public class ArrayStack implements Stack { private Object[] a; private int size; public ArrayStack(int capacity) { a=new Object[capacity]; } public boolean isEmpty(){ return (size==0); } public Object peek(){ if (size==0) throw new IllegalStateException("stack is empty"); return a[size-1]; } public Object pop(){ if (size==0) throw new IllegalStateException("stack is empty"); Object object=a[--size]; a[size]=null; return object; } public void push(Object object){ if (size==a.length) resize(); a[size++]=object; } public int size(){ return size; } private void resize(){ Object[] aa=a; a=new Object[2*aa.length]; System.arraycopy(aa,0,a,0,size); } public void reverse(){ if (size==0) throw new IllegalStateException("stack is empty"); System.arraycopy(a, 1, a, 0, --size); }}