01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
 
package algs34;
import stdlib.*;
import java.util.HashSet;
/* ***********************************************************************
 *  Compilation:  javac PhoneNumber.java
 *  Execution:    java PhoneNumber
 *  Dependencies:
 *
 *  Immutable data type for US phone numbers, but with a broken
 *  overloaded implementation of equals(). By implementing equals()
 *  with the signature
 *
 *       public boolean equals(PhoneNumber that)
 *
 *  we do not override the equals() method inherited from Object.
 *  This causes unexpected behavior when used with java.util.HashSet.
 *
 *  DO NOT USE THIS CLASS
 *
 *************************************************************************/

public final class XPhoneNumberOverload {
  private final int area;   // area code (3 digits)
  private final int exch;   // exchange  (3 digits)
  private final int ext;    // extension (4 digits)

  public XPhoneNumberOverload(int area, int exch, int ext) {
    this.area = area;
    this.exch = exch;
    this.ext  = ext;
  }

  // overloaded equals - don't do this
  public boolean equals(XPhoneNumberOverload that) {
    return (this.area == that.area) && (this.exch == that.exch) && (this.ext == that.ext);
  }

  // satisfies the hashCode contract
  public int hashCode() {
    int h = 17;
    h = ext + 31 * h;
    h = exch + 31 * h;
    h = area + 31 * h;
    return h;
  }

  // 0 for padding with leading 0s
  public String toString() {
    return String.format("(%03d) %03d-%04d", area, exch, ext);
  }

  public static void main(String[] args) {
    XPhoneNumberOverload a = new XPhoneNumberOverload(609, 258, 4455);
    XPhoneNumberOverload b = new XPhoneNumberOverload(609, 876, 5309);
    XPhoneNumberOverload c = new XPhoneNumberOverload(609, 003, 5309);
    XPhoneNumberOverload d = new XPhoneNumberOverload(215, 876, 5309);
    XPhoneNumberOverload e = new XPhoneNumberOverload(609, 876, 5309);
    Object o = e;
    StdOut.format("a = %s [hashcode=%d]\n", a, a.hashCode ());
    StdOut.format("b = %s [hashcode=%d]\n", b, b.hashCode ());
    StdOut.format("c = %s [hashcode=%d]\n", c, c.hashCode ());
    StdOut.format("d = %s [hashcode=%d]\n", d, d.hashCode ());
    StdOut.format("e = %s [hashcode=%d]\n", e, e.hashCode ());

    // show broken behavior when you use covariant equals with a Set
    HashSet<XPhoneNumberOverload> set = new HashSet<>();
    set.add(a);
    set.add(b);
    set.add(c);
    StdOut.println("Added a, b, and c");
    StdOut.println("contains a:  " + set.contains(a));
    StdOut.println("contains b:  " + set.contains(b));
    StdOut.println("contains c:  " + set.contains(c));
    StdOut.println("contains d:  " + set.contains(d));
    StdOut.println("***contains e:  " + set.contains(e));  // not in set, but it should be!
    StdOut.println("b == e:      " + (b == e));
    StdOut.println("b.equals(e): " + (b.equals(e)));
    StdOut.println("o == e:      " + (o == e));
    StdOut.println("b.equals(o): " + (b.equals(o)));
  }

}