Why are all fields in an interface implicitly static and final?

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An interface is intended to specify an interaction contract, not implementation details. A developer should be able to use an implementation just by looking at the interface, and not have to look inside the class which implements it.

An interface does not allow you to create an instance of it, because you cannot specify constructors. So it cannot have instance state, although interface fields can define constants, which are implicitly static and final.

You cannot specify method bodies or initializer blocks in an interface, although since Java 8 you can specify default methods with bodies. This feature is intended to allow new methods to be added to existing interfaces without having to update all the implementations. But you still cannot execute such a method, without first creating an instance implementing the interface.

Aside: Note that you can implement an interface with an anonymous inner class:

interface Foo {
    String bar();
}

class FooBar {
    Foo anonymous = new Foo() {
         public String bar() {
             return "The Laundromat Café";
    };
}

You have to provide the full implementation of the interface for the anonymous inner class to compile.

new Foo() is initializing the anonymous inner class with its default constructor.

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