Let’s start with a short Java question. Below you can see the interface ‘Test’. Which lines in that interface will be rejected by the compiler?

public interface Test{

	//1
	public static final int x1 = 3; 

	//2
	public static int x2 = 3; 

	//3
	static int x3 = 3; 

	//4
	int x4 = 3; 

	//5
	public int f5();

	//6
	int f6(); 

	//7
	public static int f7();

	//8
	private void f8();

	//9
	public final void f9();

	//10
	private static final int x5 = 3;
}

The answer is:

lines: 7,8,9,10

I am sure that even many of the experienced java developers will not have a 100% success answering this question because it can be confusing.

1, 2, 3 and 4 are actually all the same – only constants are allowed and by default they are. For that reason, 10 is not allowed.
5 and 6 are the same – only public and protected methods are allowed. By default they are public.

In short these are the rules for interfaces:

Member variables

• Can be only public and are by default.
• By default are static and always static
• By default are final and always final

• Can be only public and are by default.
• Can not be static
• Can not be Final

From http://www.aviyehuda.com/