Interfaces in Java

Before reading this post, I recommend you to read the following posts.
Runtime polymorphism in Java
Abstract Classes in Java:Runtime polymorphism

An interface is a collection of methods that must be implemented by the implementing class. An interface only tells the class what to do. Thus, the interfaces only have the declaration of member variables and methods. Implementing class defines all the methods declared in the interface. If a class implements an interface and does not implement all the methods then the class must be declared as abstract. Variables in interface automatically become static and final variables of the implementing class. Members of interface are implicitly public. A class cannot narrow the accessibility of an interface.

An object reference of an interface can refer to an object of implementing class. However, only methods that are present in the interface can be called using the object reference of interface.

Now lets put the interface into implementation. We consider the implementation of the Stack data structure. Below is the Stack interface and its implementing class MyStack.

interface Stack { void push(String str); String pop(); int length(); } class MyStack implements Stack { private String a[]=new String[10]; private int top=0; public void push(String str) { a[top]=str; top++; } public String pop() { top--; return a[top]; } public int length() { return top; } public void print() { for(int i=0;i<top;i++) System.out.println(a[i]); } }

The public class which uses the interface and its implementing class is as follows.

public class UseStack { public static void main(String args[]) { MyStack ms=new MyStack(); ms.push("First"); ms.push("Second"); ms.print(); System.out.println(ms.pop()); // Obj. ref. of interface refer to object of implementing class Stack s; s=new MyStack(); s.push("1"); s.push("2"); // s.print(); // Will not compile since print() is not in the interface Stack System.out.println(s.pop()); } }


Interface Vs. Abstract Class:

Both interface and abstract class cannot have objects, but can have object references. In case of interface, object reference can reference an object of the implementing class. An object reference of abstract class can reference an object of subclass. However, abstract class can have non-abstract methods and instance variables but interface can have neither of them.

Java allows interfaces to be empty. An empty interface contains no variables and methods. It is used to mark a class to be having certain behavior.

Related Posts:
Abstract Classes in Java:Runtime polymorphism
Runtime polymorphism in Java
How Interfaces are used internally in Java