OnJava8-Examples/reuse/FinalData.java

// reuse/FinalData.java
// (c)2021 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.
// The effect of final on fields
import java.util.*;

class Value {
  int i; // Package access
  Value(int i) { this.i = i; }
}

public class FinalData {
  private static Random rand = new Random(47);
  private String id;
  public FinalData(String id) { this.id = id; }
  // Can be compile-time constants:
  private final int valueOne = 9;
  private static final int VALUE_TWO = 99;
  // Typical public constant:
  public static final int VALUE_THREE = 39;
  // Cannot be compile-time constants:
  private final int i4 = rand.nextInt(20);
  static final int INT_5 = rand.nextInt(20);
  private Value v1 = new Value(11);
  private final Value v2 = new Value(22);
  private static final Value VAL_3 = new Value(33);
  // Arrays:
  private final int[] a = { 1, 2, 3, 4, 5, 6 };
  @Override public String toString() {
    return id + ": " + "i4 = " + i4
      + ", INT_5 = " + INT_5;
  }
  public static void main(String[] args) {
    FinalData fd1 = new FinalData("fd1");
    //- fd1.valueOne++; // Error: can't change value
    fd1.v2.i++; // Object isn't constant!
    fd1.v1 = new Value(9); // OK -- not final
    for(int i = 0; i < fd1.a.length; i++)
      fd1.a[i]++; // Object isn't constant!
    //- fd1.v2 = new Value(0); // Error: Can't
    //- fd1.VAL_3 = new Value(1); // change reference
    //- fd1.a = new int[3];
    System.out.println(fd1);
    System.out.println("Creating new FinalData");
    FinalData fd2 = new FinalData("fd2");
    System.out.println(fd1);
    System.out.println(fd2);
  }
}
/* Output:
fd1: i4 = 15, INT_5 = 18
Creating new FinalData
fd1: i4 = 15, INT_5 = 18
fd2: i4 = 13, INT_5 = 18
*/