Mock java.lang.Runtime with PowerMockito

My bad in the comments, apologies, I had an inner class for the test and that's why I had no trouble. I then realized this and saw your @PrepareForTest(Runtime.class) which should read @PrepareForTest(MyClass.class) (replace MyClass with whatever name you have) because Runtime is a system class. You can read more about this here and find more examples here.