Casting a primitive vs Creating a object of the primitive

Question

I'm not habitual to casting a primitive data type to an object. Saw some code like:

public static int CompareAges(Person p1, Person p2) {
    Integer age1 = p1.getAge();
    return age1.compareTo(p2.getAge());
}

The instantiation of age1 seemed extraneous, so I tried to write the code as:

public static int CompareAges(Person p1, Person p2) {
    return p1.getAge().compareTo(p2.getAge());
}

But that raised a compiler error because p1.getAge() is a primitive data type int and not an Integer, which is an object.

Intuitively, I did:

public static int CompareAges(Person p1, Person p2) {
    return ((Integer) p1.getAge()).compareTo(p2.getAge());
}

and it worked!

The question: What did I miss? Since when did we start casting primitives as value types?

Answer 1

It's what happens internally:

1. Integer age1 = p1.getAge();
       Integer != int
       Integer = Integer.valueOf(int)
   Integer age1 = Integer.valueOf(p1.getAge());

2. p1.getAge().compareTo(p2.getAge());
       int.compareTo(int)
       ^^^
       // it's just a primitive type, as a result - the compile error

3. ((Integer) p1.getAge()).compareTo(p2.getAge())
       Integer.compareTo(int)
       Integer.compareTo(Integer.valueOf(int))
   ((Integer) p1.getAge()).compareTo(Integer.valueOf(p2.getAge()))

4. (Integer) p1.getAge() ---> Integer.valueOf(p1.getAge()) 
       // why so? look at callOfCode's answer

But, in my opinion, ((Integer) p1.getAge()).compareTo(p2.getAge()) looks ugly.

I would replace it to

p1.getAge() > p2.getAge() ? 1 : (p1.getAge() < p2.getAge() ? -1 : 0)

or to

Integer.compare(p1.getAge(), p2.getAge()) // java 7+

I wouldn't like to do casting in this case.

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More about "autoboxing/unboxing" you may find here.

Answer 2

Each primitive has its own boxing type

The Boxing could be implicit (Autoboxing) or explicit like you did it in your code.

p2.getAge() is an Example of AutoBoxing or Implicit Boxing. The method compareTo takes an Object. As it is a primitive, Java converts it automatically to the correspondent Boxing Object (Integer). You did the explicit Boxing with the cast (Integer) p1.getAge()