（新手小问题）OnClickListener报空指针异常

[陈橘子]

MainActivity.java 部分代码如下

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();
        }
        bt=(Button)findViewById(R.id.button);
        bt.setOnClickListener(new OnClickListener() {
			
			@Override
			public void onClick(View arg0) {
				// TODO Auto-generated method stub
				System.out.println("dear");
			}
		});
    }

程序生成activity_main.xml，fragment_main.xml两个文件夹
我在fragment_main.xml里面写了个button。

<Button 
        android:id="@+id/button"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="button"
        />

运行起来的话，就除了如下异常：
02-05 19:35:06.791: E/AndroidRuntime(6766): Caused by: java.lang.NullPointerException

如果我把：代码修改成这样子，程序就可以正常运行起来。

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();
        }
        
    }
    @Override
    protected void onStart() {
    	// TODO Auto-generated method stub
    	super.onStart();
    	bt=(Button)findViewById(R.id.button);
        bt.setOnClickListener(new OnClickListener() {
			
			@Override
			public void onClick(View arg0) {
				// TODO Auto-generated method stub
				System.out.println("dear");
			}
		});
    }

那么现在问题来了，第一段代码中bt值为null，第二段代码却能找到bt.这里想不明白。如果在第一段代码里面修改，要怎么修改才能正常？

[陈橘子]

来人啦~！来人啦~！

[网络咖啡]

可能是R.java生成的问题，你清理一下项目试试：Project-Clean菜单

[MR__P]

@Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) { super.onCreateView(inflater, container, savedInstanceState); View rootView = inflater.inflate(R.layout.activity_main, container, false); bt=(Button)rootView.findViewById(R.id.button); bt.setOnClickListener(new OnClickListener() { @Override public void onClick(View arg0) { // TODO Auto-generated method stub System.out.println("dear"); } }); return rootView; }

[哎，真难]

因为fragment里面是不能在oncreate里面绑定id，应该在oncreateView里面绑定ID

[山鹰1985]

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();
	        }
                //用rootView来装载你的fragment布局，然后在该布局上才能找到你的Button
	        View rootView =LayoutInflater.from(this).inflate(R.layout.fragment_main, null); 
	        bt=(Button)rootView.findViewById(R.id.button);
	        bt.setOnClickListener(new OnClickListener() {
	             
	            @Override
	            public void onClick(View arg0) {
	                // TODO Auto-generated method stub
	                System.out.println("dear");
	            }
	        });
	    }