61. Primitive type should be selected from boxed primitive type

Differences between boxed primitive types and primitive types

As mentioned in Item 6, autoboxing and autounboxing obscure the difference between boxed primitive types (hereinafter boxed primitives) and primitive types (hereinafter primitives). However, there are differences between the two, and you must be aware of the differences when using them. There are three differences

primitive has only a value, but boxed primitive has a reference different from the value.
primitive can only have a functional value, but boxed primitive can have null (nonfunctional value).
primitive is superior in terms of time and memory space.

Example of error when using boxed primitive

primitive has only a value, but boxed primitive has a reference different from the value

At first glance, the code below looks like a method that makes a good comparison.

// Broken comparator - can you spot the flaw?
Comparator<Integer> naturalOrder =
    (i, j) -> (i < j) ? -1 : (i == j ? 0 : 1);

However, if you use this method like ``` naturalOrder.compare (new Integer (42), new Integer (42))` ``, it should return 0 because it is supposed to be equal, but it actually returns 1. .. The first decision (i <j) is auto-unboxed and works correctly. On the other hand, the second judgment (i == j) looks at the equivalence, that is, whether the references are the same. Therefore, this comparison is not true and 1 is returned as a result. It is usually wrong to use the == operator for boxed primitives.

To prevent the above error, use `Comparator.naturalOrder ()`, or if you write the comparator yourself, make a primitive comparison as follows.

Comparator<Integer> naturalOrder = (iBoxed, jBoxed) -> {
    int i = iBoxed, j = jBoxed; // Auto-unboxing
    return i < j ? -1 : (i == j ? 0 : 1);
};

A primitive can only have a functional value, but a boxed primitive can have null (nonfunctional value)

In the following program, nullpo occurs.

package tryAny.effectiveJava;

public class BoxedPrimitive {
    static Integer i;

    public static void main(String[] args) {
        if (i == 42)
            System.out.println("Unbelievable");
    }

}

At i == 47, we are comparing Integer and int. In a process where boxed primitive and primitive are mixed, ** In most cases, boxed primitive is auto-unboxed. ** ** Unboxing an object that references null causes a nullpo, which is happening here.

primitive is better in terms of time and memory space

The following is the program handled in Item6.

package tryAny.effectiveJava;

public class BoxedPrimitive2 {
    // Hideously slow program! Can you spot the object creation?
    public static void main(String[] args) {
        Long sum = 0L;
        for (long i = 0; i < Integer.MAX_VALUE; i++) {
            sum += i;
        }
        System.out.println(sum);
    }

}

This is much slower than when the type of a local variable is set to long, because boxing and unboxing occur repeatedly.

Summary

There were three issues, the first two had bad results and the last one had poor performance.

There are the following cases when using the boxed primitive.

When used as an element, key, or value of a collection. You can't put the primitive in the collection, so you have to use it.
When used for parameterized types. For example, you cannot declare the type ``` ThreadLocal ``, but you can declare ``ThreadLocal ` ``.
Use boxed primitive when referencing a method in reflection (?). (Item65)

Item 61: Prefer primitive types to boxed primitives