Why is HashSet size 3 after adding 4 objects?

Question

I am new to Java so please be gentle!

import java.util.HashSet;

public class HashTest {

private String str;

public HashTest(String str) {
    this.str = str;
}

public static void main(String args[]) {
    HashTest h1 = new HashTest("1");
    HashTest h2 = new HashTest("1");
    String s1 = new String("2");
    String s2 = new String("2");

    HashSet<Object> hs = new HashSet<Object>();
    hs.add(h1);
    hs.add(h2);
    hs.add(s1);
    hs.add(s2);

    System.out.print(hs.size());
 }
}

Output is 3.

My question is -- why 3 ? One of h1 and h2 and one of s1 and s2 should be inserted. Since they are using the same thing to determine equality i.e. String.hashcode function call.

Answer 1

Because you're checking the equality of the HashTest objects by comparing their strings in your .equals() method. Since both of your HashTest objects have the same string, they are equal. Now, the HashSet object replaces h1 with h2, I believe.

Edit:

Just realized the output is actually 2. That's because the second String added replaces the first as well, since the .eqauls method of a String object also compares the actual string and not the reference.

Edit 2:

Regarding your updated code where you get 3 as the output, you have not implemented an equals method. Therefore, it doesn't compare the actual str data member of the HashTest objects. Now it compares references and adds both HashTest objects since they have different references. So there exists 2 hashtest objects now, and 1 string in your HashSet now.

Answer 2

Your HashTest object extends Object (all classes extend Object by default), but not String (actually, you cannot extend String because is a final class, so when the HashSet uses equals to check if an element already exists, is Object.equals() the one executed, not the String.equals() method.You have to override that method with something like:

public boolean equals(Object o) {
    if (o instanceof HashTest) {
        return this.str.equals(((HashTest)o).str);
    }
    return false;
}

Answer 3

You have not implemented equals method in your HashTest class. Becoz of that it is treating both h1 & h2 as different objects and inserting in HashSet.

But in case of String, equals is already implemented. So it will override s1 with s2 because both strings are same only.

size of HashSet = one(h1) + one(h2) + one(s2) = 3