Using objects as keys in Java HashMap

package test;

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 the program is false I can't understand the result I implemented hashcode and equals, which are overwritten I don't know how to solve it How to use objects as keys in Java HashMap?

Solution

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 with other codes:

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