OnJava8-Examples/arrays/ArrayCopying.java

// arrays/ArrayCopying.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.
// Demonstrate Arrays.copy() and Arrays.copyOf()
import java.util.*;
import onjava.*;
import static onjava.ArrayShow.*;

class Sup { // Superclass
  private int id;
  Sup(int n) { id = n; }
  @Override public String toString() {
    return getClass().getSimpleName() + id;
  }
}

class Sub extends Sup { // Subclass
  Sub(int n) { super(n); }
}

public class ArrayCopying {
  public static final int SZ = 15;
  public static void main(String[] args) {
    int[] a1 = new int[SZ];
    Arrays.setAll(a1, new Count.Integer()::get);
    show("a1", a1);
    int[] a2 = Arrays.copyOf(a1, a1.length);   // [1]
    // Prove they are distinct arrays:
    Arrays.fill(a1, 1);
    show("a1", a1);
    show("a2", a2);

    // Create a shorter result:
    a2 = Arrays.copyOf(a2, a2.length/2);       // [2]
    show("a2", a2);
    // Allocate more space:
    a2 = Arrays.copyOf(a2, a2.length + 5);
    show("a2", a2);

    // Also copies wrapped arrays:
    Integer[] a3 = new Integer[SZ];            // [3]
    Arrays.setAll(a3, new Count.Integer()::get);
    Integer[] a4 = Arrays.copyOfRange(a3, 4, 12);
    show("a4", a4);

    Sub[] d = new Sub[SZ/2];
    Arrays.setAll(d, Sub::new);
    // Produce Sup[] from Sub[]:
    Sup[] b =
      Arrays.copyOf(d, d.length, Sup[].class); // [4]
    show(b);

    // This "downcast" works fine:
    Sub[] d2 =
      Arrays.copyOf(b, b.length, Sub[].class); // [5]
    show(d2);

    // Bad "downcast" compiles but throws exception:
    Sup[] b2 = new Sup[SZ/2];
    Arrays.setAll(b2, Sup::new);
    try {
      Sub[] d3 = Arrays.copyOf(
        b2, b2.length, Sub[].class);           // [6]
    } catch(Exception e) {
      System.out.println(e);
    }
  }
}
/* Output:
a1: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
a1: [1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]
a2: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
a2: [0, 1, 2, 3, 4, 5, 6]
a2: [0, 1, 2, 3, 4, 5, 6, 0, 0, 0, 0, 0]
a4: [4, 5, 6, 7, 8, 9, 10, 11]
[Sub0, Sub1, Sub2, Sub3, Sub4, Sub5, Sub6]
[Sub0, Sub1, Sub2, Sub3, Sub4, Sub5, Sub6]
java.lang.ArrayStoreException
*/