android - 为什么应用程序在尝试初始化 textview 时停止？

Question

我尝试了一个简单的 Activity ，但是当我按下按钮时，我的应用程序停止了。我知道这是很基本的但是

这是我的 Activity

package com.example.chelo;

import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.os.Build;

public class MainActivity extends ActionBarActivity {
    TextView tv;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView)findViewById(R.id.textView1);
        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();           
        }       
    }       
    @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);
    }

    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
            return rootView;
        }
    }
    public void Clicar(View view){

        tv.setText("WORKS");
    }

}

这是我的 xml 文件。

    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.chelo.MainActivity$PlaceholderFragment" >

    <TextView
        android:id="@+id/textView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_below="@+id/textView1"
        android:layout_marginTop="16dp"
        android:onClick="Clicar"
        android:text="Button" />

</RelativeLayout>

这是日志猫。

03-10 23:41:43.760: E/AndroidRuntime(32585): FATAL EXCEPTION: main
03-10 23:41:43.760: E/AndroidRuntime(32585): java.lang.IllegalStateException: Could not execute method of the activity
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.view.View$1.onClick(View.java:3095)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.view.View.performClick(View.java:3567)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.view.View$PerformClick.run(View.java:14224)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.os.Handler.handleCallback(Handler.java:605)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.os.Handler.dispatchMessage(Handler.java:92)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.os.Looper.loop(Looper.java:137)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.app.ActivityThread.main(ActivityThread.java:4517)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at java.lang.reflect.Method.invokeNative(Native Method)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at java.lang.reflect.Method.invoke(Method.java:511)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:993)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:760)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at dalvik.system.NativeStart.main(Native Method)
03-10 23:41:43.760: E/AndroidRuntime(32585): Caused by: java.lang.reflect.InvocationTargetException
03-10 23:41:43.760: E/AndroidRuntime(32585):    at java.lang.reflect.Method.invokeNative(Native Method)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at java.lang.reflect.Method.invoke(Method.java:511)
03-10 23:41:43.760: E/AndroidRuntime(32585):    at android.view.View$1.onClick(View.java:3090)
03-10 23:41:43.760: E/AndroidRuntime(32585):    ... 11 more
03-10 23:41:43.760: E/AndroidRuntime(32585): Caused by: java.lang.NullPointerException
03-10 23:41:43.760: E/AndroidRuntime(32585):    at com.example.chelo.MainActivity.Clicar(MainActivity.java:64)
03-10 23:41:43.760: E/AndroidRuntime(32585):    ... 14 more

Answer 1

问题似乎是textview属于fragment，而不是activity，所以这一行

tv = (TextView)findViewById(R.id.textView1);

不会工作。

尝试将其移动到 fragment 中的onCreateView()，然后像这样更改

tv = (TextView)rootView.findViewById(R.id.textView1);