Android ScrollView錯誤

Question

我正試圖恢復當我離開應用程序和旋轉應用程序時滾動視圖的位置。

出於某種原因，總是拋出這個錯誤：

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.edonfreiner.siddur/com.example.edonfreiner.siddur.Benching}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference 

下面是代碼：

package com.example.edonfreiner.siddur; 

import android.support.v4.view.ScrollingView; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.util.Log; 
import android.widget.ScrollView; 

import java.util.logging.Logger; 

public class Benching extends AppCompatActivity { 
    ScrollView mScrollView = (ScrollView) findViewById(R.id.benchingScroll); 

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


    protected void onSaveInstanceState(Bundle outState) { 
     super.onSaveInstanceState(outState); 
     Log.d("Rotated", "rotated"); 
     outState.putIntArray("ARTICLE_SCROLL_POSITION", 
       new int[]{ mScrollView.getScrollX(), mScrollView.getScrollY()}); 
    } 


    protected void onRestoreInstanceState(Bundle savedInstanceState) { 
     super.onRestoreInstanceState(savedInstanceState); 
     Log.d("Rotated1", "rotated1"); 
     final int[] position = savedInstanceState.getIntArray("ARTICLE_SCROLL_POSITION"); 
     if(position != null) 
      mScrollView.post(new Runnable() { 
       public void run() { 
        mScrollView.scrollTo(position[0], position[1]); 
       } 
      }); 
    } 




} 

它工作正常了最後2種方法，類是在清單和錯誤是從第12行（scrollview對象被實例化的地方）拋出的。id也是一個有效的id。 謝謝你這麼多

Answer 1

ScrollView mScrollView; 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_benching); 
    mScrollView = (ScrollView) findViewById(R.id.benchingScroll); 
} 

的findViewbyId是必須有一個實例方法中。 例如見我已經把它的onCreate

Answer 2

你應該寫ScrollView mScrollView = (ScrollView) findViewById(R.id.benchingScroll);內的onCreate（）後的setContentView（R.layout.activity_benching）;更改您的代碼像下面

package com.example.edonfreiner.siddur; 
import android.support.v4.view.ScrollingView; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.util.Log; 
import android.widget.ScrollView; 

import java.util.logging.Logger; 

public class Benching extends AppCompatActivity { 
    ScrollView mScrollView; 

    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_benching); 
     mScrollView = (ScrollView) findViewById(R.id.benchingScroll);  
    } 
protected void onSaveInstanceState(Bundle outState) { 
     super.onSaveInstanceState(outState); 
     Log.d("Rotated", "rotated"); 
     outState.putIntArray("ARTICLE_SCROLL_POSITION", 
       new int[]{ mScrollView.getScrollX(), mScrollView.getScrollY()}); 
    } 
    protected void onRestoreInstanceState(Bundle savedInstanceState) { 
     super.onRestoreInstanceState(savedInstanceState); 
     Log.d("Rotated1", "rotated1"); 
     final int[] position = savedInstanceState.getIntArray("ARTICLE_SCROLL_POSITION"); 
     if(position != null) 
      mScrollView.post(new Runnable() { 
       public void run() { 
        mScrollView.scrollTo(position[0], position[1]); 
       } 
      }); 
    } 
}