here is the code from Item 6, pg 24, chapter 2 of effective java 2nd edition, by Joshua Bloch.
In the pop method he defines, he uses elements[--size]
. I am wondering why he used --size
, instead elements[size--]
should return the same correct?
public class Stack {
private Object[] elements;
private int size = 0;
private static final int DEFAULT_INITIAL_CAPACITY = 16;
public Stack() {
elements = new Object[DEFAULT_INITIAL_CAPACITY];
}
public void push(Object e) {
ensureCapacity();
elements[size++] = e;
}
public Object pop() {
if (size == 0)
throw new EmptyStackException();
return elements[--size];
}
/**
* Ensure space for at least one more element, roughly
* doubling the capacity each time the array needs to grow.
*/
private void ensureCapacity() {
if (elements.length == size)
elements = Arrays.copyOf(elements, 2 * size + 1);
} }
Because arrays are 0 based indexed.
Imagine that you have a stack with 2 elements.
So the size of the stack is equals to 2 with the following representation in the array :
elements[0] = elem;
elements[1] = elem;
So you need to decrease size before pop the elem from the stack, otherwise you will try to pop elements[2]
, which doesn't exists. So an infix operator is used in this case.
return elements[--size];
is equivalent to
size--;
return elements[size];
If elements[size--];
was written, it would try to pop elements[2]
and then decrease size by 1. So an ArrayIndexOutOfBoundsException
will be thrown each time you want to pop an element from the stack.