Question

所以，我刚开始学习如何编写Java / Android代码但我一直在制作这个真正简单的代码，但我好像被卡住了......

MainAcitivty.Java

package com.example.button;

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.Button;
import android.widget.TextView;
import android.os.Build;

public class MainActivity extends ActionBarActivity {

    int counter;
    Button add, sub;
    TextView display;

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment()).commit();
            counter = 0;
            add = (Button) findViewById(R.id.bAdd);
            sub = (Button) findViewById(R.id.bSub);
            display = (TextView) findViewById(R.id.tvDisplay);

            add.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    counter+=1;
                    display.setText("Your Total is "+ counter);
                }
            });

            sub.setOnClickListener(new View.OnClickListener() {

                @Override
                public void onClick(View v) {
                    // TODO Auto-generated method stub
                    counter-=1;
                    display.setText("Your Total is "+ counter);
                }
            });

        }
    }

    @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);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    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;
        }
    }

}

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
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.button.MainActivity$PlaceholderFragment" >

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:hint="Your Total is 0" 
    android:textSize="40dp"
    android:layout_gravity="center"
    android:gravity="center"
    android:id="@+id/tvDisplay"
    android:orientation="vertical"
    />

<Button
    android:id="@+id/bSub"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/tvDisplay"
    android:layout_below="@+id/tvDisplay"
    android:hint="Subtract one"
    android:textSize="20dp" />

<Button
    android:id="@+id/bAdd"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/bSub"
    android:layout_below="@+id/bSub"
    android:hint="Add one"
    android:textSize="20dp" />

  </RelativeLayout>

AndoridManifest.xml

    <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.button"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.button.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

错误我一直在

    04-04 04:15:22.332: E/AndroidRuntime(1524): FATAL EXCEPTION: main
    04-04 04:15:22.332: E/AndroidRuntime(1524): Process: com.example.button, PID: 1524
    04-04 04:15:22.332: E/AndroidRuntime(1524): java.lang.RuntimeException: Unable to start activity     ComponentInfo{com.example.button/com.example.button.MainActivity}: java.lang.NullPointerException
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.os.Handler.dispatchMessage(Handler.java:102)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.os.Looper.loop(Looper.java:136)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.ActivityThread.main(ActivityThread.java:5017)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at java.lang.reflect.Method.invokeNative(Native Method)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at java.lang.reflect.Method.invoke(Method.java:515)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at dalvik.system.NativeStart.main(Native Method)
    04-04 04:15:22.332: E/AndroidRuntime(1524): Caused by: java.lang.NullPointerException
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at com.example.button.MainActivity.onCreate(MainActivity.java:35)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.Activity.performCreate(Activity.java:5231)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
    04-04 04:15:22.332: E/AndroidRuntime(1524):     ... 11 more

非常感谢他们的帮助：D

将Activity_main更改为Fragment

后出现的新错误

04-04 04:44:46.902: E/AndroidRuntime(1575): FATAL EXCEPTION: main
04-04 04:44:46.902: E/AndroidRuntime(1575): Process: com.example.button, PID: 1575
04-04 04:44:46.902: E/AndroidRuntime(1575): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.button/com.example.button.MainActivity}: java.lang.IllegalArgumentException: No view found for id 0x7f05003c (com.example.button:id/container) for fragment PlaceholderFragment{b1dcdaa0 #0 id=0x7f05003c}
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.os.Handler.dispatchMessage(Handler.java:102)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.os.Looper.loop(Looper.java:136)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.ActivityThread.main(ActivityThread.java:5017)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at java.lang.reflect.Method.invokeNative(Native Method)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at java.lang.reflect.Method.invoke(Method.java:515)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at dalvik.system.NativeStart.main(Native Method)
04-04 04:44:46.902: E/AndroidRuntime(1575): Caused by: java.lang.IllegalArgumentException: No view found for id 0x7f05003c (com.example.button:id/container) for fragment PlaceholderFragment{b1dcdaa0 #0 id=0x7f05003c}
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:930)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1115)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1478)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:570)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.Activity.performStart(Activity.java:5241)
04-04 04:44:46.902: E/AndroidRuntime(1575):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168)
04-04 04:44:46.902: E/AndroidRuntime(1575):     ... 11 more

Answer 1

您的EditText和命令按钮位于fragment_main.xml文件中，因此更改此行

setContentView(R.layout.activity_main);

到

setContentView(R.layout.fragment_main);

Answer 2

你是什么

setContentView(R.layout.activity_main);

按钮添加是片段布局

<Button
android:id="@+id/bAdd"

因此，您的初始化失败导致NullPointerException。

您需要将视图的初始化移至片段。

引起：java.lang.IllegalArgumentException：找不到id的视图片段的0x7f05003c（com.example.button：id / container） PlaceholderFragment {b1dcdaa0＃0 id = 0x7f05003c}

这清楚地表明您在设置为活动的布局中没有带有id容器的ViewGroup。

因为你有

getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();

Answer 3

您的Button，TextView和所有其他UI元素不在activity_main中，但在fragment_main.xml中。因此，请检查您的xml文件，并根据它查找所有UI元素的ID。

您还必须致电

setContentView(R.layout.fragment_main);

而不是

setContentView(R.layout.activity_main);

Answer 4

检查调用时使用的布局名称

setContentView(R.layout.activity_main);

对应于项目res/layout文件夹

中的文件名

如上所述，布局似乎是fragment_main.xml而不是activity_main*.xml ..

Answer 5

问题是您正在尝试从活动中执行片段按钮的单击事件处理。 bAdd和bSub按钮属于片段的布局，但您尝试从活动中访问它们（通过调用活动上的findViewById）。这些调用产生null，因为它们不在活动的布局中。

正确的解决方法是将片段布局中按钮的处理移动到片段类。这就是使用片段的方式：可以在父活动中使用的自包含片段。因此，您应将大部分代码从MainActivity.onCreate移至PlaceholderFragment.onActivityCreated。

或者，如果您并未真正计划重用PlaceholderFragment，则可以将其删除并将所有内容移动到活动的布局中。不能将片段用于简单的用例。

Answer 6

请确保在activity_main布局中有{id} FrameLayout的视图container，如下所示：

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com....MainActivity"
    tools:ignore="MergeRootFrame" />

非常简单的应用程序

6 个答案: