howto implement a class that evealuates contains in a list

Question

I have: - MyClass with hashCode() overwritten - I'll try to avoid adding a second instance of MyClass with the same content to an List using the contains() method of the List.

How can I achive this ? I thought this is done by just overriding the hashCode() method ?

I guess code says more then words

public class TestClass {

    public static void main(String[] args){

        MyClass myclass1 = new MyClass(2);
        MyClass myclass2 = new MyClass(2);

        List<MyClass> myclasses = new ArrayList<MyClass>();
        myclasses.add(myclass1);
        if(!myclasses.contains(myclass2)){
            myclasses.add(myclass2);
            System.out.println("I'll add ...");
        }

        System.out.println(myclasses.size());
    }
}

public class MyClass {

    int id = -1;
    String something = "";

    public MyClass(int idparam){
        this.id = idparam;
    }

    @Override
    public int hashCode() {
        return id;
    }

}

Updated Class:

public boolean equals(MyClass secondmyclass) {
    return secondmyclass.id == this.id;
}

I tried that too. I just updated the code above. But still it doesn't work. Can you please review the code ? (Overriding is not possible, I just added equals without annotation @override) — mcfly soft, Nov 15 '17 at 17:30
`equals` is defined with an `Object` parameter. You're currently overloading the method. — Sotirios Delimanolis, Nov 15 '17 at 17:33
Ok. I see. Thanks a lot. Always tried with the Class Type. Thanks. — mcfly soft, Nov 15 '17 at 17:36

Francisco Valle · Accepted Answer · 2017-11-15T17:53:21.787

You have to override equals and hashcode methods. The object is not contained in the list as is not equal to any instance in the list, even if it generates same hash code.

@Override
public boolean equals(Object obj) {
    boolean eq = false;
    if (obj != null && obj instanceof YOURCLASS) {
        eq = this.getId().equals((YOUCLASS)obj.getId());
    }
}

Equals method should look like this. YOURCLASS is the name of the bean added to the list. You have to compare the key fields after verifying the obj instance is not null and is of the same type of the original instance.

In your example YOURCLASS is MyClass and the comparison inside equals should be with == instead of equals, as you are not using int insteand of Integer for the field.

How do I have to implement the equal mehtod ? I updated the code. Can you review, because it still doesn't work. — mcfly soft, Nov 15 '17 at 17:33

howto implement a class that evealuates contains in a list

1 Answers1