I was going through the source code of the java.util.HashMap class and noticed that the explicit no-arg constructor expects two constants:
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);
}
But when I looked at the DEFAULT_INITIAL_CAPACITY constant, I found that it was defined as follows:
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
I've never seen this type of construct used in any product I've worked on, and couldn't find any results in the Java Language Specification or through Googling. So I looked at the byte code, but I found that using 16 vs 1 << 4 provided identical output, meaning that (at least in my minimalist case) the compiler will convert the latter to decimal notation. The bytecode of both versions includes the definition:
javap -c -verbose /---/myClass.class
----
public static final int i;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 16
So I would welcome any feedback on the necessity/use of this idiom.
It's fully equivalent, but it was written this way to make it clearer where the value came from and how it was computed -- in this case that it was intended to be 2^4.
It could have been written either way, the authors just thought this would be more self-documenting.