java实现一个栈

java实现一个栈:

public class Stack<T> {

    private ArrayList<T> mStack = new ArrayList<T>();

    public void push(T value) {
        synchronized (mStack) {
            mStack.add(value);
        }
    }

    public T pop() {
        synchronized (mStack) {
            int size = mStack.size();
            if (size > 0) {
                return mStack.get(size - 1);
            } else {
                return null;
            }
        }
    }
}