Best notes on interfaces in Java.

Interface is custom type which can be defined by the user as and when need arises.

Why and when we create interface in java?

Uses of interface in Java.

You can define interface as a fully abstract type and even if you do not define it as ‘abstract’ interface will be always abstract internally.

You can declare ‘public static’ variables explicitly (by yourself) but even if you do not define them as ‘public abstract’ the variables inside the interface will be public abstract by default i.e. in case you do not use any modifiers while declaring variables. Following is the list of modifiers that can be used with variable declaration inside the interface:

·         Private.

·         Protected.

·         Transient.

·         Volatile.

When you declare any method inside the interface then it will be ‘public abstract’ by default. So you must terminate the method termination with semicolon (;) as it is ‘abstract’.

But you can define other types of methods in interface by using any of the following methods:

·         Private.

·         Protected.

·         Final.

·         Synchronized.

·         Native.

·         Strictfp.

Inside an interface you can not define the following blocks:

·         Constructors.

·         Instance block.

·         Static blocks.

You can declare an inner class inside the interface but it will be ‘public static’ inner class.

You cannot use interface directly but you must define an class which implements the interface, the features of which you want to use. But any class which is implementing an interface must implement all unimplemented methods which are not implemented in interface but are declared in the interface.

You can avoid this implementing all the methods in interface by declaring a class as ‘abstract’.

Interface or a class can ‘implement’ any number of other interfaces but an interface cannot ‘implement’ class. In other words an interface cannot inherit the features of a class. But either class or an interface can inherit features of one or more interfaces.

You can also create a variable for an interface which will be of type interface and it can hold the reference of Objects of classes which are implementing that particular interfaces.

There is another special case of interface called as marker interface. Marker interface does not have any implementation inside it and marker interface is used to verify the condition that whether the class can perform certain tasks or not.