Question

I’m having a problem with getting an ArrayList to correctly use an overriden equals. the problem is that I’m trying to use the equals to only test for a single key field, and using ArrayList.contains() to test for the existence of an object with the correct field. Here is an example

public class TestClass  {
    private static class InnerClass{    
    private final String testKey;
    //data and such

    InnerClass(String testKey, int dataStuff) {
        this.testKey =testKey;
        //etc
    }
    @Override
    public boolean equals (Object in) {
        System.out.println("reached here");
        if(in == null) {
        return false;
        }else if( in instanceof String) {
        String inString = (String) in;
        return testKey == null ? false : testKey.equals(inString);
        }else {
        return false;
        }       
    }       
    }

    public static void main(String[] args) {    
    ArrayList<InnerClass> objectList = new ArrayList<InnerClass>();
    //add some entries
    objectList.add(new InnerClass("UNIQUE ID1", 42));
    System.out.println( objectList.contains("UNIQUE ID1")); 
    }    
}

What worries me is that not only am I getting false on the output, but I’m also not getting the “reached here” output.

Does anyone have any ideas why this override is being completely ignored? Is there some subtlety with overrides and inner classes I don’t know of?

Edit:
Having problems with the site so I cant seem to mark the answered.
Thanks for the quick response: yes an oversight on my part that it is the String .equals thta is called, not my custom one. I guess it’s old fashioned checks for now

Answer 1

If you check sources of ArrayList, you will see that it calls equals of other object. In your case it will call equals of String "UNIQUE ID1" which will check that other object is not of type String and just returns false:

public boolean contains(Object o) {
    return indexOf(o) >= 0;
}

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

For your case call contains with InnerClass that only contains id:

objectList.contains(new InnerClass("UNIQUE ID1"))

Don’t forget to implement equals for InnerClass which compares id only.