array of objects not initializing properly

Question

With this code I am trying to load a file with data into an array of objects. I haven't initalized the fields within the object properly because when I run this code I get a NullPointerException. The array is there and is even the right size but the fields aren't initialized. How should I fix this?

Here is the code:

public class aJob {
  public int job;
  {
    job = 0;
  }
  public int dead;
  {
    dead = 0;
  }
  public int profit;
  {
    profit = 0;
  }
}

public class Main {
  public static void main(String[]args) throws IOException {
    File local = readLines();
    Scanner getlength = new Scanner(local);
    int lines = 0; 

    while (getlength.hasNextLine()) {
      String junk = getlength.nextLine();
      lines++;
    }
    getlength.close();

    Scanner jobfile = new Scanner(local);  // check if empty                            

    aJob list[] = new aJob[lines];
    aJob schedule[] = new aJob[lines];
    int index = 0;
    list[index].job = jobfile.nextInt();
  }

  public static File readLines() throws IOException 
  {
    try {
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    } catch (Exception e) {
      // ignore exceptions and continue
    }

    JFileChooser chooser = new JFileChooser();
    try {
      int code = chooser.showOpenDialog(null);
      if (code == JFileChooser.APPROVE_OPTION) {
        return chooser.getSelectedFile(); 
      }
    } catch (Exception f) {
      f.printStackTrace();
      System.out.println("File Error exiting now.");
      System.exit(1);
    }
    System.out.println("No file selected exiting now.");
    System.exit(0);
    return null;
  }
}

Matzi · Accepted Answer · 2012-04-29 19:07:13Z

5

Declaring an array is not enough. You must populate it with object instances.

aJob list[] = new aJob[lines];
aJob schedule[] = new aJob[lines];

for (int i = 0; i < lines; i++){ list[i] = new aJob(); schedule[i] = new aJob(); }

answered Apr 29, 2012 at 19:07

Matzi

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Peter Lawrey · Accepted Answer · 2012-04-29 19:07:13Z

4

The problem is that the elements of the array are not initialized, i.e. still null.

aJob list[] = new aJob[lines]; // creates an array with null values.
for(int i=0;i<lines;i++) list[i] = new aJob(); // creates elements.

answered Apr 29, 2012 at 19:07

Peter Lawrey

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Jason · Accepted Answer · 2012-04-29 19:27:02Z

0

Another possibility is that you can use an ArrayList or LinkedList to take the place of the array in your program.

For example,

ArrayList<aJob> list = new ArrayList<aJob>(lines);
ArrayList<aJob> schedule = new ArrayList<aJob>(lines);

int index = 0;
list.add(0, new aJob(jobFile.nextInt());

does the same work. The last line creates a new aJob object with the value pulled from the scanner object, then inserts it at location 0.

While an array is a simple structure, using a List gives you greater flexibility, especially if you don't know just how many aJob elements you'll be making. An array requires that the size be defined at instanciation, but a List has the ability to grow to handle new elements.

answered Apr 29, 2012 at 19:27

Jason

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