initialize fileds in interfaces

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/wangbingfengf98/article/details/85542005

Fields defined in interfaces cannot be “blank finals,” but they can be initialized with non-constant expressions. For example:

// interfaces/RandVals.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Initializing interface fields with
// non-constant initializers

import java.util.*;

public interface RandVals {
  Random RAND = new Random(47);
  int RANDOM_INT = RAND.nextInt(10);
  long RANDOM_LONG = RAND.nextLong() * 10;
  float RANDOM_FLOAT = RAND.nextLong() * 10;
  double RANDOM_DOUBLE = RAND.nextDouble() * 10;
}

test above code:

// interfaces/TestRandVals.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

public class TestRandVals {
  public static void main(String[] args) {
    System.out.println(RandVals.RANDOM_INT);
    System.out.println(RandVals.RANDOM_LONG);
    System.out.println(RandVals.RANDOM_FLOAT);
    System.out.println(RandVals.RANDOM_DOUBLE);
  }
}
/* Output:
8
-32032247016559954
-8.5939291E18
5.779976127815049
*/

Knowledge:

The fields are not part of the interface. The values are stored in the static storage area for that interface.

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/interfaces/RandVals.java

3. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/interfaces/TestRandVals.java