Why am I getting a Null Pointer Exception???

[Christopher Beech]

Ok. Why does the following code give me a null pointer exception??? It's suppose to search an array for a given name. If name is found, it changes the grade to one the user gives. If name is not found, it adds the name and the grade given by the user.

 void addOrChangeRecord(String name, int grade)
  {
    // For loop to search the array.
 
    for(int i=0;i<size;i++)
    {
      int oldNumber = 0;  // Declares a variable oldNumber and makes it equal to zero.
 
      // If loop to make the array grow.
 
      if (size == directory.length);
      {
        StudentRecord[] temp = new StudentRecord[directory.length * 2];
        System.arraycopy(directory, 0, temp, 0, directory.length);
        directory = temp;
      }
 
      // If loop to check if name is already present and to change the grade if name is present.
 
      if(directory[i].getName().equals(name))
      {
        oldNumber = directory[i].getGrade();
        directory[i].setGrade(grade);
      }
      else
      {
        directory[i] = new StudentRecord(name, grade);
        size++;
      }
    }
  }   

[ March 14, 2006: Message edited by: Christopher Beech ]

[Christophe Verré]

1. What is size, what is directory, how are they initialized ?
2. Telling us where the NullPointerException happens would save some time too.

[Christopher Beech]

Directory is an array of StudentRecords.
Size is the counter variable.

Null Pointer Expcetion happens at:

   if(directory[i].getName().equals(name))
{
oldNumber = directory[i].getGrade();
directory[i].setGrade(grade);
}
else
{
directory[i] = new StudentRecord(name, grade);
size++;
} 

I think the error is in:

      else
      {
        directory[i] = new StudentRecord(name, grade);
        size++;
      }    

Being that if I comment out the above part of the code, I can change the grade.
[ March 14, 2006: Message edited by: Christopher Beech ]

[Christophe Verré]

You are making strange things

For example, using 'size' as the ending condition
for(int i=0;i<size;i++)
but incrementing it
size++;

Where do you initialize 'size' ?

[Christopher Beech]

Maybe the problem would be clearer if I post everything. Here's the StudentRecord class

public class StudentRecord
{
  private String name; //Declares a string variable named name.
  private int grade;   //Declares an int variable named grade.
 
  StudentRecord (String name, int grade)
  {
    this.name = name;   //Initializes name.
    this.grade = grade; //Initializes grade.
  }
 
  public StudentRecord ()
  { 
    grade = 0;   // Sets grade equal to 0.
    name = null; // Gives name a null value.
  }
 
  public String getName ()
  {
    return name;  //Returns name.
  }
 
  public int getGrade ()
  {
    return grade;  //Returns grade.
  }
 
  public void setGrade (int aGrade)
  {
    grade = aGrade;  //Changes grade variable to aGrade.
  }
}   

And here's the code for StudentDirectory

import java.util.*;
import java.io.*;
import java.lang.*;
 
public class StudentDirectory
{
  private StudentRecord [] directory; // Makes an array of Student Records
  private int size = 0;               // Declares an int of size and makes it 
                                      // equal to zero.
 
  StudentDirectory(String inFileName)
  {
    try
    {
      File inFile = new File(inFileName); // Tells where to find the file.
      Scanner inStream = new Scanner (inFile); // Reads data from file.
      directory = new StudentRecord [4]; //Makes an array with 20 slots for data. 
      int grades;   //Declares an int variable named grades.
      String names; //Declares an String variable called names.
 
      while (inStream.hasNext())
      {
        
        // If Loop to grow the array.
        
        if (size == directory.length);
        {
          StudentRecord[] temp = new StudentRecord[directory.length * 2];
          System.arraycopy(directory, 0, temp, 0, directory.length);
          directory = temp;
        }
 
        // Fills the array with data.
        
        names = inStream.next();
        grades = inStream.nextInt();
        directory [size] = new StudentRecord(names, grades);
        size++;
      }
    }
    catch (IOException e)
    {
      // Catches any IO Exceptions.
 
      System.out.println ("I/O ERROR");
      System.exit (1);
    }
  }
 
  int lookupRecord(String name)
  {
    // For loop to search through the array.
 
    for(int i=0;i<size;i++)
    {
      // If loop to check name and print out grade.
 
      if(directory[i].getName().equals(name))
      {
        return (directory[i].getGrade ());
      }
    }
    return -1; //Return -1 if the name is not found
  }
 
  void addOrChangeRecord(String name, int grade)
  {
    // For loop to search the array.
 
    for(int i=0;i<size;i++)
    {
      int oldNumber = 0;  // Declares a variable oldNumber and makes it equal to zero.
 
      // If loop to make the array grow.
 
      if (size == directory.length);
      {
        StudentRecord[] temp = new StudentRecord[directory.length * 2];
        System.arraycopy(directory, 0, temp, 0, directory.length);
        directory = temp;
      }
 
      // If loop to check if name is already present and to change the grade if name is present.
 
      if(directory[i].getName().equals(name))
      {
        oldNumber = directory[i].getGrade();
        directory[i].setGrade(grade);
      }
      else
      {
        directory[i] = new StudentRecord(name, grade);
        size++;
      }
    }
  }
 
  void removeRecord(String name)
  {
    // For loop to search the array.
 
    for(int i=0;i<size;i++)
    {
      // if lop to remove the entry.
 
      if(directory[i].getName().equals(name))
      {
        directory [i] = directory[size -1];
        size--;
      }
    }
  }
 
  void display ()
  {
    for (int i = 0; i < size; i++)
    {
      // For loop to print out array to screen.
      System.out.print(directory[i].getName() + ' ');
      System.out.println(directory[i].getGrade());
    }
  }
 
  void save(String outFileName)
  {
    try
    {
      // Creates a new PrintWriter to save file.
 
      PrintWriter outfile = new PrintWriter ( new FileWriter (outFileName));
 
      for (int i = 0; i < size; i++)
      {
        // For loop to save data to file.
 
        outfile.print(directory[i].getName() + ' ');
        outfile.println(directory[i].getGrade());
      }
      outfile.close (); // Closes file.
    }
    catch (IOException e)
    {
      // Catches any IO Exceptions.
 
      System.out.println ("I/O ERROR");
      System.exit (1);
    }
  }
}  

[Mark Van Tuyl]

This looks kind of odd to me

if(directory[i].getName().equals(name))
{
    oldNumber = directory[i].getGrade();
    directory[i].setGrade(grade);
}
else
{
    directory[i] = new StudentRecord(name, grade);
    size++;
}

The if statement attempts to retrieve a StudentRecord object from the array. It then attempts to compare the name in the object to the name provided. If they're not equal, it creates a StudentRecord object at that position in the array.

Why not make sure the object exists before trying to call a method on it?

if(directory[i] != null && directory[i].getName().equals(name))

[Mark Van Tuyl]

Here's an example of what I'm talking about

public class Test {
private void process() {
//Creates an array of references to MenuItem objects
//The objects themselves don't exist yet
MenuItem itemArray[] = new MenuItem[42];
 
//Since no instance of the MenuItem class has yet been
//created, this statement will result in a NullPointerException
//System.out.println(itemArray[0].getName());
 
itemArray[0] = new MenuItem("Spam");
itemArray[1] = new MenuItem("Beans");
 
//Now they've been initialized, so they can be called
System.out.println("I'll have " + itemArray[0].getName() + ", " +
itemArray[0].getName() + ", " + itemArray[0].getName() + ", " +
itemArray[0].getName() + ", " + itemArray[1].getName() + " and " +
itemArray[0].getName());
}
 
  public static void main(String [] args) {
new Test().process();
   }
}
 
class MenuItem {
public MenuItem(String name) {
this.name = name;
}
 
public String getName() {
return name;
}
 
private String name = null;
}

[Christopher Beech]

Replacing the

if(directory[i].getName().equals(name))  

with

if(directory[i] != null && directory[i].getName().equals(name))

still does not add a new record and it now takes twice as long to change the grade of an existing record.

[Joel McNary]

If you take a look at the stack trace, it will tell you exactly what line the NullPointerException happens on. Once you know that, you can take a look at the offending line and ask yourself, "what can be null here?"

If you post exactly what the offending line is, we will probably be better able to help.

[Tony Morris]

How to resolve a java.lang.NullPointerException

[Christopher Beech]

Ok. Using my original code, here's the error message I get.

NullPointerException:
at StudentDirectory.addOrChangeRecord(StudentDirectory.java:118)
at StudentDirTester.main(StudentDirTester.java:102)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)

At line 118 of the StudentDirectory:

if(directory[i].getName().equals(name))
      {
        oldNumber = directory[i].getGrade();
        directory[i].setGrade(grade);
      }
      else
      {
        directory[i] = new StudentRecord(name, grade);
        size++;

line 102 of the StudentDirTester is:

sDir.addOrChangeRecord(sName, sNum);

However, i'm 100% certain that there is no errors in the StudentDirTester, being that it was typed up by the graduate students and nobody else had any trouble.

[Jim Yingst]

[Christopher]: At line 118 of the StudentDirectory:

This is the only line we really need to know here. But you have shown us 9 lines for some reason.

Which one line (one single, solitary line, more than zero lines but less than two lines is line number 118? That is the line you need to look at.
[ March 14, 2006: Message edited by: Jim Yingst ]

[Christopher Beech]

The code at line 118 is the if(directory[i].getName().equals(name)) code. I can't understand why the compiler doesn't like the code, through. The code seems logical enough to me.

[Jim Yingst]

One of the things being referenced in that line is null. It's one of these:

directory is null

directory[i] is null

getName() is null

From your other code, it seems directory is not null, your the earlier reference to directory.length would have thrown an NPE. I recommend printing out each of these things to find out what their values are:

System.out.println("directory = " + directory);
System.out.println("directory[i] = " + directory[i]);
System.out.println("directory[i].getName() = " + directory[i].getName());

When you know what the cause really is, then you can better decide what to do about it.

[Christopher Beech]

My guess is that directory[i] is null. By the time the complier gets to the AddorChange method, directory is already filled with an array of StudentRecords. And I believe that getName() would be the name that the users gives.

[Jim Yingst]

[Christopher]: My guess is that directory[ i ] is null.

But why guess? We have to guess, because we have incomplete information. You, however, have the complete source code, and are running this on your own machine, correct? You have the ability to modify the code and find out what is going on. I just suggested a way that you could find out. Insert those print statements into the code, just before line 118. Then you will easily be able to know, for sure, which element of the code is null.
[ March 14, 2006: Message edited by: Jim Yingst ]