Question

我目前正在开发一个项目，该项目使用JFrame在GUI上运行。我想尽可能清楚地说明我的问题是什么，我正在使用3个包（实体，控制（管理），边界（GUI））。边界导入Control，Control import Entity。

在Entity包中，我得到了一个名为“Block”的形状类，其中包含：

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; 
}

在Control包中，我得到了一个名为ShapeCollection的形状类这将形状存储在ArrayList

中

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);
}
}

在控件包中我得到了另一个名为ShapeController的类：

//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);
}

在Boundary（GUI）包中我有一个名为MasterFrame的类，它必须显示由TextArea中另一个名为BlockFrame的帧创建的块。

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);
}

我在边界使用的最后一个类是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 ();
}

当您按下提交按钮时，它应该生成阻止并将其添加到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);
}

当我按下确定按钮使块成为形状时，我收到此错误：

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

导致此NullPointerException的原因是什么？

Answer 1

考虑这些方法

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无处initialized。

在构造函数

中

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

您实际上并未初始化实例成员。您正在初始化一个局部变量。

将其更改为

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

是什么导致了这个NullPointer异常？

1 个答案: