An interface is one which has abstract methods(not defined just declared)and static or non static variables.Any class can implement(inherit)the interface and make use of the methods(functions) and variables within it.

Here is an example where we have defined an interface and implements its methods (which are declared in interface) int the class.

interface Ex{

int getData();

void setData(int x);

}
class InterfaceExample implements Ex 
{
    int x;
    public int getData(){
        return x;
    }
    public void setData(int x){
        this.x=x;
    }
    public static void main(String[] args) 
    {
    InterfaceExample ex=new InterfaceExample();
    ex.setData(5);
    int num=ex.getData();
    System.out.println(num);
    }
}

Actually java does not support multiple inheritance. The reason behind this is when we extends more than one class, JVM get some ambiguous problem and will get confused as which method it will take.It creates complexity.Therefore java uses Interface.