Java 我没有错误，但为什么我的应用程序一直关闭

基于代码，我没有问题，但为什么我的应用程序在运行时一直关闭 我是java的初学者 我正在为我的论文做一个测验应用程序 ps我只复制youtube上的代码并编辑它 请帮助我打开我的应用程序

MainActivity.java

package com.example.quizexam;

import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;

import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    Button answer1, answer2, answer3;

    TextView score, question;
    private Question nQuestion = new Question();
    private String nAnswer;
    private int nScore = 0;
    private int nQuestionsLength = nQuestion.nQuestions.length;
    Random r;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        answer1 = (Button) findViewById(R.id.answer1);
        answer2 = (Button) findViewById(R.id.answer2);
        answer3 = (Button) findViewById(R.id.answer3);

        score = (TextView) findViewById(R.id.score);
        question = (TextView) findViewById(R.id.question);

        updateQuestion(r.nextInt(nQuestionsLength));

        answer1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(answer1.getText() == nAnswer){
                    nScore++;
                    score.setText("Score: " + nScore);
                    updateQuestion(r.nextInt(nQuestionsLength));
                } else {
                    gameOver();
                }
            }
        });

        answer2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(answer2.getText() == nAnswer){
                    nScore++;
                    score.setText("Score: " + nScore);
                    updateQuestion(r.nextInt(nQuestionsLength));
                } else {
                    gameOver();
                }

            }
        });

        answer3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if(answer3.getText() == nAnswer){
                    nScore++;
                    score.setText("Score: " + nScore);
                    updateQuestion(r.nextInt(nQuestionsLength));
                } else {
                    gameOver();
                }

            }
        });
    }

    private void updateQuestion(int num) {
        question.setText(nQuestion.getQuestion(num));
        answer1.setText(nQuestion.getChoice1(num));
        answer2.setText(nQuestion.getChoice2(num));
        answer3.setText(nQuestion.getChoice3(num));

        nAnswer = nQuestion.getCorrectAnswer(num);
    }
    private void gameOver(){
        AlertDialog.Builder alertDialogBuilder = new AlertDialog.Builder(MainActivity.this);
        alertDialogBuilder
                .setMessage("Game Over! Your score is " + nScore + " points.")
                .setCancelable(false)
                .setPositiveButton("NEW GAME",
                        new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {
                              startActivity(new Intent(getApplicationContext(),MainActivity.class));
                            }
                        })
                .setNegativeButton("EXIT",
                        new DialogInterface.OnClickListener() {
                            @Override
                            public void onClick(DialogInterface dialogInterface, int i) {
                            finish();
                            }
                        });
        AlertDialog alertDialog = alertDialogBuilder.create();
        alertDialog.show();

    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="16dp"
    android:paddingBottom="16dp"
    android:paddingRight="16dp"
    android:paddingLeft="16dp"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/answer3"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_marginBottom="-24dp"
        android:text="Button" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button"
        android:layout_above="@+id/answer3"
        android:layout_centerHorizontal="true"
        android:id="@+id/answer2"/>

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Button"
        android:layout_above="@+id/answer2"
        android:layout_centerHorizontal="true"
        android:id="@+id/answer1"/>

    <TextView
        android:text="TextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:textSize="24dp"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:id="@+id/score"/>

    <TextView
        android:text="TextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/score"
        android:gravity="center"
        android:textSize="24dp"
        android:layout_centerHorizontal="true"
        android:layout_above="@+id/answer1"
        android:id="@+id/question"/>


</RelativeLayout>

Logcat ps不完整

03/08 11:14:07: Launching 'app' on vivo vivo 1811.
$ adb shell am start -n "com.example.quizexam/com.example.quizexam.MainActivity" -a android.intent.action.MAIN -c android.intent.category.LAUNCHER -D
Waiting for application to come online: com.example.quizexam | com.example.quizexam.test
Waiting for application to come online: com.example.quizexam | com.example.quizexam.test
Connecting to com.example.quizexam
Capturing and displaying logcat messages from application. This behavior can be disabled in the "Logcat output" section of the "Debugger" settings page.
W/ActivityThread: Application com.example.quizexam is waiting for the debugger on port 8100...
I/System.out: Sending WAIT chunk
I/zygote64: Debugger is active
I/System.out: Debugger has connected
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
Connected to the target VM, address: 'localhost:8604', transport: 'socket'
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: waiting for debugger to settle...
I/System.out: debugger has settled (1466)
I/FtBuild: getRomVersion mRomVer=4.5
D/PhoneWindow: DEBUG_ALIENSCREEN:getRotation mRotation=0
I/DecorView: setWindowBackground mBackgroundPadding = Rect(0, 0 - 0, 0), mFramePadding = Rect(0, 0 - 0, 0), pkg = com.example.quizexam
I/zygote64: Rejecting re-init on previously-failed class java.lang.Class<androidx.core.view.ViewCompat$OnUnhandledKeyEventListenerWrapper>: java.lang.NoClassDefFoundError: Failed resolution of: Landroid/view/View$OnUnhandledKeyEventListener;
I/zygote64:     at void androidx.core.view.ViewCompat.setBackground(android.view.View, android.graphics.drawable.Drawable) (ViewCompat.java:2341)
        at void androidx.appcompat.widget.ActionBarContainer.<init>(android.content.Context, android.util.AttributeSet) (ActionBarContainer.java:62)
        at java.lang.Object java.lang.reflect.Constructor.newInstance0(java.lang.Object[]) (Constructor.java:-2)
        at java.lang.Object java.lang.reflect.Constructor.newInstance(java.lang.Object[]) (Constructor.java:334)
        at android.view.View android.view.LayoutInflater.createView(java.lang.String, java.lang.String, android.util.AttributeSet) (LayoutInflater.java:656)
        at android.view.View android.view.LayoutInflater.createViewFromTag(android.view.View, java.lang.String, android.content.Context, android.util.AttributeSet, boolean) (LayoutInflater.java:835)
        at android.view.View android.view.LayoutInflater.createViewFromTag(android.view.View, java.lang.String, android.content.Context, android.util.AttributeSet) (LayoutInflater.java:775)
        at void android.view.LayoutInflater.rInflate(org.xmlpull.v1.XmlPullParser, android.view.View, android.content.Context, android.util.AttributeSet, boolean) (LayoutInflater.java:908)
        at void android.view.LayoutInflater.rInflateChildren(org.xmlpull.v1.XmlPullParser, android.view.View, android.util.AttributeSet, boolean) (LayoutInflater.java:869)
        at android.view.View android.view.LayoutInflater.inflate(org.xmlpull.v1.XmlPullParser, android.view.ViewGroup, boolean) (LayoutInflater.java:521)
        at android.view.View android.view.LayoutInflater.inflate(int, android.view.ViewGroup, boolean) (LayoutInflater.java:429)
        at android.view.View android.view.LayoutInflater.inflate(int, android.view.ViewGroup) (LayoutInflater.java:380)
        at android.view.ViewGroup androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor() (AppCompatDelegateImpl.java:607)
        at void androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor() (AppCompatDelegateImpl.java:518)
        at void androidx.appcompat.app.AppCompatDelegateImpl.setContentView(int) (AppCompatDelegateImpl.java:466)
        at void androidx.appcompat.app.AppCompatActivity.setContentView(int) (AppCompatActivity.java:140)
        at void com.example.quizexam.MainActivity.onCreate(android.os.Bundle) (MainActivity.java:28)
        at void android.app.Activity.performCreate(android.os.Bundle, android.os.PersistableBundle) (Activity.java:7166)
        at void android.app.Activity.performCreate(android.os.Bundle) (Activity.java:7157)
        at void android.app.Instrumentation.callActivityOnCreate(android.app.Activity, android.os.Bundle) (Instrumentation.java:1220)
        at android.app.Activity android.app.ActivityThread.performLaunchActivity(android.app.ActivityThread$ActivityClientRecord, android.content.Intent) (ActivityThread.java:2854)
        at void android.app.ActivityThread.handleLaunchActivity(android.app.ActivityThread$ActivityClientRecord, android.content.Intent, java.lang.String) (ActivityThread.java:2979)
        at void android.app.ActivityThread.-wrap11(android.app.ActivityThread, android.app.ActivityThread$ActivityClientRecord, android.content.Intent, java.lang.String) (ActivityThread.java:-1)
        at void android.app.ActivityThread$H.handleMessage(android.os.Message) (ActivityThread.java:1683)
        at void android.os.Handler.dispatchMessage(android.os.Message) (Handler.java:106)
        at void android.os.Looper.loop() (Looper.java:192)
        at void android.app.ActivityThread.main(java.lang.String[]) (ActivityThread.java:6754)
        at java.lang.Object java.lang.reflect.Method.invoke(java.lang.Object, java.lang.Object[]) (Method.java:-2)
        at void com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run() (RuntimeInit.java:549)
        at void com.android.internal.os.ZygoteInit.main(java.lang.String[]) (ZygoteInit.java:828)
    Caused by: java.lang.ClassNotFoundException: Didn't find class "android.view.View$OnUnhandledKeyEventListener" on path: DexPathList[[zip file "/data/app/com.example.quizexam-WK3UcOpinvlhDThe14Llpw==/base.apk"],nativeLibraryDirectories=[/data/app/com.example.quizexam-WK3UcOpinvlhDThe14Llpw==/lib/arm64, /system/lib64, /vendor/lib64]]
        at java.lang.Class dalvik.system.BaseDexClassLoader.findClass(java.lang.String) (BaseDexClassLoader.java:125)
        at java.lang.Class java.lang.ClassLoader.loadClass(java.lang.String, boolean) (ClassLoader.java:379)
        at java.lang.Class java.lang.ClassLoader.loadClass(java.lang.String) (ClassLoader.java:312)
        at void androidx.core.view.ViewCompat.setBackground(android.view.View, android.graphics.drawable.Drawable) (ViewCompat.java:2341)
        at void androidx.appcompat.widget.ActionBarContainer.<init>(android.content.Context, android.util.AttributeSet) (ActionBarContainer.java:62)
        at java.lang.Object java.lang.reflect.Constructor.newInstance0(java.lang.Object[]) (Constructor.java:-2)
        at java.lang.Object java.lang.reflect.Constructor.newInstance(java.lang.Object[]) (Constructor.java:334)
        at android.view.View android.view.LayoutInflater.createView(java.lang.String, java.lang.String, android.util.AttributeSet) (LayoutInflater.java:656)
        at android.view.View android.view.LayoutInflater.createViewFromTag(android.view.View, java.lang.String, android.content.Context, android.util.AttributeSet, boolean) (LayoutInflater.java:835)
        at android.view.View android.view.LayoutInflater.createViewFromTag(android.view.View, java.lang.String, android.content.Context, android.util.AttributeSet) (LayoutInflater.java:775)
        at void android.view.LayoutInflater.rInflate(org.xmlpull.v1.XmlPullParser, android.view.View, android.content.Context, android.util.AttributeSet, boolean) (LayoutInflater.java:908)
        at void android.view.LayoutInflater.rInflateChildren(org.xmlpull.v1.XmlPullParser, android.view.View, android.util.AttributeSet, boolean) (LayoutInflater.java:869)
        at android.view.View android.view.LayoutInflater.inflate(org.xmlpull.v1.XmlPullParser, android.view.ViewGroup, boolean) (LayoutInflater.java:521)
        at android.view.View android.view.LayoutInflater.inflate(int, android.view.ViewGroup, boolean) (LayoutInflater.java:429)
        at android.view.View android.view.LayoutInflater.inflate(int, android.view.ViewGroup) (LayoutInflater.java:380)
        at android.view.ViewGroup androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor() (AppCompatDelegateImpl.java:607)
        at void androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor() (AppCompatDelegateImpl.java:518)
        at void androidx.appcompat.app.AppCompatDelegateImpl.setContentView(int) (AppCompatDelegateImpl.java:466)
        at void androidx.appcompat.app.AppCompatActivity.setContentView(int) (AppCompatActivity.java:140)
        at void com.example.quizexam.MainActivity.onCreate(android.os.Bundle) (MainActivity.java:28)
        at void android.app.Activity.performCreate(android.os.Bundle, android.os.PersistableBundle) (Activity.java:7166)
        at void android.app.Activity.performCreate(android.os.Bundle) (Activity.java:7157)
        at void android.app.Instrumentation.callActivityOnCreate(android.app.Activity, android.os.Bundle) (Instrumentation.java:1220)
        at android.app.Activity android.app.ActivityThread.performLaunchActivity(android.app.ActivityThread$ActivityClientRecord, android.content.Intent) (ActivityThread.java:2854)
        at void android.app.ActivityThread.handleLaunchActivity(android.app.ActivityThread$ActivityClientRecord, android.content.Intent, java.lang.String) (ActivityThread.java:2979)
        at void android.app.ActivityThread.-wrap11(android.app.ActivityThread, android.app.ActivityThread$ActivityClientRecord, android.content.Intent, java.lang.String) (ActivityThread.java:-1)
        at void android.app.ActivityThread$H.handleMessage(android.os.Message) (ActivityThread.java:1683)
        at void android.os.Handler.dispatchMessage(android.os.Message) (Handler.java:106)
        at void android.os.Looper.loop() (Looper.java:192)
        at void android.app.ActivityThread.main(java.lang.String[]) (ActivityThread.java:6754)
        at java.lang.Object java.lang.reflect.Method.invoke(java.lang.Object, java.lang.Object[]) (Method.java:-2)
        at void com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run() (RuntimeInit.java:549)
        at void com.android.internal.os.ZygoteInit.main(java.lang.String[]) (ZygoteInit.java:828)
I/zygote64: Rejecting re-init on previously-failed class java.lang.Class<androidx.core.view.ViewCompat$OnUnhandledKeyEventListenerWrapper>: java.lang.NoClassDefFoundError: Failed resolution of: Landroid/view/View$OnUnhandledKeyEventListener;
        at void androidx.core.view.ViewCompat.setBackground(android.view.View, android.graphics.drawable.Drawable) (ViewCompat.java:2341)
        at void androidx.appcompat.widget.ActionBarContainer.<init>(android.content.Context, android.util.AttributeSet) (ActionBarContainer.java:62)
        at java.lang.Object java.lang.reflect.Constructor.newInstance0(java.lang.Object[]) (Constructor.java:-2)
        at java.lang.Object java.lang.reflect.Constructor.newInstance(java.lang.Object[]) (Constructor.java:334)
        at android.view.View android.view.LayoutInflater.createView(java.lang.String, java.lang.String, android.util.AttributeSet) (LayoutInflater.java:656)
        at android.view.View android.view.LayoutInflater.createViewFromTag(android.view.View, java.lang.String, android.content.Context, android.util.AttributeSet, boolean) (LayoutInflater.java:835)
        at android.view.View android.view.LayoutInflater.createViewFromTag(android.view.View, java.lang.String, android.content.Context, android.util.AttributeSet) (LayoutInflater.java:775)
        at void android.view.LayoutInflater.rInflate(org.xmlpull.v1.XmlPullParser, android.view.View, android.content.Context, android.util.AttributeSet, boolean) (LayoutInflater.java:908)
        at void android.view.LayoutInflater.rInflateChildren(org.xmlpull.v1.XmlPullParser, android.view.View, android.util.AttributeSet, boolean) (LayoutInflater.java:869)
        at android.view.View android.view.LayoutInflater.inflate(org.xmlpull.v1.XmlPullParser, android.view.ViewGroup, boolean) (LayoutInflater.java:521)
        at android.view.View android.view.LayoutInflater.inflate(int, android.view.ViewGroup, boolean) (LayoutInflater.java:429)
        at android.view.View android.view.LayoutInflater.inflate(int, android.view.ViewGroup) (LayoutInflater.java:380)
        at android.view.ViewGroup androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor() (AppCompatDelegateImpl.java:607)
        at void androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor() (AppCompatDelegateImpl.java:518)
        at void androidx.appcompat.app.AppCompatDelegateImpl.setContentView(int) (AppCompatDelegateImpl.java:466)
        at void androidx.appcompat.app.AppCompatActivity.setContentView(int) (AppCompatActivity.java:140)
        at void com.example.quizexam.MainActivity.onCreate(android.os.Bundle) (MainActivity.java:28)
        at void android.app.Activity.performCreate(android.os.Bundle, android.os.PersistableBundle) (Activity.java:7166)
        at void android.app.Activity.performCreate(android.os.Bundle) (Activity.java:7157)
        at void android.app.Instrumentation.callActivityOnCreate(android.app.Activity, android.os.Bundle) (Instrumentation.java:1220)
        at android.app.Activity android.app.ActivityThread.performLaunchActivity(android.app.ActivityThread$ActivityClientRecord, android.content.Intent) (ActivityThread.java:2854)
        at void android.app.ActivityThread.handleLaunchActivity(android.app.ActivityThread$ActivityClientRecord, android.content.Intent, java.lang.String) (ActivityThread.java:2979)
        at void android.app.ActivityThread.-wrap11(android.app.ActivityThread, android.app.ActivityThread$ActivityClientRecord, android.content.Intent, java.lang.String) (ActivityThread.java:-1)
        at void android.app.ActivityThread$H.handleMessage(android.os.Message) (ActivityThread.java:1683)
        at void android.os.Handler.dispatchMessage(android.os.Message) (Handler.java:106)
        at void android.os.Looper.loop() (Looper.java:192)
        at void android.app.ActivityThread.main(java.lang.String[]) (ActivityThread.java:6754)
        at java.lang.Object java.lang.reflect.Method.invoke(java.lang.Object, java.lang.Object[]) (Method.java:-2)
        at void com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run() (RuntimeInit.java:549)
        at void com.android.internal.os.ZygoteInit.main(java.lang.String[]) (ZygoteInit.java:828)
    Caused by: java.lang.ClassNotFoundException: Didn't find class "android.view.View$OnUnhandledKeyEventListener" on path: DexPathList[[zip file "/data/app/com.example.quizexam-WK3UcOpinvlhDThe14Llpw==/base.apk"],nativeLibraryDirectories=[/data/app/com.example.quizexam-WK3UcOpinvlhDThe14Llpw==/lib/arm64, /system/lib64, /vendor/lib64]]
        at java.lang.Class dalvik.system.BaseDexClassLoader.findClass(java.lang.String) 

---

提前感谢您

这是因为您没有创建

随机

类实例

 Random r = new Random();

这可能解决了错误，因为stacktrace说错误是因为

r

变量为null

java.lang.NullPointerException:尝试调用虚拟方法“int java.util.Random.nextInt（int）”的尝试

---

您刚刚错过了随机对象的初始化，首先您需要创建随机对象，然后只有您可以使用它

   // create random object 
        Random r = new Random(); 

        // generating integer 
     updateQuestion(r.nextInt(nQuestionsLength));

NullPointerException表示同样的事情，您没有初始化您试图使用其方法/变量的对象

供您参考：

---

main活动第37行中的'nQuestionLength'似乎为空，但不应为空，因为finction'nextInt'获取int作为输入不为空。

“我只从youtube复制代码并编辑它”为什么这样做

 Random r = new Random();

   // create random object 
        Random r = new Random(); 

        // generating integer 
     updateQuestion(r.nextInt(nQuestionsLength));