What's causing this NullPointer Exception?

Question

I'm currently working on a project, that is runned on a GUI by using a JFrame. I want to make as clear as possible what my problem is, i'm working in 3 packages (Entity, Control(Manager), Boundary(GUI)). Boundary imports Control, Control imports Entity.

In the Entity package i got a shape class called "Block" that contains:

public class Block extends Shape 
//Shape is the parent of the class Block and a few other shapes
{
private int length, width, height;

public Block(int length, int width, int height){

    this.length= length;
    this.width= width;
    this.height= height; 
}

In the Control package i got a shape class called ShapeCollection This stores the shape in an ArrayList

import Entity.*;
import java.util.ArrayList;

public class ShapeCollection{

private ArrayList<Shape> shapecollection;


public ShapeCollection(){
    shapecollection= new ArrayList<Shape>();
}

public void addShape(Shape newShape){
    shapecollection.add(newShape);
}
}

In the control packages i got another class called ShapeController:

//This Controller is making the Shape 

import Entity.*;

public class ShapeController{
private ShapeCollection shapecollection;

public ShapeController(){
    ShapeCollection shapecollection= new ShapeCollection ();
}

public void makeBlock(double length, double width, double height)
{
    Shape shape = new Block( length,  width, height);
    shapecollection.addShape(shape);
}

In the Boundary(GUI) package i got a class called MasterFrame, this has to show a block created by a other frame called BlockFrame in a TextArea.

import Control.*;
public class MasterFrame extends javax.swing.JFrame {

ShapeController shapecontroller;

public MasterFrame () {
    initComponents();
    vormComboBox.removeAllItems();
    vormComboBox.addItem("Select a shape");
    vormComboBox.addItem("Block");
     shapecontroller = new ShapeController ();
 // Here you get referred to a other GUI to make a shape in there, now i use BlockFrame
 // The code behind this works
}

 public static MasterFrame getInstance() {
    return MasterFrameHolder.INSTANCE;
}

private static class MasterFrameHolder{
    private static final MasterFrame INSTANCE = new MasterFrame();

} 

public void addTextBlock(double length, double width, double height) {

    vormcontrole.makeBlock(length, width, height);
    InfoShapeTextArea.setText("Block" + " "+ length + " " + width + "" + height);
}

Last class i use in boundary is BlockFrame:

import Boundary.ShapeController;

public class BlockFrame extends javax.swing.JFrame {

   ShapeController shapecontroller;
   private double length;
   private double width;
   private double height;

public BlockFrame() {
    initComponents();

   shapecontroller= new ShapeController ();
}

When you press submit button it should make the Block and add it to the TextArea:

// The BlockFrame contains 3 textfields to store the doubles in and a submit button

private void BlokOkButtonActionPerformed(java.awt.event.ActionEvent evt) {                                             
    MasterFrame h = MasterFrame.getInstance();


    length = Double.parseDouble(BlockLengthTextField.getText());
    width = Double.parseDouble(BlockWidthTextField.getText());
    height = Double.parseDouble(BlokHeightTextField.getText());


    shapecontroller.makeBlock(length, width, height);
    h.addTextBlock(length, width, height);
}   

When i press the OK button to make a Block as shape i get this error:

"Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException"
at Control.ShapeController.makeBlock(VormControle.java:25)
at Boundary.BlockFrame.BlokOkButtonActionPerformed(BlockFrame.java:145)   

What is causing this NullPointerException?

Answer 1

Consider these methods

private ShapeCollection shapecollection;

public ShapeController(){
    ShapeCollection shapecollection= new ShapeCollection ();
}

public void makeBlock(double length, double width, double height)
{
    Shape shape = new Block( length,  width, height);
    shapecollection.addShape(shape);
}

shapecollection is nowhere initialized.

in the constructor

public ShapeController(){
    ShapeCollection shapecollection= new ShapeCollection ();
}

You are not actually initializing the instance member. You are initializing a local variable.

Change it to

 public ShapeController(){
          shapecollection= new ShapeCollection ();
    }