13.5.3 The Interface Members

Adding a member to an interface does not break compatibility with pre-existing binaries.

Deleting a member from an interface may cause linkage errors in pre-existing binaries. If the example program:

interface I { void hello(); }
class Test implements I {

	public static void main(String[] args) {
		I anI = new Test();
		anI.hello();
	}
	public void hello() { System.out.println("hello"); }
}

is compiled and executed, it produces the output:

hello

Suppose that a new version of interface I is compiled:

interface I { }

If I is recompiled but not Test, then running the new binary with the existing binary for Test will result in a NoSuchMethodError. (In some early implementations of Java this program still executed; the fact that the method hello no longer exists in interface I was not correctly detected.)