Question

我刚开始从developer.android.com/training/basics学习android app开发，并且我已经构建了应用程序，但是当我单击“发送”按钮时，文本将不会显示，并且错误：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);
}

}

以下是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按钮后，输出总是Hello World！不管我输入什么。

Answer 1

在setContentView()之前，findViewById()方法返回 null 像这样改变你的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();
    }

Answer 2

请拥有“activity_display_message.xml”，如下所示：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />

</LinearLayout>

请点击“activity_hello_world_main.xml”：

<?xml version="1.0" encoding="utf-8"?>  
<LinearLayout 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:orientation="horizontal">

    <EditText android:id="@+id/edit_message"
        android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:hint="@string/edit_message" >
    </EditText>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/btn_send"
        android:onClick="sendMessage" />

</LinearLayout>

请像这样使用“DisplayMessageActivity.Java”：

public class DisplayMessageActivity extends Activity {
    @SuppressLint("NewApi")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_display_message);

        Intent intent = getIntent();
        String message =   intent.getStringExtra(HelloWorldMainActivity.EXTRA_MESSAGE);

        TextView textView = new TextView(this);
        textView.setTextSize(40);
        textView.setText(message);

        setContentView(textView);
    }

请像这样：“HelloWorldMainActivity.java”：

public class HelloWorldMainActivity extends Activity {
    public static final String EXTRA_MESSAGE = "com.example.helloworld.MESSAGE";

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_hello_world_main);
    }

    public void sendMessage(View view) {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText) findViewById(R.id.edit_message);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}

让strings.xml看起来像这样：

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <string name="app_name">Hello World !!!</string>
    <string name="edit_message">Enter a message</string>
    <string name="btn_send">Send</string>
    <string name="action_settings">Settings</string>
    <string name="title_activity_main">MainActivity</string>
    <string name="title_activity_display_message">DisplayMessageActivity</string>
    <string name="hello_world">Hello world!</string>

</resources>

Android开发人员培训基础知识，文字不会显示

2 个答案: