我正在学习Android开发课程，其中一个练习就是制作一个简单的Connect 3游戏。我不确定错误在哪里，因为它不是编译错误，所以我无法弄清楚在哪里更改代码。
以下是MainActivity.java代码：
package com.anurag.connectthree;
import android.media.Image;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.GridLayout;
import android.widget.ImageView;
import android.widget.LinearLayout;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
// 0 = yellow, 1 = red => means yellow goes first
int activePlayer = 0;
//save a memory state to know when a tile has been clicked
int[] gameState = {2,2,2,2,2,2,2,2,2};
//check if game is won after min 5 moves
static int noOfMoves = 0;
//layout over the grid layout for playing again
private LinearLayout playAgainLayout = (LinearLayout)findViewById(R.id.playAgainLayout);
//the winner message on the linear layout
private TextView winnerMessage = (TextView)findViewById(R.id.winnerMessage);
//initially true, set to false after game has been won
private boolean gameIsActive = true;
public void dropIn(View view){
//tapped on view, so imageview is the view itself, then set resource to image as done on line 31 and 35
ImageView counter =(ImageView) view;
//Added tags to tiles in XML .getTag() returns tag and can act as index of gameState array
int tappedCounter = Integer.parseInt(counter.getTag().toString());
//2 means tile hasn't been been clicked
if(gameState[tappedCounter] == 2 && gameIsActive) {
gameState[tappedCounter]=activePlayer; //this will tell us which player is playing this move
counter.setTranslationY(-1000f);
if (activePlayer == 0) {
counter.setImageResource(R.drawable.yellow);
activePlayer = 1;
} else {
counter.setImageResource(R.drawable.red);
activePlayer = 0;
}
counter.animate().translationYBy(1000f).rotation(540f).setDuration(300);
}
noOfMoves++;
Log.i("Move number ",Integer.toString(noOfMoves));
if(noOfMoves>=5) checkForWin();
}
private void checkForWin() {
int i,j;
//winning combinations are {{0,1,2},{3,4,5},{6,7,8}} horizontally, {{0,3,6},{1,4,7},{2,5,8}} vertically and {{0,4,8}, {2,4,6}} diagonally
for(i=1,j=3;i<=8&&j<=8;i+=3,j+=1){
if((gameState[i-1]==gameState[i]&&gameState[i+1]==gameState[i]) //takes care of horizontal
||(gameState[j-3]==gameState[j]&&gameState[j]==gameState[j+3]) //takes care of vertical
||(gameState[0]==gameState[4]&&gameState[4]==gameState[8]) //hardcoding diagonal
||(gameState[2]==gameState[4]&&gameState[4]==gameState[6])){
if(gameState[i]==0||gameState[j]==0||gameState[4]==0) {
gameIsActive = false;
winnerMessage.setText("Yellow has won!");
if(playAgainLayout.getVisibility()==View.INVISIBLE)
playAgainLayout.setVisibility(View.VISIBLE);
}
else if(gameState[i]==1||gameState[j]==1||gameState[4]==1) {
gameIsActive = false;
winnerMessage.setText("Yellow has won!");
if(playAgainLayout.getVisibility()==View.INVISIBLE)
playAgainLayout.setVisibility(View.VISIBLE);
}
}
}
}
public void playAgain(View view){
//for playing again set the game to active again
gameIsActive=true;
//this is again set to invisible
playAgainLayout.setVisibility(View.INVISIBLE);
//game state reset to {2,2,2,2,2,2,2,2,2}
for(int i =0; i<gameState.length;i++)
gameState[i]=2;
//again yellow will start, so set to 0
activePlayer = 0;
GridLayout gridLayout = (GridLayout)findViewById(R.id.gridLayout);
//for each of the 9 children of the grid, set their image src to null
for(int i=0;i<gridLayout.getChildCount();i++){
((ImageView) gridLayout.getChildAt(i)).setImageResource(0);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i("Check if game starts :","Yes");
}
}
还给出了日志输出：
04-26 20:55:03.639 20649-20649/? E/Zygote: v2
04-26 20:55:03.639 20649-20649/? I/libpersona: KNOX_SDCARD checking this for
10231
KNOX_SDCARD not a persona
04-26 20:55:03.641 20649-20649/? E/Zygote: accessInfo : 0
04-26 20:55:03.642 20649-20649/? W/SELinux: SELinux
selinux_android_compute_policy_index : Policy Index[2], Con:u:r:zygote:s0
RAM:SEPF_SECMOBILE_7.0_0010, [-1 -1 -1 -1 0 1]
04-26 20:55:03.643 20649-20649/? I/SELinux: SELinux: seapp_context_lookup:
seinfo=untrusted, level=s0:c512,c768, pkgname=com.anurag.connectthree
04-26 20:55:03.648 20649-20649/? I/art: Late-enabling -Xcheck:jni
04-26 20:55:03.967 20649-20649/com.anurag.connectthree I/InstantRun:
starting instant run server: is main process
04-26 20:55:04.067 20649-20649/com.anurag.connectthree E/AndroidRuntime:
FATAL EXCEPTION: main
Process: com.anurag.connectthree, PID: 20649
java.lang.RuntimeException: Unable to instantiate activity
ComponentInfo{com.anurag.connectthree/com.anurag.connectthree.MainActivity}:
java.lang.NullPointerException: Attempt to invoke virtual method
'android.view.Window$Callback android.view.Window.getCallback()' on a null
object reference
at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2849)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method
'android.view.Window$Callback android.view.Window.getCallback()' on a null
object reference
at android.support.v7.app.AppCompatDelegateImplBase.<init>
(AppCompatDelegateImplBase.java:117)
at android.support.v7.app.AppCompatDelegateImplV9.<init>
(AppCompatDelegateImplV9.java:149)
at android.support.v7.app.AppCompatDelegateImplV14.<init>
(AppCompatDelegateImplV14.java:56)
at android.support.v7.app.AppCompatDelegateImplV23.<init>
(AppCompatDelegateImplV23.java:31)
at android.support.v7.app.AppCompatDelegateImplN.<init>
(AppCompatDelegateImplN.java:31)
at
android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:198)
at
android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:183)
at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:519)
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
at com.anurag.connectthree.MainActivity.<init>(MainActivity.java:26)
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1086)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2839)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
at android.app.ActivityThread.-wrap14(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6776)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
04-26 20:55:04.093 20649-20649/? I/Process: Sending signal. PID: 20649 SIG: 9
还附加activity_main XML文件：
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<android.support.v7.widget.GridLayout
android:id="@+id/gridLayout"
android:layout_width="369dp"
android:layout_height="424dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@drawable/board"
app:columnCount="3"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.0"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.492"
app:rowCount="3">
<ImageView
android:id="@+id/imageView"
android:layout_width="119dp"
android:layout_height="134dp"
android:onClick="dropIn"
android:tag="0" />
<ImageView
android:id="@+id/imageView3"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginLeft="10dp"
android:onClick="dropIn"
android:tag="1"
app:layout_column="1"
app:layout_row="0" />
<ImageView
android:id="@+id/imageView4"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginLeft="5dp"
android:onClick="dropIn"
android:tag="2" />
<ImageView
android:id="@+id/imageView10"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginTop="5dp"
android:onClick="dropIn"
android:tag="3" />
<ImageView
android:id="@+id/imageView9"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginLeft="10dp"
android:layout_marginTop="5dp"
android:onClick="dropIn"
android:tag="4" />
<ImageView
android:id="@+id/imageView8"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginLeft="5dp"
android:layout_marginTop="5dp"
android:onClick="dropIn"
android:tag="5" />
<ImageView
android:id="@+id/imageView7"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginTop="13dp"
android:onClick="dropIn"
android:tag="6" />
<ImageView
android:id="@+id/imageView6"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginLeft="9dp"
android:layout_marginTop="13dp"
android:onClick="dropIn"
android:tag="7" />
<ImageView
android:id="@+id/imageView5"
android:layout_width="119dp"
android:layout_height="134dp"
android:layout_marginLeft="5dp"
android:layout_marginTop="13dp"
android:onClick="dropIn"
android:tag="8" />
</android.support.v7.widget.GridLayout>
<LinearLayout
android:id="@+id/playAgainLayout"
android:layout_width="309dp"
android:layout_height="179dp"
android:layout_marginBottom="8dp"
android:layout_marginEnd="8dp"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:background="@android:color/holo_blue_dark"
android:orientation="vertical"
android:padding="10dp"
android:visibility="invisible"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent">
<TextView
android:id="@+id/winnerMessage"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@android:color/transparent"
android:padding="10dp"
android:text="The winner is "
android:textColor="@android:color/black"
android:textSize="30sp" />
<Button
android:id="@+id/playAgainButton"
android:layout_width="141dp"
android:layout_height="76dp"
android:layout_gravity="center_vertical|center"
android:onClick="playAgain"
android:text="Play Again"
android:textSize="17sp" />
</LinearLayout>
还有Android Mainfest XML：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.anurag.connectthree">
<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
答案 0 :(得分：0)
在onCreate()
setContentView()内查找视图
private LinearLayout playAgainLayou
private TextView winnerMessage
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
winnerMessage = (TextView)findViewById(R.id.winnerMessage);
playAgainLayout =(LinearLayout)findViewById(R.id.playAgainLayout);
}
答案 1 :(得分：0)
您有两个视图（playAgainLayout和winnerMessage）使用findViewById API调用初始化为字段声明的一部分。
当这堂课时＃39;调用构造函数，首先初始化这些字段。 这可以从堆栈跟踪中验证：
at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:190)
at com.anurag.connectthree.MainActivity.<init>(MainActivity.java:26)
这里<init>指的是构造函数调用。
findViewById仅在活动附加到窗口时才有效。
activity lifecycle中的第一个回调方法是onCreate方法。通常我们从这里调用setContentView方法来夸大我们的观点。由于您希望从布局文件activity_main.xml中填充视图，因此您应在调用setContentView(R.layout.activity_main);后找到它们。
//layout over the grid layout for playing again
private LinearLayout playAgainLayout;
//the winner message on the linear layout
private TextView winnerMessage;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// layout is inflated, now we can find the views using findViewById
playAgainLayout = (LinearLayout)findViewById(R.id.playAgainLayout);
winnerMessage = (TextView)findViewById(R.id.winnerMessage);
Log.i("Check if game starts :","Yes");
}
}
希望这有帮助。