我正在尝试启动应用，并且崩溃了

Question

Error:
01-14 20:59:18.266 27103-27103/com.example.android.cricketscore E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.android.cricketscore, PID: 27103
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.android.cricketscore/com.example.android.cricketscore.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2366)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2517)
        at android.app.ActivityThread.access$800(ActivityThread.java:162)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1412)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:189)
        at android.app.ActivityThread.main(ActivityThread.java:5529)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:950)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        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.android.cricketscore.MainActivity.<init>(MainActivity.java:19)
        at java.lang.reflect.Constructor.newInstance(Native Method)
        at java.lang.Class.newInstance(Class.java:1572)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1065)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2356)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2517) 
        at android.app.ActivityThread.access$800(ActivityThread.java:162) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1412) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:189) 
        at android.app.ActivityThread.main(ActivityThread.java:5529) 
        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:950) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745) 

主要活动。 爪哇

package com.example.android.cricketscore;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.CheckBox;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

int scoreTeamA = 0;

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

 //Figure out if Team A has scored One Run
CheckBox oneRunCheckbox = (CheckBox) findViewById(R.id.addOneForTeamA);
boolean isOneRun = oneRunCheckbox.isChecked();

/**Calculates the run of the score.
 *
 * @return total run
 */

    //Add 1 to the score if team A scored 1 run
   public void addOneForTeamA(View, view){
       scoreTeamA = scoreTeamA + 1;
       displayForTeamA(scoreTeamA);
   }


/** This method creates a summary of the total score
 * @param addOneRun is scored
 * @param score summary
 */

/**This method displays the score on the screen. */
public void displayForTeamA(int score){
    TextView scoreView = (TextView) findViewById(R.id.team_a_score);
    scoreView.setText(String.valueOf(score));
 }

 }

activity_main.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.android.cricketscore.MainActivity">

<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginBottom="16dp"
    android:layout_marginTop="16dp"
    android:fontFamily="sans-serif-medium"
    android:gravity="center_horizontal"
    android:text="Team A"
    android:textSize="14sp"
    android:id="@+id/textView2" />

<TextView
    android:id="@+id/team_a_score"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:fontFamily="sans-serif-light"
    android:gravity="center_horizontal"
    android:text="0/0"
    android:textSize="56sp"
    android:layout_marginTop="15dp"
    android:layout_alignTop="@+id/textView2"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true" />

<CheckBox
    android:id="@+id/addOneForTeamA"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:paddingLeft="24dp"
    android:text="1 Run"
    android:textSize="16sp"
    android:layout_below="@+id/team_a_score"
    android:layout_margin="16dp" />


  </RelativeLayout>

Answer 1

您正在第19行遇到NullPointerException ，如LogCat所述：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
...
at com.example.android.cricketscore.MainActivity.<init>(MainActivity.java:19)

这些行：

CheckBox oneRunCheckbox = (CheckBox) findViewById(R.id.addOneForTeamA);
boolean isOneRun = oneRunCheckbox.isChecked();

需要在您的onCreate()方法内部，如下所示：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    CheckBox oneRunCheckbox = (CheckBox) findViewById(R.id.addOneForTeamA);
    boolean isOneRun = oneRunCheckbox.isChecked();
}

Answer 2

尝试这个：

package com.example.android.cricketscore;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.CheckBox;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

int scoreTeamA = 0;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    CheckBox oneRunCheckbox = (CheckBox) findViewById(R.id.addOneForTeamA);
    boolean isOneRun = oneRunCheckbox.isChecked();
}

 //Figure out if Team A has scored One Run


/**Calculates the run of the score.
 *
 * @return total run
 */

    //Add 1 to the score if team A scored 1 run
   public void addOneForTeamA(View, view){
       scoreTeamA = scoreTeamA + 1;
       displayForTeamA(scoreTeamA);
   }


/** This method creates a summary of the total score
 * @param addOneRun is scored
 * @param score summary
 */

/**This method displays the score on the screen. */
public void displayForTeamA(int score){
    TextView scoreView = (TextView) findViewById(R.id.team_a_score);
    scoreView.setText(String.valueOf(score));
 }

 }

Answer 3

NullPointerException已在以前的答案中发现。 这是您在此处使用的错误功能。

只需删除View参数即可，因为您没有在此函数中使用它。

//Add 1 to the score if team A scored 1 run
public void addOneForTeamA(){
    scoreTeamA = scoreTeamA + 1;
    displayForTeamA(scoreTeamA);
}

无论如何，如果您想将view传递给addOneForTeamA()函数，则可以执行以下操作。

//Add 1 to the score if team A scored 1 run
public void addOneForTeamA(View view){    // remove the comma
    scoreTeamA = scoreTeamA + 1;
    displayForTeamA(scoreTeamA);
}