android - Android开发人员培训基础知识，文本不会显示

我刚刚从developer.android.com/training/basics开始研究android应用开发，
并且我已经按照所述构建了应用程序，但是当我单击“发送”按钮时，文本不会显示，并且出现错误：不幸的是，FirstApp已停止工作。

以下是DisplayMessageActivity类：

package com.example.firstapp;

import android.annotation.TargetApi;
import android.app.Activity;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.NavUtils;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;

public class DisplayMessageActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    //get the message from intent
    Intent intent=getIntent();
    String message=intent.getStringExtra(MainActivity.EXTRA_MESSAGE);

    //create the text view
    TextView textView = (TextView) findViewById(R.id.m);
    textView.setTextSize(40);
    textView.setText(message);



    super.onCreate(savedInstanceState);

    //set the text view as activity layout
            setContentView(textView);

    // Show the Up button in the action bar.
    setupActionBar();
}

/**
 * Set up the {@link android.app.ActionBar}, if the API is available.
 */
@TargetApi(Build.VERSION_CODES.HONEYCOMB)
private void setupActionBar() {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {
        getActionBar().setDisplayHomeAsUpEnabled(true);
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.display_message, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
    case android.R.id.home:
        // This ID represents the Home or Up button. In the case of this
        // activity, the Up button is shown. Use NavUtils to allow users
        // to navigate up one level in the application structure. For
        // more details, see the Navigation pattern on Android Design:
        //
        // http://developer.android.com/design/patterns/navigation.html#up-vs-back
        //
        NavUtils.navigateUpFromSameTask(this);
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

接下来是activity_display_message.xml：

<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=".DisplayMessageActivity" >

<TextView android:id="@+id/m"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

</RelativeLayout>

以前，activity_display_message中的TextView是这样的：

<TextView android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:name="@string/hello_world" />

这样，在我按下Send按钮之后，输出始终是Hello World！不管我输入什么。

最佳答案

在setContentView()之前，findViewById()方法返回 null。
像这样更改您的onCreate()方法...

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);

        //create the text view
        TextView textView = (TextView) findViewById(R.id.m);
        textView.setTextSize(40);

        //get the message from intent
        Intent intent=getIntent();
        if(intent != null) {
           String message=intent.getStringExtra(MainActivity.EXTRA_MESSAGE);
           if(message != null) {
               textView.setText(message);
           }
        }
        // Show the Up button in the action bar.
        setupActionBar();
    }