Language Fundamentals - Interface Declarations

Syntax (JJ pg142)

        modifiers interface InterfaceName extendsClause {
            // Interface body
         }

The modifiers and extendsClause are optional.

A compile time error occurs if an interface has a simple name the same as any of it's enclosing classes or interfaces (JLS §9.1)

Modifiers (JLS §9.1.1)

    public protected private
    abstract static strictfp

    private interface A {}          // compile error
    protected interface B{}         // compile error

• inner interfaces may be declared private and protected BUT only if they are defined in a class

  public interface A {
      private interface B {}          // compile error
      protected interface C {}        // compile error
  }
  
  public class A {
      private interface B {}          // compiles OK
      protected interface C {}        // compile OK
  
  }
  

• a compile error occurs if the same modifier appears more than once in an interface declaration (JLS §9.1.1)
• every interface is implicitly abstract; the modifier is obsolete and should not be used in new programs (JLS §9.1.1.1)

extendsClause

• consists of the extends keyword followed by a comma separated list of the interfaces being extended.

        interface InterfaceA extends
          interfaceX, interfaceY, ... {}

Interface body

• an interface body may contain constant declarations, abstract method declarations, inner classes and inner interfaces
• fields in an interface are implicitly static and final ie they MUST be constants (JLS§9.3)
• methods in an interface are implicitly abstract and public; they CANNOT be static (JLS§9.4)
• methods cannot be declared strictfp, native or synchronized (JLS§9.4)
• member classes declared in an interface are implicitly public and static (JLS§9.5)

Also see

• Sun Tutorial: Interfaces and packages
• Tech Tip: Abstract classes vs interfaces

Code Examples

• TestInterfaceModifiers.java
  
• TestInterfaceInClass.java

Traps

• an interface method declared as native or synchronized
• an interface method declared as static