当我将layout.setVisibilty(VISIBLE)放入checkExit()方法中时，我的应用程序崩溃了。我的FATAL EXCEPTION崩溃日志告诉我它的原因是null object reference。我不明白为什么，也不知道如何解决。
我尝试过在MainActivity中使布局静态化，我尝试过在GameView中使其静态化。我尝试过更改它的初始化位置，但是我仍然看不到它为什么被抛出。
我的MainActivity代码：
package com.example.practicingandroidwithsean.mazeone;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.animation.Animation;
import android.view.animation.AnimationUtils;
import android.widget.LinearLayout;
public class MainActivity extends AppCompatActivity {
boolean gameIsActive = true;
public void playAgain(View view){
View gameView = findViewById(R.id.gameView);
LinearLayout layout = findViewById(R.id.playAgainLayout);
Animation slideDown = AnimationUtils.loadAnimation(getApplicationContext(), R.anim.slide_down);
Animation slideUp = AnimationUtils.loadAnimation(getApplicationContext(), R.anim.slide_up);
if(gameView.getVisibility() == View.INVISIBLE){
gameIsActive = false;
layout.startAnimation(slideUp);
layout.setVisibility(View.VISIBLE);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
LinearLayout layout = findViewById(R.id.playAgainLayout);
layout.setVisibility(View.INVISIBLE);
}
}
我的GameView代码...这是一切主要发生的地方：
public class GameView extends View {
public GameView(Context context) {
super(context);
}
private enum Direction{ //data type of self defined constants
UP, DOWN, LEFT, RIGHT
}
private static Cell[][] cells;
public static Cell player, exit, trail;
private static final int COLS = 12, ROWS = 18;
private static final float WALL_THICKNESS = 4;
private static ArrayList<Cell> trailList = new ArrayList<>();
private float cellSize, hMargin, vMargin;
private int counter = 0;
private Paint wallPaint, playerPaint, exitPaint, trailPaint;
private static Random random;
private TextView TextViewTime;
private int seconds, minutes;
private boolean gameIsActive = true;
LinearLayout layout;
View gameView;
public GameView(Context context, AttributeSet attrs) {
super(context, attrs);
wallPaint = new Paint();
wallPaint.setColor(Color.WHITE);
wallPaint.setStrokeWidth(WALL_THICKNESS);
playerPaint = new Paint();
playerPaint.setColor(Color.RED);
exitPaint = new Paint();
exitPaint.setColor(Color.BLUE);
// trailPaint = new Paint();
// trailPaint.setColor(Color.YELLOW);
random = new Random();
this.seconds = 0;
this.minutes = 0;
layout = findViewById(R.id.playAgainLayout);
gameView = findViewById(R.id.gameView);
// timer(findViewById(R.id.Timer));
createMaze();
}
public void createMaze(){
Stack<Cell> stack = new Stack<>();
Cell current, next;
cells = new Cell[COLS][ROWS];
for(int x=0; x<COLS; x++){
for(int y=0; y<ROWS; y++){
cells[x][y] = new Cell(x, y); //2 Dimensional array of cell objects for the whole msze
}
}
player = cells[0][0]; //This is where the player starts
exit = cells[COLS-1][ROWS-1]; //This is the exit
current = cells[0][0];
current.visited = true;
do {
next = getNeighbour(current);
if (next != null) {
removeWall(current, next);
stack.push(current);
current = next;
current.visited = true;
} else
current = stack.pop(); //gives us the top element of the stack and removes it from the
}while(!stack.empty());
Animation slideUp = AnimationUtils.loadAnimation(getContext(), R.anim.slide_up);
gameView.startAnimation(slideUp);
}
public void checkExit() {
if (player == exit && counter < 3) {
counter++;
Animation slideDown = AnimationUtils.loadAnimation(getContext(), R.anim.mazeslidedown);
gameView.startAnimation(slideDown);
gameView.setVisibility(INVISIBLE);
layout.setVisibility(VISIBLE);
}
}
在player == exit时，视图会滑出。这没有问题，但是当我尝试使layout.setVisibility(Visible)起作用时，player==exit会崩溃。下面是错误。
Process: com.example.practicingandroidwithsean.mazeone, PID: 4295
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.LinearLayout.setVisibility(int)' on a null object reference
at com.example.practicingandroidwithsean.mazeone.GameView.checkExit(GameView.java:285)
at com.example.practicingandroidwithsean.mazeone.GameView.movePlayer(GameView.java:273)
at com.example.practicingandroidwithsean.mazeone.GameView.onTouchEvent(GameView.java:349)
at android.view.View.dispatchTouchEvent(View.java:12540)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3159)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2844)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3159)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2844)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3159)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2844)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3159)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2844)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3159)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2844)
at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:3159)
at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2844)
at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:601)
at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1871)
at android.app.Activity.dispatchTouchEvent(Activity.java:3384)
at androidx.appcompat.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:69)
at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:563)
at android.view.View.dispatchPointerEvent(View.java:12788)
at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:5670)
at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:5465)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4958)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:5011)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4977)
at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:5114)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4985)
at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:5171)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4958)
at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:5011)
at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4977)
at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4985)
at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4958)
at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:7736)
at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:7676)
at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:7637)
at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:7847)
at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:197)
at android.view.InputEventReceiver.nativeConsumeBatchedInputEvents(Native Method)
at android.view.InputEventReceiver.consumeBatchedInputEvents(InputEventReceiver.java:186)
at android.view.ViewRootImpl.doConsumeBatchedInput(ViewRootImpl.java:7810)
at android.view.ViewRootImpl$ConsumeBatchedInputRunnable.run(ViewRootImpl.java:7874)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
at android.view.Choreographer.doCallbacks(Choreographer.java:723)
at android.view.Choreographer.doFrame(Choreographer.java:652)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
at android.os.Handler.handleCallback(Handler.java:789)
E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:98)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6944)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
Application terminated.
预期的结果是，视图将滑移，然后滑动playAgainLayout。然后，这允许我的playAgain按钮起作用（我将在MainActivity.class中创建它）。如果有帮助知道，activity_main.xml包含一个链接到gameView的视图。
我的activity_main.xml：
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
android:background="@android:color/black"
tools:context=".MainActivity">
<Chronometer
android:id="@+id/chronometer"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginEnd="8dp"
android:layout_marginBottom="8dp"
android:textColor="@android:color/white"
android:textSize="45sp"
app:layout_constraintBottom_toTopOf="@+id/gameView"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<view
android:id="@+id/gameView"
class="com.example.practicingandroidwithsean.mazeone.GameView"
android:layout_width="0dp"
android:layout_height="0dp"
android:layout_marginStart="8dp"
android:layout_marginTop="140dp"
android:layout_marginEnd="8dp"
android:layout_marginBottom="8dp"
android:background="@drawable/border"
android:visibility="visible"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.1"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<LinearLayout
android:id="@+id/playAgainLayout"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginStart="8dp"
android:layout_marginTop="8dp"
android:layout_marginEnd="8dp"
android:layout_marginBottom="8dp"
android:background="@drawable/border"
android:orientation="vertical"
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/wellDoneTV"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="10dp"
android:text="Well Done!"
android:textAlignment="center"
android:textColor="@android:color/white"
android:textSize="50sp" />
<TextView
android:id="@+id/gameTime"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="5dp"
android:text="0:00"
android:textAlignment="center"
android:textColor="@android:color/white"
android:textSize="30sp" />
<Button
android:id="@+id/playAgain"
style="@style/Widget.AppCompat.Button.Colored"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="@drawable/button_border"
android:onClick="playAgain"
android:text="Button"
android:textColor="@android:color/white"
android:textSize="25sp" />
</LinearLayout>
</androidx.constraintlayout.widget.ConstraintLayout>
答案 0 :(得分：0)
根据您的代码，可能是您的.xml文件中的此ID不同：-
LinearLayout layout = findViewById(R.id.playAgainLayout);
layout.setVisibility(View.INVISIBLE);