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Thinking in Java
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Initializing fields in interfaces

Fields defined in interfaces are automatically static and final. These cannot be “blank finals,” but they can be initialized with nonconstant expressions. For example:

// Initializing interface fields with
// non-constant initializers.
import java.util.*;

public interface RandVals {
  Random rand = new Random();
  int randomInt = rand.nextInt(10);
  long randomLong = rand.nextLong() * 10;
  float randomFloat = rand.nextLong() * 10;
  double randomDouble = rand.nextDouble() * 10;
} ///:~

Since the fields are static, they are initialized when the class is first loaded, which happens when any of the fields are accessed for the first time. Here’s a simple test:

import com.bruceeckel.simpletest.*;

public class TestRandVals {
  private static Test monitor = new Test();
  public static void main(String[] args) {
    monitor.expect(new String[] {
      "%% -?\\d+",
      "%% -?\\d+",
      "%% -?\\d\\.\\d+E?-?\\d+",
      "%% -?\\d\\.\\d+E?-?\\d+"
} ///:~

The fields, of course, are not part of the interface but instead are stored in the static storage area for that interface.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire