Inner Classes - Beginning Java 8 Language Features - page 46

Beginning Java 8
Scripting in Java

The fact that the scope of a local inner class is limited to its enclosing block has some implications on how to
declare a local inner class. Consider the following class declaration:

package com.jdojo.innerclasses;

public class SomeTopLevelClass {
// Some code for SomeTopLevelClass goes here

public void someMethod() {
class SomeLocalInnerClass {
// Some code for SomeLocalInnerClass goes here
}

// SomeLocalInnerClass can only be used here
}
}

SomeTopLevelClass is a top-level class. The someMethod() method of SomeTopLevelClass declares the
SomeLocalInnerClass local inner class. Note that the name of the local inner class, SomeLocalInnerClass , can only
be used inside the someMethod() method. This implies that objects of the SomeLocalInnerClass can only be created
and used inside the someMethod() method. This limits the use of a local inner class to only being used inside its
enclosing block—in your case the someMethod() method. At this point, it may seem that a local inner class is not
very useful. However, Listing 2-4 demonstrated that the code for the local inner class TitleIterator can be called
from another class, TitleListTest . This was possible because the local inner class TitleIterator implemented the
Iterator interface.
To use a local inner class outside its enclosing block, the local inner class must do one or both of the following:

• Implement a public interface

• Inherit from another public class and override some of its superclass methods

The name of the interface or another class must be available outside the enclosing block that defines the local
inner class. Listing 2-3 and Listing 2-4 illustrate the first case where a local inner class implements an interface.
Listing 2-5 and Listing 2-6 illustrate the second case, where a local inner class inherits from another public class.
Listing 2-7 provides a test class to test a local inner class. The example is trivial. However, it illustrates the concept
of how to use a local inner class by inheriting it from another class. Note that you may get a different output when
you run the program in Listing 2-7.

Listing 2-5. Declaring a Top-Level Class, Which Is Used as the Superclass for a Local Class
// RandomInteger.java
package com.jdojo.innerclasses;

import java.util.Random;

public class RandomInteger {
protected Random rand = new Random();