Question

I’ve got a list filled with ints:

List collection = new ArrayList();
collection.add(this.score1);
collection.add(this.score2);
collection.add(this.score3);

Now I want to compare the highest score with each individual score, to see which one(s) is the highest. Intuitively I tried to do it like so:

String highestScore;

if(Collections.max(collection) == this.score1) {
highestScore = "Score 1";
}

if(Collections.max(collection) == this.score2) {
highestScore += ", Score 2";

}

if(Collections.max(collection) == this.score3) {
highestScore += ", Score 3";

}

However,

Collections.max(collection) == this.score1
Collections.max(collection) == this.score2
Collections.max(collection) == this.score3

all give me the error:

Incompatible operand types Comparable and int

However this seems to work:

int highestScoreValue =  Collections.max(collection);

Now how can that be?

Why is Java allowing an int to be set to Collections.max(collection), however doesn’t allow an int to be compared with Collections.max(collection)?

Answer 1

Primitive types can’t be stored in Java collections, they automatically get boxed inside respective classes.

Whenever you do list.add(score), score is first wrapped into an Integer object, then added to the collection.

Now, when you call Collections.max an obect of type <T extends Object & Comparable<? super T>> is returned, and you are comparing it to a primitive type with == but this operator is only able to compare references to objects or directly primitive types and in your situation, since your List parametric type is unspecified, the compiler is not able to unbox the Integer instance returned to an int.

Possible solutions are to

• cast the returned value to an Integer, thus allowing unboxing
• use equals method instead that == operator so that the score will be boxed to an Integer and then compared to the result of max