关于Java HashMap源

Question

So, we have a HashMap, and a transient Entry[] table inside it. In many methods, for example, in clear(), we copy table:

public void clear() {
    modCount++;
    Entry[] tab = table;
    for (int i = 0; i < tab.length; i++)
        tab[i] = null;
    size = 0;
}

But why we do Entry[] tab = table? What's wrong in next code?

public void clear() {
    modCount++;
    for (int i = 0; i < table.length; i++)
        table[i] = null;
    size = 0;
}

As far as i know, tab is only a reference to table, and, for at first sight, is just a wasting of space.

Answer 1

 for (int i = 0; i < table.length; i++)
        tab[i] = null;

You are right, tab and table both point to the same object. The problem I see is using table in the for-conditional and tab in the for-body. You should use one or the other, but not both.

tab is unnecessary, but sometimes people do this for readability reasons. Doesn't make sense here, since table is more readable. If I were code reviewing this, I would strongly prefer the second version.

Answer 2

It's just one locally scoped object reference. Not a lot of space there.