Over a million developers have joined DZone.
Platinum Partner

Java Interface Rules

· Java Zone

The Java Zone is brought to you in partnership with AppDynamics. Discover how AppDynamics steps in to upgrade your performance game and prevent your enterprise from these top 10 Java performance problems.

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

From http://www.aviyehuda.com/

The Java Zone is brought to you in partnership with AppDynamics. AppDynamics helps you gain the fundamentals behind application performance, and implement best practices so you can proactively analyze and act on performance problems as they arise, and more specifically with your Java applications. Start a Free Trial.

Topics:

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}