What is the purpose of a static method in interface from Java 8?

Question

Why are static methods supported from Java 8? What is the difference between the two lines in main method in below code?

package sample; public class A {     public static void doSomething()     {         System.out.println("Make A do something!");     } }  public interface I {     public static void doSomething()     {         System.out.println("Make I do something!");     } }  public class B {     public static void main(String[] args) {         A.doSomething(); //difference between this         I.doSomething(); //and this     } } 

As we can see above, I is not even implemented in B. What purpose would it serve to have a static method in an interface when we can write the same static method in another class and call it? Was it introduced for any other purpose than modularity. And by modularity, I mean the following:

public interface Singable {     public void sing();     public static String getDefaultScale()     {         return "A minor";     } } 

Just to put like methods together.

Answer 1

In the past, if you had an interface Foo and wanted to group interface-related utils or factory methods, you would need to create a separate utils class FooUtils and store everything there.

Those classes would not have anything in common other than the name, and additionally, the utils class would need to be made final and have a private constructor to forbid unwanted usage.

Now, thanks to the interface static methods, you can keep everything in one place without creating any additional classes.

It's also important to not forget all good practices and not throw everything mindlessly to one interface class - as pointed out in this answer

Answer 2

There are mainly two reasons for static method inside interfaces: create instances of those interfaces (and the code is clearly where it has to be); like Predicate::isEqual that would create a Predicate based provided Object; or Comparator::comparing, etc. And the second reason would be utility methods that are general per all those types; like Stream::of

Still an interface has to be clear and does not have to create additional clutter in the API. Even the jdk code has Collectors - static factory methods, but a Collector interface at the same time for example. Those methods could be merged into Collector interface, but that would make the interface more clunky than it has to be.