Java 8
introduces “Default Method” or (Defender methods) new feature, which allows a
developer to add new methods to the
Interfaces
without breaking the existing implementation of these Interface.
For example,
‘List’ or ‘Collection’ interfaces do not have ‘forEach’ method declaration.
Thus, adding such method will simply break the collection framework
implementations.
See how it
works :
public
interface studentClassOne{
default void classTest() {
System.out.println("classTest in
studentClassOne");
}
}
public
interface studentClassTwo{
default void classTest() {
System.out.println("classTest in
studentClassOne");
}
}
public class
student implements studentClassOne, studentClassTwo{
}
The above
code will fail to compile with the following error,
java: class
student inherits unrelated defaults for classTest() from types studentClassOne
and studentClassTwo
In order to
fix this class, we need to provide default method implementation with super
Interface:
public class
student implements studentClassOne, studentClassTwo{
default void classTest() {
vehicle.super.classTest();
}
}
 
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Why we needed to implements the default method?
Re-engineering an existing JDK framework is always very complex.If we modify one interface in
JDK interface in JDK
framework
breaks all classes and extends the interface ,when we want to adding any new
method could be break millions of code.
Therefore
Defaults method has introduced in Java 8.
Default
methods can be provide to an interface without affecting implementaing calsses
as it includes an implementation.
