Fields

An interface can declare fields, which are implicitly public, static, and final.

• That is, interface fields are constants that can't be changed by their implementing classes or anyone else.

• None of these modifier keywords is required, but any are allowed and no others are allowed.

• Interface fields are final; interfaces have no constructors and cannot define instance initializers, so initialize interface fields in their declarations.

public interface ConstantsInterface {
  // All of theses are valid interface field declarations
  // All are public, static, and final:
  public static final int A = 1;
  public static int B = 2;
  public final int C = 3;
  static final int D = 4;
  static int E = 5;
  final int F = 6;
  public int G = 7;
  int H = 8;
}

Interface fields can be accessed directly via the interface, or via an implementing class or object.

public class ConstantsImplementer implements ConstantsInterface {
  public void aMethod() {
    System.out.println(this.A);
    System.out.println(B);
    System.out.println(ConstantsInterface.C);
//    this.D = 100;  // WILL NOT COMPILE: final field can't be reassigned.
  }
}

public class ConstantsTester {
  public static void main(String[] args) {
    ConstantsImplementer imp = new ConstantsImplementer();
    int x = imp.A;
    int y = ConstantsInterface.B;
    int z = ConstantsImplementer.C;
  }
}

Drill

What will happen when you try to compile and run this code?

interface SomeInterface {
  int value = 0;
}
public class SomeClass implements SomeInterface {

  public static void main(String[] args) {
    SomeClass c = new SomeClass();
    c.doStuff();
  }

  private void doStuff() {
    value = 1;
    System.out.println("Doing stuff: " + value);
  }

}

1. [ ] It will print "Doing stuff: 0" 2. [ ] It will print "Doing stuff: 1" 3. [ ] It will not compile 4. [ ] It will throw an exception at runtime

(Solution: SomeClass.java)

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