Question

HashMap internally has its own static final variables for its working.

        static final int DEFAULT_INITIAL_CAPACITY = 16;

Why can’t they use byte datatype instead of using int since the value is too small.

Answer 1

Using byte or short for variables and constants instead of int is a premature optimization that has next to no effect.

Most arithmetic and logical instructions of the JVM work only with int, long, float and double, other data types have to be cast to (usually) ints in order for these instructions to be executed on them.

The default type of number literals is int for integral and double for floating point numbers. Using byte, short and float types can thus cause some subtle programming bugs and generally worsens code readability.

A little example from the Java Puzzlers book:

public static void main(String[] args) {
    for (byte b = Byte.MIN_VALUE; b < Byte.MAX_VALUE; b++) {
        if (b == 0x90)
            System.out.print("Joy!");
    }
}

This program doesn’t print Joy!, because the hex value 0x90 is implicitly promoted to an int with the value 144. Since bytes in Java are signed (which itself is very inconvenient), the variable b is never assigned to this value (Byte.MAX_VALUE = 127) and therefore, the condition is never satisfied.

All in all, the reduction of the memory footprint is simply too small (next to none) to justify such micro-optimisation. Generally, explicit numeric types of different size are not necessary and suitable for higher level programming. I personally think that only case where smaller numeric types are acceptable are byte arrays.