Using Objects as keys in Java HashMap

Question

package test;

import java.util.HashMap;

public class test {
    public static void main(String args[]) {
        HashMap<ID, String> h = new HashMap<ID, String> ();
        String b;

        ID id1 = new ID(100);
        ID id2 = new ID(200);
        ID id3 = new ID(200);

        h.put(id1, "apple");
        h.put(id2, "pear");

        **System.out.println(h.containsKey(id3));**
    }
}

class ID {
    Integer code;

    public ID(Integer id) {
        this.code = id;
    }

    @Override
    public int hashCode()
    {
        return code.hashCode();
    }

    @Override
    public boolean equals(Object o)
    {
        return this.code.equals(o);
    }
}

the output of this program is "false". I can't understand of this result. I implemented hashCode and equals which are overrided. I don't have an idea how to solve. How to use objects as keys in java HashMap?

@saka1029 He puts another ID instance with the same integer value. So it should print "true" if he fixes his problem in the equals method. — Tom
– Tom, Commented Jun 6, 2015 at 10:36

Eran · Accepted Answer · 2015-06-06 08:07:07Z

7

Your equals implementation is wrong :

public boolean equals(Object o)
{
    return this.code.equals(o); // this will never return true,
                                // since you are comparing an Integer instance
                                // to an ID instance
}

You should compare this code to the other code:

public boolean equals(Object o)
{
    if (!(o instanceof ID))
        return false;
    ID oid = (ID) o;
    return this.code.equals(oid.code);
}

edited Jun 6, 2015 at 8:07

answered Jun 6, 2015 at 7:59

Eran

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Comments

Tom · Accepted Answer · 2015-06-06 10:35:29Z

0

Wrong Implementation of equals method. According to your implementation it executes the Integer class equals method. We should handle the logic without execute library classes equals methods.

@Override 
public boolean equals(Object o) {
    if (o == null || ((o instanceof ID) && o.hashCode() != this.hashCode())) {
        return false;
    }
    return true;
}

edited Jun 6, 2015 at 10:35

Tom

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answered Jun 6, 2015 at 10:19

user4935529

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