追加字符串到StringBuffer，給NullPointerException

Question

我有一個問題，我想追加一個字符串到StringBuffer對象，但它給我的代碼中的NullPointerException。請檢查並告訴代碼出錯的地方，以及最佳的解決方案是什麼？

錯誤堆棧：

11-03 18:13:29.672: ERROR/AndroidRuntime(17973): FATAL EXCEPTION: main 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example/com.example.TestCryptoActivity}: java.lang.NullPointerException 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2663) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.ActivityThread.access$2300(ActivityThread.java:125) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.os.Handler.dispatchMessage(Handler.java:99) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.os.Looper.loop(Looper.java:123) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.ActivityThread.main(ActivityThread.java:4627) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at java.lang.reflect.Method.invokeNative(Native Method) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at java.lang.reflect.Method.invoke(Method.java:521) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at dalvik.system.NativeStart.main(Native Method) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973): Caused by: java.lang.NullPointerException 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at com.example.TestCryptoActivity.onCreate(TestCryptoActivity.java:83) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2627) 
11-03 18:13:29.672: ERROR/AndroidRuntime(17973):  ... 11 more 

代碼：

String str; 
    StringBuffer strBuf; 
    @Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.main); 

     String data = "xyzzy734499639E0022505@a2+;%d3-"; 


     try { 
      str = getHashCode(data); 
      strBuf.append(str); -----------------> This is the error position 
     } catch (NoSuchAlgorithmException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     }catch(Exception e) 
     { 
      e.printStackTrace(); 
     } 


     for(int i=1;i<31;i++) 
     { 
      if(i<10) 
      { 
       str = String.valueOf(30)+String.valueOf(30+i)+str; 
       try { 
        str = new String(getHashCode(str)); 
       } catch (NoSuchAlgorithmException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
       } 
      }else 
      { 
       char[] num = String.valueOf(i).toCharArray(); 
       String firstIndex = String.valueOf(num[0]); 
       String secondIndex = String.valueOf(num[1]); 
       str = firstIndex + secondIndex+ str; 
       try { 
        str = new String(getHashCode(str)); 
       } catch (NoSuchAlgorithmException e) { 
        // TODO Auto-generated catch block 
        e.printStackTrace(); 
       } 
      } 
      strBuf.append(str); 
      System.out.println("The final string after hashing:"+str); 
     } 
    } 
    public String getHashCode(String str) throws NoSuchAlgorithmException 
    { 

     MessageDigest md = MessageDigest.getInstance("SHA-512"); 
     md.update(str.getBytes()); 

     byte byteData[] = md.digest(); 

     //convert the byte to hex format method 1 
     StringBuffer sb = new StringBuffer(); 
     for (int i = 0; i < byteData.length; i++) { 
      sb.append(Integer.toString((byteData[i] & 0xff) + 0x100, 16).substring(1)); 
     } 

     System.out.println("Hex format : " + sb.toString()); 
     return sb.toString(); 
    } 

Answer 1

未初始化。

strBuf = new StringBuffer(); 

Answer 2

將您的代碼中的第二行更改爲此。

StringBuffer strBuf=new StringBuffer(); 

希望這會對你有用。 :)

Answer 3

public class MainActivity extends Activity ActivityBuffer strBuf;

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 
StringBuffer strBuf=new StringBuffer(); 

這也適用於您的代碼。