Java ArrayList IndexOf - Finding Object Index

Question

Lets say I have a class

public class Data{
    public int k;
    public int l;
    public Data(int k, int l){
      this.k = k; 
      this.l = l;
    }
    public boolean equals(Date m){
      if(this.k == m.k && this.l = m.l)
           return true;
      return false;
    }
}

And I add a few Data objects to a ArrayList:

ArrayList<Data> holder = new ArrayList<Data>;
Data one = new Data(0,0);
Data two = new Data(0,4);
Data three = new Data(0,5);

Why does indexOf not find this?:

holder.indexOf(new Data(0,4)); //returns -1

Is indexOf any better than going through the whole array list myself? Or am I missing something.

Answer 1

The indexOf() method does go through the entire list. Here's an excerpt from Java 7 source code:

public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

It'd be better to let Java go through it than write it yourself. Just make sure that your equals method is sufficient at finding the object you want. You'll also want to override hashCode() as well.

I won't write your equals method out, but I would recommend that you at least:

• Check for null
• Test if the instances you're comparing are the same
• You don't need to do if(boolean_expr) { return true; }; just return the boolean expression.
• Make sure you're actually overriding your equals method - the signature of that requires an Object parameter, not Date.