Java 禁用或/和启用按钮时Android应用程序崩溃

主要活动：

    public class MainActivity extends Activity  implements OnKeyboardActionListener{
    public int start = 0;
private View view;
static Button submit;
final Activity activity = this;
public int stop;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState == null) {
        getFragmentManager().beginTransaction()
        .add(R.id.container, new PlaceholderFragment()).commit();
    }

    //EditText editText = (EditText) findViewById(R.id.result);
    submit = (Button) activity.findViewById(R.id.submit);

}

@Override
public boolean onCreateOptionsMenu(Menu menu) {

    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

/** *包含简单视图的占位符片段。 */

计时器：

package com.example.sum;

import android.os.CountDownTimer;
import android.widget.TextView;

public class timer {

    void timer(final TextView time){
        new CountDownTimer(3000, 1000) {

             public void onTick(long millisUntilFinished) {
                 time.setText("" + millisUntilFinished / 1000);
             }

             public void onFinish() {
                 stop stop = new stop();
                 stop.stop();
                 time.setText("done!");

             }
          }.start();
    }




}

package com.example.sum;

import android.app.Activity;
import android.widget.Button;

public class stop extends MainActivity {

    void stop() {
        // TODO Auto-generated constructor stub
        System.out.println(start);
        //submit.setEnabled(true);
        stop =0;
        //submit.setText("NG ");
        start = 0;
    }

}

---

请共享崩溃的堆栈跟踪。还有，你的占位符碎片在哪个文件中？@cybersam你是什么意思？

package com.example.sum;

import android.os.CountDownTimer;
import android.widget.TextView;

public class timer {

    void timer(final TextView time){
        new CountDownTimer(3000, 1000) {

             public void onTick(long millisUntilFinished) {
                 time.setText("" + millisUntilFinished / 1000);
             }

             public void onFinish() {
                 stop stop = new stop();
                 stop.stop();
                 time.setText("done!");

             }
          }.start();
    }




}

package com.example.sum;

import android.app.Activity;
import android.widget.Button;

public class stop extends MainActivity {

    void stop() {
        // TODO Auto-generated constructor stub
        System.out.println(start);
        //submit.setEnabled(true);
        stop =0;
        //submit.setText("NG ");
        start = 0;
    }

}