Implement stack structure with Java

Stack is an advanced data structure, the stack stack is the top element of the operation, the following is a simple stack structure using the Java language implementation
Class mystack{
private int size;//stack size
Private object[] elementdata;//elements in stack
private int top;//Stack top pointer

public MyStack(int size){    this.size = size;    this.top = 0;    this.elementData = new Object[size];}public boolean push(Object o){    if (ensureCapacity(top+1)){        top++;        elementData[top] = o;        return true;    }    return false;}public Object pop(){    if (top >= 0){        Object o = elementData[top];        elementData[top] = null;        top--;        return o;    }    return null;}public boolean isEmpty(){    return top == 0;}private boolean ensureCapacity(int capacity) {    if (capacity >= this.size){        return false;    }    return true;}

}

Implement stack structure with Java