NullPointerException while setting objects to an array of null

Question

I'm getting a NullPointerException and after doing research I can't figure out why. The error message reads:

java.lang.NullPointerException
    at TextDecoder.readMessagesFromFile(TextDecoder.java:32)
    at TextDecoder.main(TextDecoder.java:54)

I've commented next to lines 54 and 32.

This is my constructor (all of these methods come from the same class):

  public TextDecoder() {
        messages = new TextMessage[10];
        msgCount = 0;
      }

My Main method is the following (I included class declaration):

public class TextDecoder {
public static void main(String[] args) throws Exception {

    if (args.length != 1) {
      System.out.println("There is an incorrect amount of command-line arguments. Command-line Argument length: " + args.length);
      System.exit(1);
    } 
    File msgFile = new File(args[0]);
    if (msgFile == null) {
      System.out.println("File not found.");
      System.exit(1);
    }
    readMessagesFromFile(msgFile); // Line 32
}
}

My readMessagesFromFile is the following:

public static void readMessagesFromFile(File inputMsgFile) throws Exception {

Scanner input = new Scanner(inputMsgFile);

 while (input.hasNextLine()) {

   /* Some code here */
// Recipient and keyPresses are defined in here and are not null

   }
   messages[msgCount] = new TextMessage(recipient, keyPresses); // Line 54
   msgCount++;
 }
 input.close();

Possible duplicate of What is a NullPointerException, and how do I fix it? — Lorelorelore
– Lorelorelore, Commented Oct 2, 2018 at 15:26
Please post a minimal reproducible example, you are leaving out essentially pieces of your code. It looks like messages is static, if so it is likely wrong to initialize it from the TextDecoder constructor; or it indicates that messages should not be static. — Mark Rotteveel
– Mark Rotteveel, Commented Oct 2, 2018 at 15:34

Andy Turner · Accepted Answer · 2018-10-02 15:25:29Z

3

public TextDecoder() {
    messages = new TextMessage[10];
    msgCount = 0;
  }

This is a constructor, which is invoked with new TextDecoder().

You don't invoke this, so messages is never initialized. (But you shouldn't be setting static fields in a constructor anyway).

Either make the readMessagesFromFile method an instance method (remove the static; and create an instance of TextDecoder on which to invoke the method); or initialize the state statically.

answered Oct 2, 2018 at 15:25

Andy Turner

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