Java 8 Features - Understanding Default methods

What are Default Method?

Default methods are the method defined in the interfaces with method body and using the default keyword. Thus we can add instance methods to the interfaces. Default method can call methods from the interfaces they are enclosed in. Also in default method, this keyword refers to the declaring interface.

Need for Default Method-

With Java 8, lambda expressions were introduced. So in existing Interfaces like Iterator interface, new methods were needed to be added like Spliterator<T> spliterator(). If this was done, then the code written in previous versions of java would break when they start using Java 8 because their interface implementation would not have these new methods. So for backward compatibility and allowing code written in previous versions to make use of Lambda expressions. So with default methods it is possible to evolve interfaces more easily.

Some key points when using default method-

Default methods can have access specified as public only. They cannot be declared as protected or private. Also since all interfaces are public by default, decalring the default methods as public explicitly is not required.
Default methods cannot be abstract and should always have the method body.
The class implementing the Interface having default method can override it.
Default methods cannot be Final or Synchronized.
Static methods can never be default methods. Default methods are always instance methods.

Rules for defining multiple default methods-

When defining multiple interfaces having same default method name, we may come accross the diamond problem.Lets look at such a scenario and how to resolve it.

Define the Employee Interface with default method showName.


Class Employee
{
default void showName()
{
System.out.println("employee")
}
}

Define the Parent Interface with default method showName.


Class Parent
{
default void showName()
{
System.out.println("parent")

}

If we now create a Person class implementing the above interfaces and call the default method-


Class Person implements Employee, Parent
{
public static void main(String args[])
{
new Person().showName();
}
}

We get the exception-

Error: java: class Person inherits unrelated defaults for showName() from types Employee and Parent

This can be resolved in two ways-

By overriding and creating own implementation of the showName method

Class Person implements Employee, Parent
{
	default void showName()
	{
		System.out.println("Person")
	}
}

This will print

Person

Overriding and using super keyword explicitly mentioning which interface method needs to be called.
```
Class Person implements Employee, Parent
{
	default void showName()
	{
		Parent.super.showName();
	}
}
```
This will print
```
Parent
```

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Java 8 Features - Understanding Default methods

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