Using an object's instance as a HashMap key

Question

I have found similar questions, but none of them have really helped me so far (which probably means I am doing something wrong, which is why I'm here).

I have a HashMap Map<Integer, List<Book>> that is supposed to have a Student's ID as the key and the list of Book he has in his possession as the value. Although only passing the ID as the key works, I feel like it isn't an object-oriented enough approach and that I should use Student as the key (so it'd look like so: Map<Student, List<Book>>). I tried to use it, but then I got a bunch of errors when I tried to return a given Student's list of Book, most likely because the Student wasn't found. My professor suggested me to @Override Java's hashCode and equals methods, which makes sense, since that's how HashMap compares keys, but I haven't had success in that (I am not sure what exactly I need to compare in order to see if two Student are the same; supposedly, their ID alone should work). Here are the overridden methods:

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + id;
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Student other = (Student) obj;
    if (id != other.id)
        return false;
    return true;
}

What am I doing wrong? Any light you could shed on this would be extremely helpful. Thanks in advance!

The error is elsewhere. Post a complete minimal example reproducing the problem. — JB Nizet
– JB Nizet, Commented Mar 11, 2017 at 14:26
if (id != other.id) return false; return true; Why did you prefer this to return id.equals(other.id);? — Lew Bloch
– Lew Bloch, Commented Mar 11, 2017 at 16:33

Rahul · Accepted Answer · 2017-03-11 14:31:45Z

1

The error seems to be in your equals method, provided your id has values outside integer caching range i.e. -128 - 127

 if (id != other.id) // <-- Error
        return false;

From your example, it seems like id is an Integer object

== checks whether the references are equal, i.e. whether they point to the same object.

For primitive types, == checks whether the values are equal.

java.lang.Integer is a reference type. int is a primitive type.

Although in case of Integer, == only works for numbers between -128 and 127 as JVM caches those values

You need to change it to

if (!id.equals(other.id))
            return false;

answered Mar 11, 2017 at 14:31

Rahul

16.4k4 gold badges44 silver badges64 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

BarrySW19 Over a year ago

Possible, but as those look like auto-generated methods I'd guess that id is a primitive.

Rahul Over a year ago

As the OP has used an Integer as a key in the map, makes me believe that id is an Integer instance

Lew Bloch Over a year ago

if (!id.equals(other.id)) return false; Why did you prefer this to return id.equals(other.id);?

Rahul Over a year ago

@LewBloch could work well for now, but as soon as you add more fields for comparison in your equals method, you will have to change the statement return id.equals(other.id); otherwise remaining comparison statements will not be executed. Hence, the correct form is better from code extensibility point of view

Lew Bloch Over a year ago

That wasn't the case here, but let's say that instead of the non-object-oriented id we use three name parts, we could just use: boolean is = first.equals(other.first) && second.equals(other.second) && third.equals(other.third);. I agree that sometimes if chains are better, but very few equality comparisons of value types involve more than three fields.

|

Collectives™ on Stack Overflow

Using an object's instance as a HashMap key

1 Answer 1

9 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

9 Comments

Your Answer

Sign up or log in

Post as a guest

Related