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Tip of the Day
Language: Java
Expertise: Intermediate
Oct 30, 1998

Why Use Inner Classes?

Inner classes often make your programming task easier by eliminating the requirement to write parameterized constructors for your classes and to pass references as parameters. A method in an inner class has direct access to the instance variables of an object of the type for which it is an inner class. A method in an outer class (or top-level class) requires a reference to the object to be able to access the instance members of the object. This often requires a parameterized constructor, which receives and saves such references for later use. The definition of an inner class and the instantiation of an object of that class are often simpler than the corresponding tasks for an outer class. This code was tested using JDK1.1.6 under Win95:
 
import java.util.*;
import java.io.*;

class InnerClasses01 {
	int myData;
  
	public static void main(String[] args){ //main method
		new InnerClasses01();
	}//end main
	//-----------------------------------------------------//
	InnerClasses01(){//constructor
		myData = 10;
    
		//Instantiate an inner class object and invoke its
		// displayData() method.  No parameter passing required
		new AnInnerClass().displayData();
    
		//Instantiate an outer class object and invoke its
		// displayData() method.  Reference to this object must
		// be passed when object is instantiated.
		new AnOuterClass(this).displayData();
	}//end constructor
	//-----------------------------------------------------//

	class AnInnerClass{//no parameterized constructor needed
		void displayData(){
		System.out.println(myData);
		}//end displayData()
	}//end AnInnerClass

}//End InnerClasses01 class
//=======================================================//

class AnOuterClass{
	InnerClasses01 otherObject;//reference variable required
  
	//A parameterized constructor is required
	AnOuterClass(InnerClasses01 reference){
		otherObject = reference;
	}//end constructor
	//-----------------------------------------------------//
  
	void displayData(){
		System.out.println(otherObject.myData);
	}//end displayData()
}//end AnOuterClass
Richard G.
 
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