Java Interface

Interface is also known as kind or type of a class. It contains only abstract methods and final fields.

Interface do not specify any code to implement methods or initialization of data. These are always implemented in the class which defines the implementation of these methods.

Characteristics of Interface

• They contains only abstract methods and final fields (constants)
• Interface can extends other interfaces but cannot define superinterface methods.
• Interface can extends multiple interfaces seperated by commas
• A class can also implement multiple interfaces seperated by commas
• Interface cannot extend other classes.
• Interface cannot be instantiated (object not created)
• Interface does not contain any constructor like class.

Uses of interface

Interfaces are mostly used in java for following purposes

Syntax for creating interface

interface InterfaceName
{       
    // methods declarations                    
    return_type methodName1 (parameterList);
    return_type methodName2 (parameterList);
    .
    .
    return_type methodNameN (parameterList);
    
    
    // field declarations
    type final_field = value;
    type final_field2 = value;
    .
    .
    type final_fieldN = value;
}

Variables

By default variables declared inside the interface becomes final fields. They must be initialized to constant value otherwise gives an error as the black field variable have not been intialized.

Methods

Interface contains only declaration of methods without writting its body with statements. By default declared methods are considered as a abstract methods. Easy class which implements that interface must write all methods body.

Example of interface

interface Mobile
{
    int price = 5600;
    String brand = "MyPhone";
    void displayDetails();
    void startPhone();
}

class PhoneUser implements Mobile
{
    public void displayDetails()
    {
	System.out.println("\nPhone brand : "+brand);
	System.out.println("\nPhone price : "+price);
    }
    public void startPhone()
    {
	System.out.println("\nStarting the phone ...");
    }

    public static void main(String args[])
    {
	PhoneUser n1 = new PhoneUser();
	n1.displayDetails();
	n1.startPhone();
    }
}

Output

In above example we have created interface Mobile, which is implemented in the class PhoneUser. All methods are implemented inside the class and called them using object of class.