a negative index value would produce the same result.

An ArrayIndexOutOfBounds exception

A similar result occurs if you attempt to access an element in an ordinary array object outside the bounds of the index values determined by the size ofthe array. However, in that case, the name of the exception is ArrayIndexOutOfBounds .

Back to Question 7

Answer 6

B. Runtime Error

Explanation 6

The infamous NullPointerException

Interestingly, one of the first things that you read when you start reading Java books, is that there are no pointers in Java . It is likely that shortly thereafter when you begin writing, compiling, and executing simple Javaprograms, one of your programs will abort with an error message looking something like that shown below :

What is a NullPointerException?

Stated simply, a NullPointerException occurs when you attempt to perform some operation on an object using a reference that doesn'trefer to an object.

That is the case in this program

The following code fragment declares a local reference variable and initializes its value to null .

(A reference variable in Java must either refer to a valid object, or specifically refer to no object (null). Unlike a pointer in C and C++, aJava reference variable cannot refer to something arbitrary.)

In this case, null means that the reference variable doesn't refer to a valid object.

No ArrayList object

Note that the code in the above fragment does not instantiate an object of the class ArrayList and assign that object's reference to the reference variable.

(The reference variable doesn't contain a reference to an object instantiated from the class named ArrayList , or an object instantiated from any class for that matter.)

Call a method on the reference

However, the code in the next fragment attempts to add a String object's reference to a nonexistent ArrayList object by calling the add method on the reference containing null.

This results in the NullPointerException shown earlier .

What can you do with a null reference?

The only operation that you can perform on a reference variable containing null is to assign an object's reference to the variable. Any other attemptedoperation will result in a NullPointerException .

Back to Question 6

Answer 5

C. ABC DEF GHI

Explanation 5

The purpose of this program is to

• Continue to illustrate the use of java packages, and
• Illustrate the use of the Java import directive.

Program contains an import directive