请问为何我的JUnit Test出现了NullPointerException的错误呢

[orocsy]

stack.clear();这里出现的错误

import static org.junit.Assert.*;
import junit.framework.TestCase;
import org.junit.Test;


public class StackTest extends TestCase {

	private SelfDefinedStack stack_test;
	
	public  void Test()
	{
		stack_test = new SelfDefinedStack();
		testsize();
		testclear();
		testpush();
		testtop();
		testpop();
		
	}
	/** 
	    * Test of size() method of class SelfDefinedStack 
	    * This method will push an element into the stack and verify that the stack size will 
	    * increase by 1 
	    */
	public void testsize()
	{
		int count = 0;
		count = stack_test.size();
		System.out.println("size of the stack is "+count);
		
		stack_test.push("test_stack1");
		int newcount = stack_test.size();
		System.out.println("size of the stack is "+newcount);
		if(newcount != count+1)
		{
			System.out.println("sizetest failed");
		}
		else
		{
			System.out.println("sizetest passed");
		}
	}
	/** 
	    * Test of push() method of class SelfDefinedStack 
	    * This method will push an element into the stack and verify that the stack size will 
	    * increase by 1
	    */
	public void testpush()
	{
		stack_test.clear();
		if(stack_test.size() != 0)
		{
			System.out.println("testpush() failed on size");
		}
		else
		{
			stack_test.push("test_stack2");
			if(stack_test.size() != 1)
			{
				System.out.println("testpush() failed");
			}
			else
			{
				System.out.println("testpush() passed");
			}
		}
	}
	/** 
	    * Test of pop() method of class SelfDefinedStack 
	    * This method will pop out the element on the top of the stack and verify the returned item
	    */
	public void testpop()
	{
		stack_test.clear();
		if(stack_test.size() != 0)
		{
			System.out.println("testpop() failed on size");
		}
		else
		{
			stack_test.push("test_stack3");
			if(stack_test.size() != 1)
			{
				System.out.println("testpush() failed");
			}
			String popElem = stack_test.pop().toString();
			if(popElem == null)
			{
				System.out.println("testtop() failed: return element is null");
			}
            else if (popElem != "test_stack3")
            {
            	System.out.println("testpop() failed wrong element returned ");
            }
            else if(stack_test.size() != 0)
            {
            	System.out.println("testpop() failed: size is not 0");
            }
            System.out.println("testpop() passed");
		}
	}
	 /** 
	   * Test of top() method of class SelfDefinedStack 
	   * This method will verify that the element on the top of the stack will be retrieved and the
	   * stack won't change
	   */
	public void testtop()
	{
	    stack_test.clear();
	    if(stack_test.size() != 0)
	    {
	    	System.out.println("testtop() failed on size");
	    }
	    else
	    {
	    	stack_test.push("test_stack4");
	    	stack_test.push("test_stack5");
	    	String topElem = stack_test.top().toString();
	    	if(stack_test.size() != 2)
	    	{
	    		System.out.println("testtop() failed on size");
	    	}
	    	else if(topElem == null)
	    	{
	    		System.out.println("testtop() failed: Element topped from stack is null");
	    	}
	    	else if(!topElem.equals("test_stack5"))
	    	{
	    		System.out.println("testtop() failed: Element topped is not the first element of the stack");
	    	}
	    	else
	    		System.out.println("testtop() passed");
	    }
	}
	/** 
	   * Test of clear() method of class SelfDefinedStack 
	   * This method will verify that all elements in the stack have been removed
	   */
	public void testclear()
	{
		stack_test.push("test_stack6");
		if(stack_test.size() <= 0)
		{
			System.out.println("testclear() failed on push");
		}
		else
		{
			stack_test.clear();
			if(stack_test.size() != 0)
			{
				System.out.println("testclear() failed");
			}
			else
				System.out.println("testclear() passed");
		}
		
	}
	

}

以上是测试类
以下是实现的简单堆栈功能

import java.util.LinkedList;
/**
 * This class 
 * @author shiyucai
 *
 */
public class SelfDefinedStack implements Stack{

	/**
	 * 
	 */
	@Override
	public void push(Object obj) {
     	assert obj!=null;	
		assert wellformed();
		num++;
		list.addFirst(obj);
	}
    /**
     * 
     */
	@Override
	public Object pop() {
		// TODO Auto-generated method stub
		assert wellformed();
		return list.pollFirst();
	}
   /**
    * 
    */
	@Override
	public int size() {
		// TODO Auto-generated method stub
		assert wellformed();
		return list.size();
	}
    /**
     * 
     */
	@Override
	public Object top() {
		// TODO Auto-generated method stub
		assert wellformed();
		if(num!=0)
		return list.getFirst();
		else
			return "empty stack";
	}
	
	 public void clear() {
	      list = new LinkedList<Object>();
	      num = 0;
	    }
	
	public boolean isEmpty() {return list.isEmpty();}
	protected boolean wellformed(){

		if (this.size() < 0 || this.size() > num) { 
            return false; 
        } 
        return true;
	  }
	public String Link()
	{
		LinkedList<Object> lnk = new LinkedList<Object>();
		if(num >= 1)
		{
			lnk = list;
		}
		else
		{
			return "Empty Stack";
		}
		return lnk.toString();
	}
   private LinkedList<Object> list = new LinkedList<Object>();
   private int num = 0;
}

public interface Stack 
{
    public void push(Object obj);
    public Object pop();
    public int size();
    public Object top();
    
}

import javax.swing.JApplet;
public class StackJApplet extends JApplet implements ActionListener{
	public StackJApplet() {
	}
	Toolkit kit = Toolkit.getDefaultToolkit();
	private JTextField txtEnterANumber;
	private Dimension d = kit.getScreenSize();
	private SelfDefinedStack stack = new SelfDefinedStack();
	private JPanel panel;
	private JLabel lblPleaseEnterThe, lblNewLabel ;
	private JButton btnTop,btnPush,btnPop,btnNewButton;
	private JTextArea txtrEmptyStack;
	public void init() {
		setSize(d.width/4,d.height/3);
		panel = new JPanel();
		panel.setLayout(new BorderLayout());
		panel.setLocation(d.width/2, d.height/2);
		panel.setBackground(Color.YELLOW);
		panel.setForeground(Color.WHITE);
		getContentPane().add(panel, BorderLayout.CENTER);
		panel.setLayout(new FlowLayout(FlowLayout.CENTER, 5, 5));
		lblPleaseEnterThe = new JLabel("please enter the number you want to push");
		lblPleaseEnterThe.setForeground(Color.BLUE);
		panel.add(lblPleaseEnterThe);
		txtEnterANumber = new JTextField();
		panel.add(txtEnterANumber);
		txtEnterANumber.setColumns(10);
		btnTop = new JButton("top");
		btnTop.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				TopButtonaction(e);
			}
		});
		panel.add(btnTop, BorderLayout.WEST);
		btnPush = new JButton("Push");
		btnPush.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				PushButtonaction(e);
			}
		});
		btnPop = new JButton("Pop");
		btnPop.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				PopButtonaction(e);
			}
		});
		panel.add(btnPop);
		panel.add(btnPush);
		txtrEmptyStack = new JTextArea();
		txtrEmptyStack.setText("Empty Stack");
		txtrEmptyStack.setRows(10);
		txtrEmptyStack.setLineWrap(true);
		txtrEmptyStack.setFont(new Font("Lucida Grande", Font.BOLD, 13));
		txtrEmptyStack.setEditable(false);
		txtrEmptyStack.setColumns(20);
		panel.add(txtrEmptyStack);
		
		lblNewLabel = new JLabel("<html><font size = 4 color = Blue>Welcome!");
		lblNewLabel.setForeground(Color.BLUE);
		panel.add(lblNewLabel, BorderLayout.SOUTH);
		
		btnNewButton = new JButton("Clear Stack");
		btnNewButton.addActionListener(new ActionListener() {
			public void actionPerformed(ActionEvent e) {
				ClearButtonaction(e);
			}
		});
		panel.add(btnNewButton, BorderLayout.SOUTH);

	}
	protected void ClearButtonaction(ActionEvent e) {
		// TODO Auto-generated method stub
		if(stack.isEmpty())
		{
			lblNewLabel.setText("<html><font size = 4 color = Red>Stack is already cleared");
		}
		else
		{
			stack.clear();
			lblNewLabel.setText("<html><font size = 4 color =Blue>StackCleared");
			txtrEmptyStack.setText("Empty Stack");
		}
	}
	protected void PopButtonaction(ActionEvent e) {
		// TODO Auto-generated method stub
		if(stack.isEmpty())
		{
			lblNewLabel.setText("<html><font size = 4 color = Red>Stack's Empty Can't Pop");
		}
		else
		{
			Object obj = stack.pop();
			lblNewLabel.setText("<html><font size = 4 color = Blue>"+obj.toString()+"is poped");
			newText();
		}
	}
	protected void PushButtonaction(ActionEvent e) {
		// TODO Auto-generated method stub
		if(txtEnterANumber.getText().equals(""))
		{
			lblNewLabel.setText("<html><font size = 4 color = Red>Please input a number to push");
		}
		else
		{
			stack.push(txtEnterANumber.getText());
			lblNewLabel.setText("<html> <font size = 4 color = Blue>"+txtEnterANumber.getText()+"is pushed");
			newText();
		}
		txtEnterANumber.setText("");
	}
	protected void TopButtonaction(ActionEvent e) {
		// TODO Auto-generated method stub
		if(stack.isEmpty())
		{
			lblNewLabel.setText("<html><font size = 4 color = Red>Empty Stack Can't Top");
		}
		else
		{
			stack.top();
			lblNewLabel.setText("<html><font size = 4 color = Blue>"+stack.top().toString()+"is topped");
			newText();
		}
	}
	protected void MouseClickedaction(MouseEvent e) {
		// TODO Auto-generated method stub
		if(txtEnterANumber.getText().equals(""))
		{
			txtEnterANumber.setText("Enter a number");
		}
		else
		{
			txtEnterANumber.setText("");
		}
	}
	public void newText() {
	    if (!stack.isEmpty()) {
	    	txtrEmptyStack.setText(stack.Link() + "\r\n" + "Size of Stack: " + stack.size() + 
	        "\r\nTop of Stack: " + stack.top());
	    } else {
	    	txtrEmptyStack.setText("Empty Stack");
	    }
	  }
	}

[orocsy]

引用 3 楼 boblxj0815 的回复:

楼主，当你 Run as JUnit Test时，在你的StackTest类中的Test函数是不会被调用的。 只会调用你写的test***的测试函数类。 所以，stack_test这个变量在JUnit Test 调用 test*** 函数时，会是被认为没有初始化。 所以会抛出NullPoint这个异常。 解决方案: 定义时就初始化stack_test 这个变量。 private SelfDefinedStack stack_test; 修改为： private SelfDefinedStack stack_test = new SelfDefinedStack();; 然后再运行，就没有任何问题了。 楼主的test***函数写的都没有问题，如果 楼主的unit test 自己写个main函数，调用Test()函数，并且最后Run as Java Application,而不是Run as JUnit Test，那么，也不会报NullPointer的异常，而且会正常打印出 test pass的日志。

解释的很详细，也麻烦你了这么认真看，问题果然解决了，高手谢谢

[orocsy]

同时也谢谢楼上两位

[失落夏天]

我猜最大的可能是 stack 这个对象要么已经被释放了，要么就是没有被实例化

[邵云龙]

debug跟一遍，注意下stack的引用什么时候变null了

[boblxj0815]

楼主，当你 Run as JUnit Test时，在你的StackTest类中的Test函数是不会被调用的。 只会调用你写的test***的测试函数类。 所以，stack_test这个变量在JUnit Test 调用 test*** 函数时，会是被认为没有初始化。 所以会抛出NullPoint这个异常。 解决方案: 定义时就初始化stack_test 这个变量。 private SelfDefinedStack stack_test; 修改为： private SelfDefinedStack stack_test = new SelfDefinedStack();; 然后再运行，就没有任何问题了。 楼主的test***函数写的都没有问题，如果 楼主的unit test 自己写个main函数，调用Test()函数，并且最后Run as Java Application,而不是Run as JUnit Test，那么，也不会报NullPointer的异常，而且会正常打印出 test pass的日志。