Android：向 MainActvity 添加方法后應用程序崩潰

Question

我正在開發一個應用程序來計時和得分籃球比賽，當我添加倒計時部分和一個按鈕來控制MainActivity.java文件的計數時，該應用程序構建良好但每次都崩潰。 這是我使用的代碼。 這是我每次嘗試啟動應用程序時生成的錯誤

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.abdulkarim.courtcounter, PID: 19740
                  java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.abdulkarim.courtcounter/com.example.abdulkarim.courtcounter.MainActivity}: java.lang.NullPointerException
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2306)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2471)
                      at android.app.ActivityThread.access$900(ActivityThread.java:175)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:146)
                      at android.app.ActivityThread.main(ActivityThread.java:5603)
                      at java.lang.reflect.Method.invokeNative(Native Method)
                      at java.lang.reflect.Method.invoke(Method.java:515)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
                      at dalvik.system.NativeStart.main(Native Method)
                   Caused by: java.lang.NullPointerException
                      at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:116)
                      at android.support.v7.app.AppCompatDelegateImplV9.<init>(AppCompatDelegateImplV9.java:147)
                      at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:27)
                      at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:50)
                      at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:201)
                      at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:181)
                      at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:521)
                      at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
                      at com.example.abdulkarim.courtcounter.MainActivity.<init>(MainActivity.java:14)
                      at java.lang.Class.newInstanceImpl(Native Method)
                      at java.lang.Class.newInstance(Class.java:1208)
                      at android.app.Instrumentation.newActivity(Instrumentation.java:1067)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2297)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2471) 
                      at android.app.ActivityThread.access$900(ActivityThread.java:175) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1308) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:146) 
                      at android.app.ActivityThread.main(ActivityThread.java:5603) 
                      at java.lang.reflect.Method.invokeNative(Native Method) 
                      at java.lang.reflect.Method.invoke(Method.java:515) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099) 
                      at dalvik.system.NativeStart.main(Native Method)

主活動.java

package com.example.abdulkarim.courtcounter;

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;

public class MainActivity extends AppCompatActivity {

    int scoreTeamA = 0, scoreTeamB = 0;
    TextView minutes = (TextView) findViewById(R.id.min_counter);
    TextView seconds = (TextView) findViewById(R.id.sec_counter);
    long milliLeft, min, sec;
    CountDownTimer gameTime;
    final Button timeoutButton = (Button) findViewById(R.id.timeout_button);

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

    /**
     * This method is called when the +3 points button is clicked
     */
    public void threePointsA(View view){
        scoreTeamA = scoreTeamA + 3;
        displayScoreA(scoreTeamA);
    }

    /**
     * This method is called when the +2 points button is clicked
     */
    public void twoPointsA(View view){
        scoreTeamA = scoreTeamA + 2;
        displayScoreA(scoreTeamA);
    }

    /**
     * This method is called when the Free Throw button is clicked
     */
    public void freeThrowA(View view){
        scoreTeamA = scoreTeamA + 1;
        displayScoreA(scoreTeamA);
    }

    /**
     * This method is called when the +3 points button is clicked
     */
    public void threePointsB(View view){
        scoreTeamB = scoreTeamB + 3;
        displayScoreB(scoreTeamB);
    }

    /**
     * This method is called when the +2 points button is clicked
     */
    public void twoPointsB(View view){
        scoreTeamB = scoreTeamB + 2;
        displayScoreB(scoreTeamB);
    }

    /**
     * This method is called when the Free Throw button is clicked
     */
    public void freeThrowB(View view){
        scoreTeamB = scoreTeamB + 1;
        displayScoreB(scoreTeamB);
    }

    /**
     * This method is called when the reset button is clicked
     */
    public void reset(View view){
        scoreTeamA = 0;
        scoreTeamB = 0;
        displayScoreA(scoreTeamA);
        displayScoreB(scoreTeamB);
    }

    /**
     * This method initializes the countdown timer
     */
    public void gameTime(long timeLengthMilli) {
        gameTime = new CountDownTimer(timeLengthMilli, 1000) {

            @Override
            public void onTick(long milliTillFinish) {
                milliLeft = milliTillFinish;
                min = (milliTillFinish / (1000 * 60));
                sec = ((milliTillFinish / 1000) - min * 60);
                minutes.setText(Long.toString(min));
                seconds.setText(Long.toString(sec));
                Log.i("Tick", "Tock");
            }
            public void onFinish() {
                minutes.setText("00");
                seconds.setText("00");
            }
        }.start();

    }

    public void timerPause() {
        gameTime.cancel();
    }

    private void timerResume() {
        Log.i("min", Long.toString(min));
        Log.i("Sec", Long.toString(sec));
        gameTime(milliLeft);
    }

    /**
     *This method is called when the timeOut button is clicked
     */
    public void timeOut(View view){
        timeoutButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if(timeoutButton.getText().equals("Start")){
                    Log.i("Started", timeoutButton.getText().toString());
                    timeoutButton.setText("Timeout");
                    gameTime(60*1000);
                } else if (timeoutButton.getText().equals("Pause")){
                    Log.i("Timeout", timeoutButton.getText().toString());
                    timeoutButton.setText("Resume");
                    timerPause();
                } else if (timeoutButton.getText().equals("Resume")){
                    timeoutButton.setText("Timeout");
                    timerResume();
                }
            }
        });
    }
    /**
     * This method handles the display of scores for team A
     */
    private void displayScoreA(int score){
        TextView scoreTextView = (TextView) findViewById(R.id.teamA_score_text_view);
        scoreTextView.setText(String.valueOf(score));
    }
    /**
     * This method handles the display of scores for team A
     */
    private void displayScoreB(int score){
        TextView scoreTextView = (TextView) findViewById(R.id.teamB_score_text_view);
        scoreTextView.setText(String.valueOf(score));
    }
}

main_activity.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.example.abdulkarim.courtcounter.MainActivity">

    <RelativeLayout
        android:id="@+id/r1_layout"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true">
        <TextView
            android:id="@+id/min_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginLeft="20dp"
            android:layout_marginRight="75dp"
            android:paddingTop="16dp"
            android:text="Mins"
            android:textAllCaps="true"
            android:textColor="@android:color/black"
            android:textSize="10sp" />

        <TextView
            android:id="@+id/sec_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentTop="true"
            android:paddingTop="16dp"
            android:layout_toRightOf="@id/min_text_view"
            android:text="Secs"
            android:textAllCaps="true"
            android:textColor="@android:color/black"
            android:textSize="10sp" />

        <TextView
            android:id="@+id/min_counter"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/min_text_view"
            android:layout_marginTop="8dp"
            android:fontFamily="sans-serif-light"
            android:text="00"
            android:textColor="#000000"
            android:textSize="56sp" />

        <TextView
            android:id="@+id/blinker_text_view"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/min_text_view"
            android:layout_marginTop="8dp"
            android:layout_toRightOf="@id/min_counter"
            android:fontFamily="sans-serif-light"
            android:paddingLeft="16dp"
            android:paddingRight="16dp"
            android:text=":"
            android:textColor="#000000"
            android:textSize="56sp" />

        <TextView
            android:id="@+id/sec_counter"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@id/min_text_view"
            android:layout_marginTop="8dp"
            android:layout_toRightOf="@id/blinker_text_view"
            android:fontFamily="sans-serif-light"
            android:text="00"
            android:textColor="#000000"
            android:textSize="56sp" />
    </RelativeLayout>

    <LinearLayout
        android:id="@+id/l1_layout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_below="@id/r1_layout"
        android:orientation="horizontal">

        <LinearLayout
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:orientation="vertical">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:fontFamily="sans-serif-medium"
                android:gravity="center_horizontal"
                android:text="Team A"
                android:textColor="#616161"
                android:textSize="14sp" />

            <TextView
                android:id="@+id/teamA_score_text_view"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:fontFamily="sans-serif-light"
                android:gravity="center_horizontal"
                android:text="0"
                android:textColor="#000000"
                android:textSize="56sp" />

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginBottom="8dp"
                android:layout_marginLeft="24dp"
                android:layout_marginRight="24dp"
                android:layout_marginTop="24dp"
                android:fontFamily="sans-serif-medium"
                android:onClick="threePointsA"
                android:text="+3 Points"
                android:textAllCaps="true"
                android:textColor="#616161"
                android:textSize="14sp" />

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginBottom="8dp"
                android:layout_marginLeft="24dp"
                android:layout_marginRight="24dp"
                android:fontFamily="sans-serif-medium"
                android:onClick="twoPointsA"
                android:text="+2 Points"
                android:textAllCaps="true"
                android:textColor="#616161"
                android:textSize="14sp" />

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginBottom="8dp"
                android:layout_marginLeft="24dp"
                android:layout_marginRight="24dp"
                android:fontFamily="sans-serif-medium"
                android:onClick="freeThrowA"
                android:text="Free Throw"
                android:textAllCaps="true"
                android:textColor="#616161"
                android:textSize="14sp" />
        </LinearLayout>

        <View
            android:layout_width="1dp"
            android:layout_height="match_parent"
            android:layout_marginTop="16dp"
            android:background="@android:color/darker_gray"></View>

        <LinearLayout
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"

            android:orientation="vertical">

            <TextView
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:fontFamily="sans-serif-medium"
                android:gravity="center_horizontal"
                android:text="Team B"
                android:textColor="#616161"
                android:textSize="14sp" />

            <TextView
                android:id="@+id/teamB_score_text_view"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginTop="16dp"
                android:fontFamily="sans-serif-light"
                android:gravity="center_horizontal"
                android:text="0"
                android:textColor="#000000"
                android:textSize="56sp" />

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginBottom="8dp"
                android:layout_marginLeft="24dp"
                android:layout_marginRight="24dp"
                android:layout_marginTop="24dp"
                android:fontFamily="sans-serif-medium"
                android:onClick="threePointsB"
                android:text="+3 Points"
                android:textAllCaps="true"
                android:textColor="#616161"
                android:textSize="14sp" />

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginBottom="8dp"
                android:layout_marginLeft="24dp"
                android:layout_marginRight="24dp"
                android:fontFamily="sans-serif-medium"
                android:onClick="twoPointsB"
                android:text="+2 Points"
                android:textAllCaps="true"
                android:textColor="#616161"
                android:textSize="14sp" />

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginBottom="8dp"
                android:layout_marginLeft="24dp"
                android:layout_marginRight="24dp"
                android:fontFamily="sans-serif-medium"
                android:onClick="freeThrowB"
                android:text="Free Throw"
                android:textAllCaps="true"
                android:textColor="#616161"
                android:textSize="14sp" />
        </LinearLayout>

    </LinearLayout>


    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true">
        <Button
            android:id="@+id/timeout_button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginRight="8dp"
            android:layout_marginBottom="32dp"
            android:layout_marginLeft="16dp"
            android:text="Start"
            android:fontFamily="sans-serif-medium"
            android:textAllCaps="true"
            android:textColor="#616161"
            android:textSize="14sp"
            android:onClick="timeOut"/>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="32dp"
        android:layout_marginRight="16dp"
        android:layout_marginLeft="8dp"
        android:fontFamily="sans-serif-medium"
        android:onClick="Reset"
        android:text="Reset"
        android:textAllCaps="true"
        android:textColor="#616161"
        android:textSize="14sp" />
    </LinearLayout>

</RelativeLayout>

Answer 1

您不能在onCreate方法之前調用findViewById 。
在onCreate移動視圖引用的初始化。

public class MainActivity extends AppCompatActivity {

    int scoreTeamA = 0, scoreTeamB = 0;
    TextView minutes; 
    TextView seconds; 
    long milliLeft, min, sec;
    CountDownTimer gameTime;
    Button timeoutButton;

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

        minutes = (TextView) findViewById(R.id.min_counter);
        seconds = (TextView) findViewById(R.id.sec_counter);
        timeoutButton = (Button) findViewById(R.id.timeout_button);
    }

Answer 2

1. 在全局中聲明所有變量

2. 在 onCreate 方法中移動findViewById 。 您應該在 onCreate 方法中初始化它們，而不是在全局中。

    public class MainActivity extends AppCompatActivity { int scoreTeamA = 0, scoreTeamB = 0; TextView minutes; TextView seconds; long milliLeft, min, sec; CountDownTimer gameTime; final Button timeoutButton; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); minutes = (TextView) findViewById(R.id.min_counter); seconds = (TextView) findViewById(R.id.sec_counter); timeoutButton = (Button) findViewById(R.id.timeout_button); } // others code... }