异步任务的空指针异常

Question

我正在处理我的网络服务，我遇到了一个奇怪的问题，我无法解决这个问题，因为我是 android 开发的新手。在异步任务的onPostExecute() 方法中显示吐司时出现空指针异常。请有人帮我解决这个问题

if (result.booleanValue()) // here at this line I am getting the error in logcat//

提前谢谢....！

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    ///////////male female buttons code/////////////
    Button regmalebtn = (Button) findViewById(R.id.regmalebtn); 
    regmalebtn.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            gender="M";
            System.out.println("gender value after  male button click ="+gender);
            Log.v(TAG,"gender value on  male button click ="+gender );
        }
    });
    Button regfemalebtn = (Button)findViewById(R.id.regfemalebtn);
    regfemalebtn.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub
            gender ="F";
            // System.out.println("gender value on  female button click ="+gender);
            Log.v(TAG,"gender value on  female button click ="+gender );
        }
    });

    ////////////male female button code will ends here/////////////

    Button signin = (Button) findViewById(R.id.regsubmitbtn);
    signin.setOnClickListener(new OnClickListener() {
        public void onClick(View v) {
            new RegisterTask().execute();
        };
    });
}


private class RegisterTask extends AsyncTask<Void, Void, Boolean> {
    private final ProgressDialog dialog = new ProgressDialog(Register.this);

    protected void onPreExecute() {
        this.dialog.setMessage("Registering...");
        this.dialog.show();
    }


    //  @SuppressWarnings("unused")
    protected Boolean doInBackground(final Void unused) {
        return this.register(); //don't interact with the ui!
    }

    private Boolean register() {
        ************some code for webservices************
    }

    return null;

}

protected void onPostExecute(final Boolean result) {
    if (this.dialog.isShowing()) {
        this.dialog.dismiss();
    }
    if (result.booleanValue()) {
        // show register success dialog
        // I m getting the error at above line

        Toast.makeText(Register.this, "Registerd", Toast.LENGTH_SHORT).show();
    }
    else {
        Toast.makeText(Register.this, "Try Again", Toast.LENGTH_SHORT).show();
    }
}

@Override
protected Boolean doInBackground(Void... params) {
    // TODO Auto-generated method stub
    return register();
}


}   
}

这些是 logcat 错误

06-28 16:55:56.685: WARN/dalvikvm(708): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
06-28 16:55:56.695: ERROR/AndroidRuntime(708): FATAL EXCEPTION: main
06-28 16:55:56.695: ERROR/AndroidRuntime(708): java.lang.NullPointerException
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at com.soap.Register$RegisterTask.onPostExecute(Register.java:193)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at com.soap.Register$RegisterTask.onPostExecute(Register.java:1)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at android.os.AsyncTask.finish(AsyncTask.java:417)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at android.os.AsyncTask.access$300(AsyncTask.java:127)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:429)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at android.os.Looper.loop(Looper.java:123)
06-28 16:55:56.695: ERROR/AndroidRuntime(708):     at android.app.ActivityThread.main(ActivityThread.java:4627)

Answer 1

尝试使用您的activity.class 的名称作为Toa​​st 的参数。我认为在你的情况下是 Register.class 而不是 Register.this

Answer 2

您显然是从 register 方法返回 null 。在调用 result.booleanValue() 之前返回非空值或进行空值检查。

你可以这样做：

if (result == null) {

}
else if (result.booleanValue()) {

}
else {

}

对了，为什么需要返回一个布尔值而不是布尔值（原始类型，不能为空）。

Answer 3

尝试将上下文传递给变量。或尝试Register.this.getApplicationContext() 或BaseContext。检查对话框是否为空，传递值并在postExecute() 中创建新对话框，如果这与您的要求不重叠:)

Answer 4

我写了这个

protected void onPostExecute(String unused) {
            dismissDialog(DIALOG_DOWNLOAD_PROGRESS);

            Toast.makeText(
                    myclassname.this,
                    "Downloading of " + FILENAME
                            + " complete.", Toast.LENGTH_LONG).show();
            refreshList();


        }

这对我有用..

see this link for more detail
http://hi.baidu.com/hi_android/blog/item/fea149f90da7fdedfd037f56.html
http://www.screaming-penguin.com/node/7746

Answer 5

通过像这样启动 Toast，我摆脱了同样的 Toast/Async/Nullpointe 问题：

Toast.makeText(getApplicationContext(), "Hello Toast!", Toast.LENGTH_SHORT).show();