java - 如何排除空指针异常？

Question

下面的程序抛出 NullPointerException。在 Log cat 中它显示:

01-09 20:40:34.366: D/RemoteIt(2809): java.lang.NullPointerException

单击按钮时，它不会转到 Mousefragment 类。我尝试解决该问题，但无法解决 - 如何解决此问题？

public class connect extends ListActivity implements OnClickListener{
  WifiApManager wifiApManager;
  TextView tv;
  Button ipscan,con;
  ListView lv;
  EditText tbIp;
  private static final String TAG = "RemoteIt";
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.connect);
    tv =(TextView) findViewById(R.id.iptv);
    ipscan=(Button) findViewById(R.id.scan);
    con=(Button) findViewById(R.id.connect);
    tbIp=(EditText) findViewById(R.id.etIp);
    ipscan.setOnClickListener(this);  
    con.setOnClickListener(this);  
    lv = getListView();
  }
  class scan extends AsyncTask<Void, Void, ArrayList<ClientScanResult>>{ 
    public Context context;
    public scan(Context c)  // constructor to take Context
    {
      context = c;   // Initialize your Context variable
    }

    protected ArrayList<ClientScanResult> doInBackground(Void... params) {
      wifiApManager = new WifiApManager(context);  // use the variable here
      return wifiApManager.getClientList(false);

    }

    protected void onPostExecute(ArrayList<ClientScanResult> clients){
      CustomArrayAdapter cus = new CustomArrayAdapter(connect.this,clients);
      lv.setAdapter(cus);   
    }
  }
  @Override
  public void onClick(View v) {
    // TODO Click Action...
    if(v == ipscan){
      scan myScan = new scan(this); // pass the context to the constructor
      myScan.execute();
    }
    if(v == con){
      onConnectButton();
    }
  }
  private void onConnectButton() {
    // TODO When Btn s Clicked...

    String ip = this.tbIp.getText().toString();
    if (ip.matches("^[0-9]{1,4}\\.[0-9]{1,4}\\.[0-9]{1,4}\\.[0-9]{1,4}$")) {
      try {
        Settings.setIp(ip);
        Intent intent = new Intent(connect.this,MouseFragment.class);
        connect.this.startActivity(intent); 
        //Intent i = new Intent(this, MouseFragment.class);
        //this.startActivity(i);
        this.finish();
      } catch (Exception ex) {

        Toast.makeText(this, this.getResources().getText(R.string.invalid_ip), Toast.LENGTH_LONG).show(); **//this toast is displayed**
        Log.d(TAG, ex.toString());
      }
    } else {
      //this.tvError.setText("Invalid IP address");
      //this.tvError.setVisibility(View.VISIBLE);
      Toast.makeText(this, this.getResources().getText(R.string.in_valid), Toast.LENGTH_LONG).show();
    }
  }; 

  /** OS kills process */
  public void onDestroy() {
    super.onDestroy();
  }

  /** App starts anything it needs to start */
  public void onStart() {
    super.onStart();
  }

  /** App kills anything it started */
  public void onStop() {
    super.onStop();
  }
}

编辑

    01-09 21:23:28.663: W/System.err(5657): java.lang.NullPointerException
01-09 21:23:28.665: W/System.err(5657):     at com.arul.remoteit.Settings.setIp(Settings.java:67)
01-09 21:23:28.668: W/System.err(5657):     at com.arul.remoteit.connect.onConnectButton(connect.java:81)
01-09 21:23:28.668: W/System.err(5657):     at com.arul.remoteit.connect.onClick(connect.java:72)
01-09 21:23:28.669: W/System.err(5657):     at android.view.View.performClick(View.java:4445)
01-09 21:23:28.670: W/System.err(5657):     at android.view.View$PerformClick.run(View.java:18429)
01-09 21:23:28.672: W/System.err(5657):     at android.os.Handler.handleCallback(Handler.java:733)
01-09 21:23:28.673: W/System.err(5657):     at android.os.Handler.dispatchMessage(Handler.java:95)
01-09 21:23:28.673: W/System.err(5657):     at android.os.Looper.loop(Looper.java:136)
01-09 21:23:28.674: W/System.err(5657):     at android.app.ActivityThread.main(ActivityThread.java:5081)
01-09 21:23:28.675: W/System.err(5657):     at java.lang.reflect.Method.invokeNative(Native Method)
01-09 21:23:28.677: W/System.err(5657):     at java.lang.reflect.Method.invoke(Method.java:515)
01-09 21:23:28.678: W/System.err(5657):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:781)
01-09 21:23:28.679: W/System.err(5657):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
01-09 21:23:28.680: W/System.err(5657):     at dalvik.system.NativeStart.main(Native Method)
01-09 21:25:34.395: W/IInputConnectionWrapper(5657): getTextBeforeCursor on inactive InputConnection
01-09 21:25:34.513: W/IInputConnectionWrapper(5657): getTextBeforeCursor on inactive InputConnection

单击按钮多少次，就会显示异常并显示 invalid_ip toast 消息

Answer 1

如上所述，您的问题询问如何解决此问题。

您需要弄清楚抛出Exception 的行中的null 是什么。为此，您需要查看堆栈跟踪以查看哪一行导致了问题。然后，您可以使用调试器(或一张纸和一支铅笔)逐步执行程序，或者仅添加打印语句来找出哪个变量为 null。

当您知道哪个变量为 null 时，您可以回溯程序以找出为什么它是 null。当您知道为什么它是 null 时，您就可以解决您的问题。