Below is the syntax highlighted version of CovariantPhoneNumber.java
from §3.4 Hash Tables.
/****************************************************************************** * Compilation: javac CovariantPhoneNumber.java * Execution: java CovariantPhoneNumber * Dependencies: * * Immutable data type for US phone numbers, but with a broken * covariant implementation of equals(). By implementing equals() * with the signature * * public boolean equals(CovariantPhoneNumber 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 * ******************************************************************************/ import java.util.HashSet; public final class CovariantPhoneNumber { private final int area; // area code (3 digits) private final int exch; // exchange (3 digits) private final int ext; // extension (4 digits) public CovariantPhoneNumber(int area, int exch, int ext) { this.area = area; this.exch = exch; this.ext = ext; } // covariant equals - don't do this public boolean equals(CovariantPhoneNumber that) { return (this.area == that.area) && (this.exch == that.exch) && (this.ext == that.ext); } // satisfies the hashCode contract public int hashCode() { return 10007 * (area + 1009 * exch) + ext; } // 0 for padding with leading 0s public String toString() { return String.format("(%03d) %03d-%04d", area, exch, ext); } public static void main(String[] args) { CovariantPhoneNumber a = new CovariantPhoneNumber(609, 258, 4455); CovariantPhoneNumber b = new CovariantPhoneNumber(609, 876, 5309); CovariantPhoneNumber c = new CovariantPhoneNumber(609, 203, 5309); CovariantPhoneNumber d = new CovariantPhoneNumber(215, 876, 5309); CovariantPhoneNumber e = new CovariantPhoneNumber(609, 876, 5309); StdOut.println("a = " + a); StdOut.println("b = " + b); StdOut.println("c = " + c); StdOut.println("d = " + d); StdOut.println("e = " + e); // show broken behavior when you use covariant equals with a Set HashSet<CovariantPhoneNumber> set = new HashSet<CovariantPhoneNumber>(); 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))); } }