android - NullPointerException 和 Intent Android

Question

我有一个开关类来确定手机或平板电脑，并且在创建 Intent 时出现空指针异常。我只是想知道是什么导致了这种情况，因为这两种 Activity 都存在，并且开关正常工作，因为它在平板电脑手机上时会在开关上出错。

这是启动相应 Activity 的初始 Activity 的代码:

package jack.beastapps.TimerPlus;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.os.Bundle;
import android.util.DisplayMetrics;

public class SplashScreen extends Activity {
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
}


public boolean isTablet() { 
try { 
    Context context = this;
    // Compute screen size 
    DisplayMetrics dm = context.getResources().getDisplayMetrics(); 
    float screenWidth  = dm.widthPixels / dm.xdpi; 
    float screenHeight = dm.heightPixels / dm.ydpi; 
    double size = Math.sqrt(Math.pow(screenWidth, 2) + 
                            Math.pow(screenHeight, 2)); 

    // Tablet devices should have a screen size greater than 6 inches 
    return size >= 6; 
} catch(Throwable t) { 
    return false; 
}
}{
if ( isTablet() == true ) {
        Intent tablet = new Intent(SplashScreen.this, TabletActivity.class);
       startActivity(tablet);
}
else {
        Intent phone = new Intent(SplashScreen.this, PhoneActivity.class);
        startActivity(phone);
}

这是启动时强制关闭的 logcat:

09:34:08.454: E/AndroidRuntime(12322): FATAL EXCEPTION: main
04-07 09:34:08.454: E/AndroidRuntime(12322): java.lang.RuntimeException: Unable to         instantiate activity        
ComponentInfo{jack.beastapps.TimerPlus/jack.beastapps.TimerPlus.SplashScreen}:   java.lang.NullPointerException
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1880)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.ActivityThread.access$600(ActivityThread.java:123)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.os.Handler.dispatchMessage(Handler.java:99)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.os.Looper.loop(Looper.java:137)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.ActivityThread.main(ActivityThread.java:4424)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at java.lang.reflect.Method.invokeNative(Native Method)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at java.lang.reflect.Method.invoke(Method.java:511)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:787)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at dalvik.system.NativeStart.main(Native Method)
04-07 09:34:08.454: E/AndroidRuntime(12322): Caused by: java.lang.NullPointerException
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.content.ContextWrapper.getPackageName(ContextWrapper.java:127)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.content.ComponentName.<init>(ComponentName.java:75)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.content.Intent.<init>(Intent.java:3122)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at jack.beastapps.TimerPlus.SplashScreen.<init>(SplashScreen.java:36)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at java.lang.Class.newInstanceImpl(Native Method)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at java.lang.Class.newInstance(Class.java:1319)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
04-07 09:34:08.454: E/AndroidRuntime(12322):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1871)
04-07 09:34:08.454: E/AndroidRuntime(12322):    ... 11 more

Answer 1

this 不能用在从构造函数调用的方法中，因为对象还不存在。您应该在 onCreate() 中创建 Intent。在 onCreate() 之前， Activity 中没有代码(构造函数或静态初始化程序除外)运行。所以在 onCreate() 之前不需要它。