Question

我很确定我的XML或清单没有问题。问题必须出在我的代码中（因为我稍微修改了我的代码并且它停止了工作）。该应用程序甚至在开始之前就崩溃了。我得到的错误是：

java.lang.RuntimeException：无法启动活动 ComponentInfo {com.trainer.braintrainer / com.trainer.braintrainer.MainActivity}： java.lang.NullPointerException：尝试调用虚方法'void android.widget.Button.setText（java.lang.CharSequence）'为null 对象参考在 android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2416）在 android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2476）在android.app.ActivityThread.-wrap11（ActivityThread.java）在 android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1344）在android.os.Handler.dispatchMessage（Handler.java:102）在android.os.Looper.loop（Looper.java:148）在android.app.ActivityThread.main（ActivityThread.java:5417） at java.lang.reflect.Method.invoke（Native Method）在 com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:616）引起：java.lang.NullPointerException：尝试调用虚拟方法'void android.widget.Button.setText（java.lang.CharSequence）'on 空对象引用在 com.trainer.braintrainer.MainActivity.generateQuestion（MainActivity.java:60）在 com.trainer.braintrainer.MainActivity.onCreate（MainActivity.java:101）在android.app.Activity.performCreate（Activity.java:6237）在 android.app.Instrumentation.callActivityOnCreate（Instrumentation.java:1107）在 android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2369）在 android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2476）在android.app.ActivityThread.-wrap11（ActivityThread.java）在 android.app.ActivityThread $ H.handleMessage（ActivityThread.java:1344）在android.os.Handler.dispatchMessage（Handler.java:102）在android.os.Looper.loop（Looper.java:148）在android.app.ActivityThread.main（ActivityThread.java:5417） at java.lang.reflect.Method.invoke（Native Method）在 com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:726）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:616)06-16 08：01：16.547 24091-24091 / com.trainer.braintrainer I / Process：发送信号。 PID：24091 SIG：9

以下是我的主要java代码：

package com.trainer.braintrainer;

import android.os.CountDownTimer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import org.w3c.dom.Text;

import java.util.ArrayList;
import java.util.Random;

public class MainActivity extends AppCompatActivity {
    ArrayList<Integer> answers=new ArrayList<Integer>();
    Random rand = new Random();

    int locationOfCorrectAnswer;
    Button startButton;
    Button button0;
    Button button1;
    Button button2;
    Button button3;
    int score=0;
    int numberOfQuestions=0;
    TextView answerMessage;
    String message;
    TextView scoreText;
    TextView sumTextView;
    TextView timerText;

    public void generateQuestion(){

        int a=rand.nextInt(21);
        int b=rand.nextInt(21);

        sumTextView.setText(Integer.toString(a) + " + " + Integer.toString(b));

        locationOfCorrectAnswer=rand.nextInt(4);
        answers.clear();
        int inCorrectAns;
        for (int i=0;i<=3;i++){
            if (i==locationOfCorrectAnswer){
                answers.add(a+b);
            }else{
                inCorrectAns=rand.nextInt(50);
                while (inCorrectAns==a+b){
                    inCorrectAns=rand.nextInt(50);

                }
                answers.add(inCorrectAns);
            }
        }
        button0.setText(Integer.toString(answers.get(0)));
        button1.setText(Integer.toString(answers.get(1)));
        button2.setText(Integer.toString(answers.get(2)));
        button3.setText(Integer.toString(answers.get(3)));
    }

    public void startButton(View view){
        startButton.setVisibility(View.INVISIBLE);
    }
    public void answerFunction(View view){
         int tappedLocation= (int)view.getTag();

        if (tappedLocation==locationOfCorrectAnswer){
            message="Correct!";
            score++;
        }else{
            message="Wrong!";
        }
        answerMessage.setText(message);
        scoreText.setText(Integer.toString(score) + "/" + Integer.toString(numberOfQuestions));
        numberOfQuestions++;
        generateQuestion();
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        startButton=(Button)findViewById(R.id.button5);
        answerMessage=(TextView)findViewById(R.id.messageView);
        sumTextView=(TextView)findViewById(R.id.sumTextView);
        scoreText=(TextView)findViewById(R.id.scoreText);
        timerText=(TextView)findViewById(R.id.timerText);

        generateQuestion();
        new CountDownTimer(30100,1000){

            @Override
            public void onTick(long millisUntilFinished) {
                int seconds=(int) millisUntilFinished/1000;
                timerText.setText(Integer.toString(seconds)+"s");
            }

            @Override
            public void onFinish() {
                answerMessage.setText("Done");
            }
        }.start();
        button0=(Button)findViewById(R.id.ans1);
        button1=(Button)findViewById(R.id.ans2);
        button2=(Button)findViewById(R.id.ans3);
        button3=(Button)findViewById(R.id.ans4);
    }
}

Answer 1

您正在调用方法generateQuestion（）;在为按钮赋值之前：button1，button2等。当你在这些按钮上执行setText时，它会抛出一个空对象引用错误。你能做的是：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    startButton=(Button)findViewById(R.id.button5);
    answerMessage=(TextView)findViewById(R.id.messageView);
    sumTextView=(TextView)findViewById(R.id.sumTextView);
    scoreText=(TextView)findViewById(R.id.scoreText);
    timerText=(TextView)findViewById(R.id.timerText);

    button0=(Button)findViewById(R.id.ans1);
    button1=(Button)findViewById(R.id.ans2);
    button2=(Button)findViewById(R.id.ans3);
    button3=(Button)findViewById(R.id.ans4);

    generateQuestion();
    new CountDownTimer(30100,1000){

        @Override
        public void onTick(long millisUntilFinished) {
            int seconds=(int) millisUntilFinished/1000;
            timerText.setText(Integer.toString(seconds)+"s");
        }

        @Override
        public void onFinish() {
            answerMessage.setText("Done");
        }
    }.start();

}
}

这是在调用generateQuestion（）

之前为按钮分配相应的视图

不幸的是，该应用已停止Android Studio

1 个答案: