android - 单击开始按钮时，Android应用程序崩溃

Question

我正在尝试在android中开发游戏，但是我还算是新手!无论如何，我的游戏包含两个 Activity (1.主菜单2.游戏)。游戏从主菜单 Activity 开始，并且当用户单击ID =(button2)的按钮时，它应启动游戏的第二个 Activity :
这是我MainMenu的.xml代码:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context=".MainMenu"
android:background="@drawable/starting_page">

<Button
    android:layout_width="400dp"
    android:layout_height="45dp"
    android:text="Exit Game"
    android:id="@+id/button"
    android:background="#f6d89d00"
    android:textSize="30sp"
    android:layout_alignParentBottom="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_marginBottom="39dp" />

<Button
    android:layout_width="400dp"
    android:layout_height="45dp"
    android:text="Start Game"
    android:id="@+id/button2"
    android:background="#f6d86d00"
    android:textSize="30sp"
    android:layout_above="@+id/button"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:clickable="true"
    android:enabled="true"
    android:visibility="visible"
    android:focusableInTouchMode="true" />

<CheckBox
    android:layout_width="100dp"
    android:layout_height="wrap_content"
    android:text="Sound"
    android:id="@+id/checkBox"
    android:textSize="20dp"
    android:longClickable="false"
    android:layout_alignParentBottom="true"
    android:layout_alignRight="@+id/button"
    android:layout_alignEnd="@+id/button"
    android:enabled="true" />
</RelativeLayout>

到目前为止，他是该 Activity 中的代码:

package com.example.user.catchthefly;

import android.support.v7.app.ActionBarActivity; 
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.content.Intent;
import android.widget.ImageView;
import android.graphics.Color;
import static android.graphics.Color.*;

public class MainMenu extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main_menu);
    final Button btn1 = (Button) findViewById(R.id.button2);
    final Button btn2 = (Button) findViewById(R.id.button);
    btn2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            finish();
            System.exit(0);
        }
    });
    btn1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            Intent intent = new Intent(MainMenu.this, MainGame.class);
            MainMenu.this.startActivity(intent);
            finish();
        }
    });
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main_menu, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}
}

在下面，我提供了AndroidManifest.xml，以防万一您需要它来总结出什么问题:

 <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.user.catchthefly" >

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".MainMenu"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name=".MainGame"
        android:label="@string/app_name" >
    </activity>
    <activity
        android:name=".ScorePage"
        android:label="@string/app_name" >
    </activity>
</application>

</manifest>

日志猫

07-21 13:50:53.828      811-811/com.example.user.catchthefly E/Trace﹕ error opening trace file: No such file or directory (2)
07-21 13:50:55.478      811-811/com.example.user.catchthefly D/dalvikvm﹕ GC_FOR_ALLOC freed 55K, 7% free 2564K/2740K, paused 36ms, total 39ms
07-21 13:50:55.588      811-811/com.example.user.catchthefly D/dalvikvm﹕ GC_FOR_ALLOC freed 2K, 6% free 2901K/3080K, paused 26ms, total 27ms
07-21 13:50:55.868      811-811/com.example.user.catchthefly D/libEGL﹕ loaded /system/lib/egl/libEGL_emulation.so
07-21 13:50:55.888      811-811/com.example.user.catchthefly D/﹕ HostConnection::get() New Host Connection established 0x2a154d08, tid 811
07-21 13:50:55.938      811-811/com.example.user.catchthefly D/libEGL﹕ loaded /system/lib/egl/libGLESv1_CM_emulation.so
07-21 13:50:55.979      811-811/com.example.user.catchthefly D/libEGL﹕ loaded /system/lib/egl/libGLESv2_emulation.so
07-21 13:50:56.308      811-811/com.example.user.catchthefly W/EGL_emulation﹕ eglSurfaceAttrib not implemented
07-21 13:50:56.328      811-811/com.example.user.catchthefly D/OpenGLRenderer﹕ Enabling debug mode 0
07-21 13:50:56.548      811-814/com.example.user.catchthefly D/dalvikvm﹕ GC_CONCURRENT freed 359K, 14% free 2937K/3416K, paused 6ms+46ms, total 117ms
07-21 13:51:49.189      811-811/com.example.user.catchthefly D/AndroidRuntime﹕ Shutting down VM
07-21 13:51:49.199      811-811/com.example.user.catchthefly W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x40a71930)
07-21 13:51:49.229      811-811/com.example.user.catchthefly E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.user.catchthefly/com.example.user.catchthefly.MainGame}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2106)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5041)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.app.Activity.findViewById(Activity.java:1839)
            at com.example.user.catchthefly.MainGame.<init>(MainGame.java:25)
            at java.lang.Class.newInstanceImpl(Native Method)
            at java.lang.Class.newInstance(Class.java:1319)
            at android.app.Instrumentation.newActivity(Instrumentation.java:1054)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2097)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
            at android.app.ActivityThread.access$600(ActivityThread.java:141)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:5041)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
            at dalvik.system.NativeStart.main(Native Method)

问题是，当我启动程序时一切正常，但是当我单击“开始游戏”按钮(ID为(Button2))时，游戏在模拟器中崩溃了。
我花了数小时试图找出问题所在，但似乎无法发现问题所在
因此欢迎任何帮助
非常感谢你!!!

解决方案:
我已经在onCreate方法上方的MainGame.class中启动了按钮，布局，textviews，imageviews，这就是程序崩溃的原因
谢谢您的回答 :)

Answer 1

Caused by: java.lang.NullPointerException
  at android.app.Activity.findViewById(Activity.java:1839)
  at com.example.user.catchthefly.MainGame.<init>(MainGame.java:25)

在 MainGame中，初始化成员变量时调用 findViewById()太早。

将 findViewById()移到 onCreate()之后到 setContentView()。