java.lang.NullPointerException

Question

public boolean test(Example testExample)
    {
// calculate posterior probs 后验概率
double [] posteriorProbs = calculateProbs(testExample);
// predicted class
int predictedClass = argMax(posteriorProbs);
        int freqnum[]=new int[numCategories];//计算词频
        System.out.print("Document: " + testExample.name + "\nResults: ");
        
                 StringBuffer src=null;
                 File f=testExample.getDocument().file;
                try{
                BufferedReader in=new BufferedReader(new FileReader(f));
                String str=new String();
                while((str=in.readLine())!=null)
                  src=src.append(str);////报错处 !!!
                  in.close();
                }
                catch(IOException e){
                  e.printStackTrace();
                }    for (int j=0; j<numCategories; j++)
         {
             System.out.print(cat[j] + "(" + posteriorProbs[j] + ")    ");
                  
             freqnum[j]=StringUtils.countMatches(src.toString(),cat[j]);
                  System.out.println("frequent num="+freqnum[j]);
            }
             System.out.println("Predicted class: " +cat[predictedClass]  + "\n");
             //System.out.println("Predicted class: " +cat[predictedClass]  + "\n");
        
if (debug) {
    System.out.print("Document: " + testExample.name + "\nResults: ");
    for (int j=0; j<numCategories; j++) {
  System.out.print(cat[j] + "(" + posteriorProbs[j] + ")\t");
    }
    System.out.println("\nCorrect class: " + testExample.getCategory() + ", Predicted class: " + predictedClass  + "\n");
}
return (predictedClass == testExample.getCategory());    }
**************************************************************
报错 :java.lang.NullPointerException at ir.classifiers.NaiveBayes.test(NaiveBayes.java:131) at ir.classifiers.TestNaiveBayes.main(TestNaiveBayes.java:34)如下:   while(i<examples.size()){
      Example e =(Example)examples.get(i);
      BC.test(e);//BC是NaiveBayes类
      i++;
      }
大虾们,帮帮忙

Answer 1

StringBuffer src=null;
这里错了 src没有实例化,它是null 因此下面的src也就无法append!

Answer 2

while((str=in.readLine())!=null)
                  src=src.append(str);////报错处 !!!
                  in.close();
                }
in.close()应该放在循环外面，
in.close后，in.readLine()就会报错

Answer 3

我这个程序也报这个错误不知道怎么回事,请个位高手为小弟指点一下吧,
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.*; 
class myButton extends JButton
{
int ;
myButton()
{
=(int)(Math.floor(Math.random()+0.3)); }
}
class shengchengmigong extends JPanel
{
static myButton []mybutton;
shengchengmigong()
{
this.setLayout(new GridLayout(10,10,0,0));
myButton []mybutton=new myButton[144];
for(int i=0;i<=99;i++)
{
mybutton[i]=new myButton();
}
int i=0;
while(i<=99)
{
if(i==88||i==11)
{}
else if(i<=9||i>=90)
{mybutton[i].setBackground(Color.black);}
else if(i%10==0)
{mybutton[i].setBackground(Color.black);}
else if((i+1)%10==0)
{mybutton[i].setBackground(Color.black);}
else if(mybutton[i].==1)
{mybutton[i].setBackground(Color.black);}

else
{}
this.add(mybutton[i]);
i++;

}

}
}
class mymigong  implements ActionListener
{
JButton sc;
JButton qj;
JFrame f;
shengchengmigong mymigong;
mymigong()
{
sc=new JButton("生成迷宫");
qj=new JButton("求解");

sc.addActionListener(this);
qj.addActionListener(this);
JPanel jp=new JPanel();
jp.setLayout(new FlowLayout(FlowLayout.CENTER));
jp.add(sc); jp.add(qj);
f=new JFrame("小小迷宫");
f.setBounds(0,0,500,550);
f.setLayout(new BorderLayout());
shengchengmigong mymigong=new shengchengmigong();
f.add(mymigong,BorderLayout.CENTER);
f.add(jp,BorderLayout.NORTH);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.setVisible(true);

}
public void actionPerformed(ActionEvent e)
{
if(e.getSource()==sc)
{
mymigong=new shengchengmigong();

f.add(mymigong,BorderLayout.CENTER);
}
else if(e.getSource()==qj)
{

ArrayList<myButton> arraylist=new ArrayList<myButton>(1);
try{
if(arraylist.add(shengchengmigong.mybutton[11]));System.out.println("true");

shengchengmigong.mybutton[11].setBackground(Color.red);
}
catch(Exception ee)
{System.out.println(ee.toString());} }
}



}public class migong
{
public static void main(String args[])
{
new mymigong();
}
}