Stack底层源码

public class Vector<E> extends AbstractList<E> implements List<E>{
    
    protected Object[] elementData;//new Object[10]{"杨勇1","杨勇2","杨勇3".....}
    protected int elementCount;//3
    
    public synchronized void addElement(E obj) {
        modCount++;
        ensureCapacityHelper(elementCount + 1);
        elementData[elementCount++] = obj;
    }
    
    public synchronized int size() {
        return elementCount;
    }
    
    //index - 2
    public synchronized E elementAt(int index) {
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " + elementCount);
        }

        return elementData(index);
    }
    
    //index - 2
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }
    
    //index - 2
    public synchronized void removeElementAt(int index) {
        modCount++;
        if (index >= elementCount) {
            throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                     elementCount);
        }
        else if (index < 0) {
            throw new ArrayIndexOutOfBoundsException(index);
        }
        int j = elementCount - index - 1;
        if (j > 0) {
            System.arraycopy(elementData, index + 1, elementData, index, j);
        }
        elementCount--;
        elementData[elementCount] = null; /* to let gc do its work */
    }
}

public class Stack<E> extends Vector<E> {

    public Stack() {
    }

    //item - 杨勇3
    public E push(E item) {
        addElement(item);

        return item;
    }
    
    public boolean empty() {
        return size() == 0;
    }
    
    public synchronized E pop() {
        E obj;
        int len = size();//len - 3

        obj = peek();//obj - "杨勇3"
        removeElementAt(len - 1);

        return obj;
    }
    
    public synchronized E peek() {
        int len = size();//len - 3

        if (len == 0)
            throw new EmptyStackException();
        return elementAt(len - 1);
    }
    
}

public class Test01 {

	public static void main(String[] args) {
		
		Stack<String> stack = new Stack<>();
		
		stack.push("杨勇1");
		stack.push("杨勇2");
		stack.push("杨勇3");
		
        while(!stack.empty()){
			String pop = stack.pop();
			System.out.println(pop);
		}
	}
}