当我点击Android应用程序上的按钮时,我收到NullPointerException

Question

我正在开发一个简单的测验应用程序,向用户提出一个问题,可以选择True或False来回答问题.我添加了一个"显示答案"按钮,但当我点击该按钮时,我得到以下NullPointerException.我想我在activity_cheat.xml中遗漏了一些内容,但我无法弄清楚是什么.

05-31 19:30:59.238    5978-5978/com.bignerdranch.android.geoquiz.geoquiz E/AndroidRuntime? FATAL EXCEPTION: main
    Process: com.bignerdranch.android.geoquiz.geoquiz, PID: 5978
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(int)' on a null object reference
            at com.bignerdranch.android.geoquiz.geoquiz.CheatActivity$1.onClick(CheatActivity.java:33)
            at android.view.View.performClick(View.java:5197)
            at android.view.View$PerformClick.run(View.java:20926)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:145)
            at android.app.ActivityThread.main(ActivityThread.java:5942)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1400)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1195)

CheatActivity.java

package com.bignerdranch.android.geoquiz.geoquiz;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;


public class CheatActivity extends Activity {
    private boolean mAnswerIsTrue;

    private TextView mAnswerTextView;
    private Button mShowAnswer;

    public static final String EXTRA_ANSWER_IS_TRUE =  "com.bignerdranch.android.geoquiz.answer_is_true";

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

        mAnswerIsTrue = getIntent().getBooleanExtra(EXTRA_ANSWER_IS_TRUE, false);


        mShowAnswer = (Button)findViewById(R.id.showAnswerButton);
        mShowAnswer.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (mAnswerIsTrue) {
                    mAnswerTextView.setText(R.string.true_button);
                } else {
                    mAnswerTextView.setText(R.string.false_button);
                }
            }
        });
    }

}

activity_cheat.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="match_parent"
              android:layout_height="match_parent">
              android:gravity="center">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="24dp"
        android:text="@string/warning_text_view" />

    <TextView
        android:id="@+id/answerTextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="24dp" />

    <Button
        android:id="@+id/showAnswerButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/show_answer_button" />


</LinearLayout>

Answer 1

您忘了将mAnswerTextView绑定到UI:

加

mAnswerTextView = (TextView)findViewById(R.id.answerTextView);

之后

setContentView(R.layout.activity_cheat);

Answer 2

mAnswerTextView从未初始化.这就是你获得NPE的原因.

加

mAnswerTextView = (TextView)findViewById(R.id.answerTextView);

之前

 if (mAnswerIsTrue)

Answer 3

mAnswerTextView一片空白.加,

mAnswerTextView = (TextView) findViewById(R.id.answerTextView);

在你的onClickListener之前.