Autoboxing

Caveats of autoboxing and unboxing

• Arrays are never autoboxes or auto-unboxed, e.g. if you have an array of integers int[] x, and try to put its address into a variable of type Integer[], the compiler will not allow your program to compile.

• Autoboxing and unboxing also has a measurable performance impact. That is, code that relies on autoboxing and unboxing will be slower than code that eschews such automatic conversions.

• Additionally, wrapper types use much more memory than primitive types. On most modern comptuers, not only must your code hold a 64 bit reference to the object, but every object also requires 64 bits of overhead used to store things like the dynamic type of the object.

  • For more on memory usage, see this link or this link.

Widening

For example, doubles in Java are wider than ints. If we have the function shown below:

public static void blahDouble(double x) {
    System.out.println(“double: “ + x);
}

We can call it with an int argument:

int x = 20;
blahDouble(x);

The effect is the same as if we’d done blahDouble((double) x). Thanks Java!