为什么我会在这一行收到 NullPointerException？

Question

虽然我给出了值，但我得到了 NullPointerException。我有一个名为 model1 的 DefaultTableModel。我想将第 4 列和第 5 列的值相乘并将其添加到第 6 列。我做了三行。前两行 4,5 处的值在第 6 列上相乘并相加，但第三行的值未显示在该列上并生成异常。

这是错误块-

for(int i = 0; i <= model1.getRowCount();i++){
    Double d = Double.parseDouble((String) model1.getValueAt(i, 4));
    Double d2 = Double.parseDouble((String) model1.getValueAt(i, 5)); //exception
    Double d3 = d * d2;
    model1.setValueAt(d3, i, 6);
}

这里是例外-

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1838)
    at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110)
    at java.lang.Double.parseDouble(Double.java:538)
    at Busy.saver(Busy.java:246)
    at Busy.lambda$inventory(Busy.java:233)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6533)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6298)
    at java.awt.Container.processEvent(Container.java:2237)
    at java.awt.Component.dispatchEventImpl(Component.java:4889)
    at java.awt.Container.dispatchEventImpl(Container.java:2295)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4889)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4526)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4467)
    at java.awt.Container.dispatchEventImpl(Container.java:2281)
    at java.awt.Window.dispatchEventImpl(Window.java:2746)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    at java.awt.EventQueue.access0(EventQueue.java:97)
    at java.awt.EventQueue.run(EventQueue.java:709)
    at java.awt.EventQueue.run(EventQueue.java:703)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
    at java.awt.EventQueue.run(EventQueue.java:731)
    at java.awt.EventQueue.run(EventQueue.java:729)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)
    BUILD SUCCESSFUL (total time: 48 seconds)

为什么我会收到此异常？

这是 MCVE-

import java.awt.*;
import java.io.Serializable;
import javax.swing.*;
import javax.swing.table.*;
public class Mcve extends JPanel{
    String data[][] ={{""}};
    String row[] = {"#","Item Name","HSN/SAC","Description","Qty.","Rate","Price"};
    DefaultTableModel model = new DefaultTableModel(data, row);
    JTable table = new JTable(model);
    JScrollPane pane = new JScrollPane(table);

    public void mainPage() {
        Mcve obj = new Mcve();
        JFrame frame = new JFrame("MCVE");
        frame.setExtendedState(JFrame.MAXIMIZED_BOTH);
        frame.setDefaultCloseOperation(frame.EXIT_ON_CLOSE);
        JInternalFrame iframe = new JInternalFrame();
        JButton save , add;
        save = new JButton("Save");
        add = new JButton("Add Item");
        save.setSize(75,40);
        add.setSize(85,40);
        iframe.setTitle("Inventory");
        iframe.add(pane);
        iframe.add(save, BorderLayout.WEST);
        iframe.add(add, BorderLayout.SOUTH);
        frame.add(iframe);
        iframe.setClosable(true);
        save.setVisible(true);
        add.setVisible(true);
        iframe.setVisible(true);
        frame.setVisible(true);
        save.addActionListener((ActionEvent) ->{
            saver();
        });
        add.addActionListener((ActionEvent) -> {
            rowAdder();
        });
    }
    public void saver(){
        for(int i = 0; i<=model.getRowCount();i++){
            Double d= Double.parseDouble((String) model.getValueAt(i,4));
            Double d2= Double.parseDouble((String) model.getValueAt(i,5));
            Double d3=d*d2;
            model.setValueAt(d3,i,6);
        }
    }
    public void rowAdder(){
        String row[][] = {};
        model.addRow(row);
    }
    public static void main(String[] args) {
        Mcve obj = new Mcve();
        obj.mainPage();
    }
}

Answer 1

列是零索引的。所以第 4 列和第 5 列是 索引 3 和 4。

如果您的第六列是结果的占位符，则它可能是空的，因此错误地调用 model1.getValueAt(i, 5) 会返回 null，因为该单元格是空的。

Double d = Double.parseDouble((String) model1.getValueAt(i, 3));
Double d2 = Double.parseDouble((String) model1.getValueAt(i, 4));
Double d3 = d * d2;
model1.setValueAt(d3, i, 5);

Answer 2

仅使用 Stacktrace 和文档：

我们可以看到这是Double.parseDouble的调用抛出了异常

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at
sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1838)
at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110) at
java.lang.Double.parseDouble(Double.java:538) at
Busy.saver(Busy.java:246)

Double.parseDouble 的文档告诉你：

Throws:
- NullPointerException - if the string is null

因此您将 null 值传递给 Double.parseString。

我的原因是当你点击按钮保存时你仍然处于 "edit" 模式，你不会提交值所以单元格仍然被模型视为空（null） .你可以在saver方法的开头强制版本结束。

TableCellEditor editor = table.getCellEditor();
if (editor != null) {
    editor.stopCellEditing();
}

来自 Guillaume answer.

当然，如果单元格不是数值（空或非数字），这不会阻止异常。所以解决方案是捕获每一行的异常。

for (int i = 0; i <= model.getRowCount(); i++) {
    try{
        Double d = Double.parseDouble((String) model.getValueAt(i, 4));
        Double d2 = Double.parseDouble((String) model.getValueAt(i, 5));
        Double d3 = d * d2;
        model.setValueAt(d3, i, 6);
    } catch (NullPointerException | NumberFormatException e){
        model.setValueAt("N/A", i, 6);
    }
}

然后，您可以检查以更正您将从您正在使用的循环中获得的 java.lang.ArrayIndexOutOfBoundsException。

Answer 3

for(int i = 0; i<=model1.getRowCount();i++)你循环一次太多了。你需要去 i<model1.getRowCount())

正如 AxelH 在他的评论中提到的，这不会解决您的 NPE 问题。但这仍然是您的代码中的一个错误，一旦您解决了 NPE，就会导致问题。

为什么我会在这一行收到 NullPointerException？

Why am I getting NullPointerException at this line?

java

swing

jtable

nullpointerexception