Java Interface

Interface is a collection of function declarations (abstract functions). The function declaration has name of the function and type of arguments that will be passed to the function. Interface is related to a particular task. For example, developer may create an interface for printing the document (say Print). Then 'Print' interface consists of a set of function declarations that are able to print document.

To create an interface, following two steps are required:

Step 1: Define an interface with declaration of the functions (abstract class)
Step 2: Implement an interface in the class
Following is explanation of each step with example:

Step 1: Define an interface with declaration of the function
To define a function, use the keyword 'interface', for example:

interface Print {
	void PrintData(int PrintMode);
}

In this example, interface name is 'Print'. It consists of a function named 'PrintData' and one argument named 'PrintMode' of type 'int'.

Step 2: Implement an interface in the class
An interface is implemented by a class.
A class has to implement all abstract functions of the interface.

class Computer implements Print {
	public void PrintData (int a) {
		System.out.println ("Printer with mode" + a);
	} 
}

In this example, a class named 'Computer' is defined which implements the interface 'Print'.
If a class implements an interface, then it is necessary for a class to provide body of each abstract function of the interface.
Class 'Computer' defines body of the abstract function 'PrintData' (or you can say, the class implements the abstract function 'PrintData').

Consider one more example

interface Disp{
	void DispData();
}
interface Print {
	void PrintData(int PrintMode);
}
class Computer implements Print, Disp {
	public void PrintData (int a) {
		System.out.println ("Printer with mode" + a);
	}
	public void DispData () {
		System.out.println ("Hello! Learner");
	}	
	public static void main(String args[])
	{
		Computer c1 = new Computer();
		c1.PrintData(12);
		c1.DispData();
	}
}

In this program, two interfaces Disp and Print are defined. Class Computer implements both interfaces. It provides function body of both functions declared in interfaces.

From Java 8 onwards

An interface may have body of a function. But it should be declared as default.

Example

interface Disp {
  default void DispData()
  {
	  System.out.println("Function body within interface"); // Defining body of function in interface
  }
}
interface Print {
	void PrintData(int PrintMode);
}
class Computer implements Print, Disp {
	public void PrintData (int a) {
		System.out.println ("Printer with mode" + a);
	}
	public static void main(String args[])
	{
		Computer c1 = new Computer();
		c1.PrintData(12);
		c1.DispData(); //DispData function body is defined in interface
	}
}

Interface Disp consists of one function named DispData () with its body .

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