Collections - Beginning Java 8 Language Features - page 588

if (obj instanceof BadKey) {
BadKey bk = (BadKey) obj;
if (bk.getId() == this.id) {
return true;
}
}
return false;
}
@Override
public String toString() {
return String.valueOf(this.id);
}
}
The BadKey class stores an integer value. It is a mutable class. You can modify its state by calling the setId()
method and supplying a new value for its id . It overrides the equals() and hashCode() methods of the Object class.
The implementation of the hashCode() method is simple. It returns the value of the id instance variable as the hash
code value. The equals() method checks if the id instance variable's value for two BadKey objects are the same or not.
If two BadKey objects have the same id , they are considered equal.
Consider the program in Listing 12-35 that uses BadKey objects in a Set . Can you spot a problem by looking at the
program and the output? Don't worry if you do not see the problem. I will explain it.
Listing 12-35. Using BadKey Objects in a Set
// BadKeyTest.java
package com.jdojo.collections;
import java.util.HashSet;
import java.util.Set;
public class BadKeyTest {
public static void main(String[] args) {
Set<BadKey> s = new HashSet<>();
BadKey bk1 = new BadKey(100);
BadKey bk2 = new BadKey(200);
// Add two objects bk1 and bk2 to the set
s.add(bk1);
s.add(bk2);
System.out.println("Set contains:" + s);
System.out.println("Set contains bk1: " + s.contains(bk1));
// Set the id for bk1 to 300
bk1.setId(300);
System.out.println("Set contains:" + s);
System.out.println("Set contains bk1: " + s.contains(bk1));
}
}