Question

正如标题所暗示的那样，我正在尝试使用JavaFx制作一个以随机方式回答是或否的程序。我使用了一点Java来实现它的随机方面。但是每次编译它都会产生异常。

这里是代码：

package decision2;
import javafx.application.*;
import javafx.stage.*;
import javafx.scene.*;
import javafx.scene.control.*;
import javafx.scene.layout.*;
import javafx.event.*;
import java.util.Random;
import javafx.geometry.*;

public class decision2 extends Application {
    TextField question;
    Label label;
    Random rnd;
    int rndnumber;

    public static void main (String [] args) {
     launch (args);
    }

    public void start (Stage myStage){
        myStage.setTitle ("Decision Maker v1.0");
        FlowPane node = new FlowPane (10,10);
        node.setAlignment (Pos.CENTER);
        Scene scene = new Scene (node,230,140);
        myStage.setScene(scene);

        question = new TextField();
        question.setPromptText("Enter your question...");
        question.setPrefColumnCount(15);
        question.setOnAction(new EventHandler <ActionEvent>() {
            public void handle (ActionEvent ae) {
                rndnumber = rnd.nextInt (2);
                //If the number is 1, it prints out yes, no otherwise.
                if (rndnumber==1) {
                    label.setText("Yes!");
                }
                else {
                    label.setText("No.");
                }
            }
        });

        Button btn = new Button ("Ask Question");
        btn.setOnAction (new EventHandler <ActionEvent> () {
            public void handle (ActionEvent ae) {
                rndnumber = rnd.nextInt (2);
                if (rndnumber==1) {
                    label.setText("Yes!");
                } else {
                    label.setText("No.");
                }
            }
        });

        Separator sp = new Separator ();
        sp.setPrefWidth (180);
        node.getChildren().addAll (question,btn,sp,label);
        myStage.show();
    }
}

这是编译输出

Executing C:\Users\EskimoOfHoth\Documents\NetBeansProjects\decision2\dist\run1520470956\decision2.jar using platform C:\Program Files\Java\jdk1.7.0_75\jre/bin/java
Exception in Application start method
java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.javafx.main.Main.launchApp(Main.java:698)
    at com.javafx.main.Main.main(Main.java:871)
Caused by: java.lang.RuntimeException: Exception in Application start method
    at com.sun.javafx.application.LauncherImpl.launchApplication1(LauncherImpl.java:403)
    at com.sun.javafx.application.LauncherImpl.access$000(LauncherImpl.java:47)
    at com.sun.javafx.application.LauncherImpl$1.run(LauncherImpl.java:115)
    at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException: Children: child node is null: parent = FlowPane@141567b[styleClass=root]
    at javafx.scene.Parent$1.onProposedChange(Parent.java:316)
    at com.sun.javafx.collections.VetoableObservableList.addAll(VetoableObservableList.java:106)
    at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:160)
    at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:309)
    at decision2.decision2.start(decision2.java:58)
    at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
    at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:219)
    at com.sun.javafx.application.PlatformImpl$4$1.run(PlatformImpl.java:182)
    at com.sun.javafx.application.PlatformImpl$4$1.run(PlatformImpl.java:179)
    at java.security.AccessController.doPrivileged(Native Method)
    at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:179)
    at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:76)
    at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
    at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:17)
    at com.sun.glass.ui.win.WinApplication$3$1.run(WinApplication.java:67)
    ... 1 more
Java Result: 1

Answer 1

请初始化Label label。当您尝试将其添加到null时（以及当您尝试为标签设置文字时），它仍为FlowLayout，

尝试制作是/否应答程序，但在编译期间会出现异常

1 个答案: