Question

今天在写android时，只要给监听事件就会出错，提示“很抱歉，XXX已停止运行”！代码如下：

package com.example.klsahkl;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
public class MainActivity extends Activity {
private Button b1 ;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

b1 = (Button)findViewById(R.id.bt);
//只要设置按键监听就会提示“很抱歉，XXX已停止运行。”
b1.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
// TODO Auto-generated method stub

}
});

setContentView(R.layout.activity_main);
}

@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;
}

}
提示错误：
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.klsahkl/com.example.klsahkl.MainActivity}: java.lang.NullPointerException

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)

Answer 1

把 setContentView(R.layout.activity_main);移到super的下面就可以了。

Answer 2

setContentView(R.layout.activity_main)没写，所以没绑定那个button，当你设置监听器时这个button就会报空指针异常，懂了不？ 够详细吧