13.4.7 Field Declarations

Adding a field to a class will not break compatibility with any pre-existing binaries that are not recompiled, even in the case where a class could no longer be recompiled because a field access previously referenced a field of a superclass with an incompatible type. The previously compiled class with such a reference will continue to reference the field declared in a superclass. Thus compiling and executing the code:

class Hyper { String h = "hyper"; }
class Super extends Hyper { String s = "super"; }
class Test {
	public static void main(String[] args) {
		System.out.println(new Super().h);
	}
}

produces the output:

hyper

Changing Super to be defined as:

class Super extends Hyper {
	String s = "super";
	int h = 0;
}

recompiling Hyper and Super, and executing the resulting new binaries with the old binary of Test produces the output:

hyper

The field h of Hyper is output by the original binary of main no matter what type field h is declared in Super. While this may seem surprising at first, it serves to reduce the number of incompatibilities that occur at run time. (In an ideal world, all source files that needed recompilation would be recompiled whenever any one of them changed, eliminating such surprises. But such a mass recompilation is often impractical or impossible, especially in the Internet. And, as was previously noted, such recompilation would sometimes require further changes to the source code.)

Deleting a field from a class will break compatibility with any pre-existing binaries that reference this field, and a NoSuchFieldError will be thrown when such a reference from a pre-existing binary is linked. Only private fields may be safely deleted from a widely distributed class.