Stackの実装

TestStack.java

import java.util.*;
import org.junit.*;
import static org.junit.Assert.*;

// Last In First Out
public class TestStack {
  @Test
  public void testStack(){
    Stack<Integer> stack = new Stack<Integer>();
    stack.push(1);
    stack.push(2);
    Integer obj = stack.peek();
    assertEquals("2", obj.toString());
  }

  @Test
  public void testStack2(){
    Stack<Integer> stack = new Stack<Integer>();
    stack.push(1);
    stack.push(2);
    stack.push(3);
    stack.pop();
    assertEquals(2, stack.length());
  }

  @Test
  public void testStack3(){
    Stack<Integer> stack = new Stack<Integer>();
    stack.push(1);
    stack.push(2);
    stack.push(3);
    Integer obj = stack.pop();
    assertEquals("3", obj.toString());
  }
}

class Stack<E> {
  static class Node<E> {
    public Node<E> next = null;
    public E data;
  }

  Node<E> top;

  void push(E e){
    Node<E> node = new Node<E>();
    node.data = e;
    node.next = top;
    top = node;
  }

  E peek(){
    return top.data;
  }

  E pop(){
    if(top != null){
      E obj = top.data;
      top = top.next;
      return obj;
    }
    return null;
  }

  int length(){
    if(top == null) {
      return 0;
    }
    int length = 1;
    Node<E> node = top;
    while(node.next != null){
      node = node.next;
      length++;
    }
    return length;
  }
}