why it shows NullPointerException?

Question

I have an object which is Student student; which has some fields like name,family,... I want to make a user name for this student and by clicking on the one button ,I want to show the student's user name on the console but it will show these sentences on the console(I have copied and pasted just a part of that)----->

java.lang.NullPointerException at registration.Students.nextUserName(Students.java:149) at ui.AddStudent.submit(AddStudent.java:254) at ui.AddStudent.jButton1ActionPerformed(AddStudent.java:186) at ui.AddStudent.access$000(AddStudent.java:24) at ui.AddStudent$1.actionPerformed(AddStudent.java:76) at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995) at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318) at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387) at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236) at java.awt.Component.processMouseEvent(Component.java:6038) at javax.swing.JComponent.processMouseEvent(JComponent.java:3265) . . .

a part of my student class which also extends Human:

    public String nextUserName() {
    StringBuffer user = new StringBuffer();
    SystemManagement students = new SystemManagement();
    String name1 = students.getName();
    char char1 = name1.charAt(0);
    String family1 = students.getFamily();
    char char2 = family1.charAt(1);
    setUserName(name1 + "." + family1);
    if (user.length() != 0 && user.toString().equals(getUserName())) {
        char[] allowedCharacters1 = {char1, char2, '1', '.'};
        SecureRandom random = new SecureRandom();

        for (int i = 0; i < user.length(); i++) {
            user.append(allowedCharacters1[random.nextInt(allowedCharacters1.length)]);
        }
        setUserName(user.toString());
    }

    return getUserName();

}

public String currentUserName() {
    return getUserName();
}

public static Students getInstance() {
    if (instance == null) {
        instance = new Students();
    }
    return instance;
}

my Human class:

 private String userName;

 public Human(String userName) {

    this.userName = Students.getInstance().nextUserName();
}

public void setUserName(String userName) {

    this.userName = userName;
}

public String getUserName() {
    return userName;
}

Thanks I have changed my netUserName() method to this but it return null instead of exception.

public String nextUserName() {
  StringBuffer user = new StringBuffer();
  SystemManagement students = new SystemManagement();
  String name1 = students.getName();
  String family1 = students.getFamily();
  if(name1 !=null && family1 !=null ) {
    char char1 = name1.charAt(0);  
    char char2 = family1.charAt(1);
    setUserName(name1 + "." + family1);
    if (user.length() != 0 && user.toString().equals(getUserName())) {
      char[] allowedCharacters1 = {char1, char2, '1', '.'};
      SecureRandom random = new SecureRandom();
      for (int i = 0; i < user.length(); i++) {
        user.append(allowedCharacters1[random.nextInt(allowedCharacters1.length)]);
      }
      setUserName(user.toString());
    }
  }
  return getUserName();
}

Answer 1

When creating a new SystemManagment what does it return exactly?

The null is on line 3 or 4 of the nextUserName method, check if !=null before starting.

Answer 2

The exception message shows that somewhere on line 149 of Students.java (which is somewhere in the nextUserName() method) you're trying to access a property or method of a null object. For example, if students.getName(); returns null, then you'll get this exception on the next line when you try to call chatAt() on it.

Without the source of SystemManagement it's hard to go into more detail, but if you find which line is 149 you should be able to trace back where the null comes from.

Answer 3

It shows NullPointerException because you have a null pointer on line 149. I don't know which one is line 149, so let's consider the options:

StringBuffer user = new StringBuffer();

Can't be that line. It's creating a new object.

SystemManagement students = new SystemManagement();

Can't be that line, either, for the same reason. (Although I'm puzzled what a system management is, and why "students" would be a good name for such an object.)

String name1 = students.getName();

Can't be that line. We already know students can't be null (or else the previous line would have thrown a different exception without allowing execution to reach this line at all).

char char1 = name1.charAt(0);

Maybe that one. Use your debugger to inspect the value of name1. Is it null? What name do you expect to get from a brand new system management?

String family1 = students.getFamily();

Can't be that one.

char char2 = family1.charAt(1);

Maybe that one. What family do you expect from a brand new system management?

setUserName(name1 + "." + family1);

Can't be this one. If either of those two string variables were null, you'd have gotten the exception earlier, when you called charAt on them.

Everything after that line either uses the two string objects or it uses newly created objects, so we can eliminate the rest of the method from consideration. Focus your attention on the two lines I pointed out (or on the one line that you know to be line 149).

---

You've changed your code to check that name1 and family1 aren't null before you use them, but I don't think you've really considered the underlying problem. What is a new SystemManagement supposed to give you for user name and family name? Will they ever be anything other than null?

Obviously, one or both of those values is null, so you skip over everything else in the method up to the last line, where you call getUserName. Since you've skipped the call to setUserName, what value is getUserName supposed to return instead? (If you don't know, go find out.)

When you ask "Why does my program do X?" answer with another question: "Why shouldn't it do X?" You're the one writing the code, so the program is ultimately doing exactly what you told it to do. If it's not doing what you expected, then you gave it wrong instructions. Go through your code, one line at a time if you have to, and figure out where things went wrong. Make sure you understand exactly what each line of your code will do. If you have a line and you don't understand it, go read the documentation, ask your instructor, or ask here. Walk through the execution on paper, keeping track of each variable's value and crossing out old values as you re-assign them. If you have multiple instances of a class, make sure you keep track of which instance you're working with at any given time.

(Someday, you might hit a problem that's due to external factors. A buggy compiler, bad RAM, poorly implemented specifications, etc. Today is not that day. If you suspect any of those, make sure you've first thoroughly ruled out any possibility of it being your own mistake instead.)