你会想到我有多少时间来回答NullPointerException。虽然它有答案，但新手程序员很难找到原因并修复它。请帮忙。
这是代码 -
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import javax.script.*;
public class JCalculator extends javax.swing.JFrame{
JButton n1,n2,n3,n4,n5,n6,n7,n8,n9,n10,n11,n12,n13,n14,n15,n16,n17,n18;
public void calc(){
JFrame frame = new JFrame("JCalculator");
frame.setExtendedState(Frame.MAXIMIZED_BOTH);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JTextField tf = new JTextField();
JPanel panel = new JPanel();
panel.setLayout(new BorderLayout());
panel.setLayout(new GridLayout(4,4,4,4));
//1.declare and instantiate buttons
n1 = new JButton("1");
n2 = new JButton("2");
n3 = new JButton("3");
n4 = new JButton("4");
n5 = new JButton("5");
n6 = new JButton("6");
n7 = new JButton("7");
n8 = new JButton("8");
n9 = new JButton("9");
n10= new JButton("0");
n11= new JButton("+");
n12= new JButton("-");
n13= new JButton("*");
n14= new JButton("/");
n15= new JButton("=");
n16= new JButton("C");
n17= new JButton(".");
n18= new JButton("Del");
//2.add to frame
panel.add(tf);
panel.add(n1);
panel.add(n2);
panel.add(n3);
panel.add(n4);
panel.add(n5);
panel.add(n6);
panel.add(n7);
panel.add(n8);
panel.add(n9);
panel.add(n10);
panel.add(n11);
panel.add(n12);
panel.add(n13);
panel.add(n14);
panel.add(n15);
panel.add(n16);
panel.add(n17);
panel.add(n18);
frame.add(panel);
//4.AddActionListener
n1.addActionListener((ActionEvent) -> {
String num1 = n1.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n2.addActionListener((ActionEvent) -> {
String num1 = n2.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n3.addActionListener((ActionEvent) -> {
String num1 = n3.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n4.addActionListener((ActionEvent) -> {
String num1 = n4.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n5.addActionListener((ActionEvent) -> {
String num1 = n5.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n6.addActionListener((ActionEvent) -> {
String num1 = n6.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n7.addActionListener((ActionEvent) -> {
String num1 = n7.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n8.addActionListener((ActionEvent) -> {
String num1 = n8.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n9.addActionListener((ActionEvent) -> {
String num1 = n9.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n10.addActionListener((ActionEvent) -> {
String num1 = n10.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n11.addActionListener((ActionEvent) -> {
String num1 = n11.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n12.addActionListener((ActionEvent) -> {
String num1 = n12.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n13.addActionListener((ActionEvent) -> {
String num1 = n13.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n14.addActionListener((ActionEvent) -> {
String num1 = n14.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n15.addActionListener((ActionEvent) -> {
String num = tf.getText();
tf.setText("");
ScriptEngineManager mgr = new ScriptEngineManager();
ScriptEngine engine = mgr.getEngineByName("JavScript");
try {
String s = engine.eval(num).toString();
tf.setText(s);
}catch (ScriptException e) {
e.printStackTrace();
}
});
n16.addActionListener((ActionEvent) -> {
String num = n16.getText();
num = null;
tf.setText(num);
});
n17.addActionListener((ActionEvent) -> {
String num1 = n17.getText();
String num = tf.getText();
num = num.concat(num1);
tf.setText(num);
});
n18.addActionListener((ActionEvent) -> {
String num = tf.getText();
String num1 = "";
for(int i = 0; i<num.length()-1; i++){
tf.setText(num1+num.charAt(i));
}
});
frame.setVisible(true);
tf.setVisible(true);
n1.setVisible(true);
n2.setVisible(true);
n3.setVisible(true);
n4.setVisible(true);
n5.setVisible(true);
n6.setVisible(true);
n7.setVisible(true);
n8.setVisible(true);
n9.setVisible(true);
n10.setVisible(true);
n11.setVisible(true);
n12.setVisible(true);
n13.setVisible(true);
n14.setVisible(true);
n15.setVisible(true);
n16.setVisible(true);
n17.setVisible(true);
}}
当我按下＆＃34; =＆＃34;按钮引发此异常 -
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at JCalculator.lambda$calc$14(JCalculator.java:147)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
at javax.swing.AbstractButton$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.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
at java.awt.EventQueue$4.run(EventQueue.java:731)
at java.awt.EventQueue$4.run(EventQueue.java:729)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$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: 29 seconds)
为什么我收到此错误？根本原因是什么？请清楚地解释一下。
答案 0 :(得分：1)
空指针异常是最容易修复的错误之一。
错误发生在
String s = engine.eval(num).toString();
表示引擎未正确启动。好像脚本名称有拼写错误。应该是 javascript
包含任何空指针异常导航到行号，按顺序检查对象，即（左 - 右），检查可能的空值，将断点放在同一行并在调试模式下运行。