Creating an equals method

Question

As per an assignment, I have to create a method called "equals" to do the following:

The method compares the instance variables of the calling object with instance variables of the parameter object for equality and returns true if the dollars and the cents of the calling object are the same as the dollars and the cents of the parameter object. Otherwise, it returns false.

UPDATE: I made a mistake and put the wrong thing, updated it. Got a new error:

Money.java:115: error: long cannot be dereferenced
  if (dollars.equals(otherObject.dollars) &&
             ^
Money.java:116: error: long cannot be dereferenced
     cents.equals(otherObject.cents))
          ^
2 errors

With the method:

   public boolean equals(Money otherObject)
{
  boolean status;

  if (dollars.equals(otherObject.dollars) &&
     cents.equals(otherObject.cents))
    return status = true;
  else
    return status = false;
}

Does your `Money` class define a `symbol` instance variable? — rgettman, Mar 17 '14 at 19:06
Be careful with the ``dollars == otherObject.dollars`` comparison. If ``dollars`` is a primitive type (i.e. ``int``, or ``long``, etc) your comparison is fine, but if ``dollars`` is an object type (say, a ``Dollar`` class, for example), then your comparison might not be good enough, and you should also use ``dollars.equals(otherObject.dollars)``. — gdiazc, Mar 17 '14 at 19:10
@Gonzalo - The cents and dollars are both longs. Thanks for the reminder about the return, forgot. — WillBro, Mar 17 '14 at 19:14
(otherObject.symbol.GETSYMBOL() when you try to access private members of the class — Bogdan M., Mar 17 '14 at 19:17
http://users.csc.calpoly.edu/~kmammen/documents/java/howToOverrideEquals.html — Engineer2021, Mar 17 '14 at 19:31

score 1 · Answer 1 · edited May 23 '17 at 10:25

1

So both of your errors are trying to tell you that for whatever reason, your Money class doesn't have a field called symbol. So I'd confirm what the name of that field is before anything else and correct that.

You also need a return status; line, as outlined in the comments below your question.

Finally, and perhaps requiring the biggest change of all, you don't have the method signature for equals() correct. As you can see from the JavaDoc, the method is meant to take an Object as a parameter, not another instance of Money. Typically you'd do something like the following:

@Override
public boolean equals(Object object) {
    if (object == null || !(object instanceof Money)) {
        return false;
    }

    Money other = (Money) object;

    return this.value.equals(other.value)
            && this.secondField.equals(other.secondField)
            && this.primitiveField == other.primitiveField;
}

Since primitive types (ints, longs, etc.) don't have any methods on them (including .equals(), you have to compare these fields using == rather than .equals(), as shown in the example above.

This isn't the most complete solution (there's plenty of good information answering this question), and you should also override Object.hashCode() if you're modifying equals(), to keep them consistent.

edited May 23 '17 at 10:25

Community

1
1

answered Mar 17 '14 at 19:15

Edd

3,724
3
26
33

I checked, I don't know why I put symbol. My 2 instance fields are longs called "dollars" and "cents". This has cause a new error though so I'mma make an edit. Must of absent-mindedly put symbol. – WillBro Mar 17 '14 at 19:18
@WillBro Updated to include comparison of primitive (e.g. `long`) fields, which is what's causing your new error. – Edd Mar 17 '14 at 19:28
Need to check if `object` is null first and if it is return false. – Engineer2021 Mar 17 '14 at 19:28
Also need to check `this.getClass() != other.getClass()`.. instanceOf does the same thing too – Engineer2021 Mar 17 '14 at 19:29
1

Thanks @GIJoe - was typed in a bit of a hurry, and I have to admit to generally using IDE generated `equals()` and `hashCode()` methods as starting points most of the time! – Edd Mar 17 '14 at 19:32

score 1 · Accepted Answer · answered Mar 17 '14 at 19:25

1

Are dollars and cents primitive long types?

You'd want to use (dollars == otherObject.dollars) && (cents == otherObject.cents) if they are. You can't call a method (such as equals()) on a primitive.

answered Mar 17 '14 at 19:25

Darryl Gerrow

167
3

Creating an equals method

2 Answers2