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我想從FragmentPageAdapter的TextView上改變顏色。當我打電話onHomeScreen或onSettings/onFriends屏幕時，我收到一個NullPointerException。這是解決textviews錯誤的方法嗎？它們在mainmenu.xml中被刪除，並且在onCreate方法中更改顏色/文本時不會發生異常。onPageSelected上的ViewPager和碎片異常。
我的MainMenu.java類。
package com.nordoid.*;
import android.graphics.Color;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v4.view.ViewPager.OnPageChangeListener;
import android.widget.TextView;
public class MainMenu extends FragmentActivity implements OnPageChangeListener {
private MyAdapter mAdapter;
private ViewPager mPager;
// MainMenu bar
TextView home;
TextView settings;
TextView friends;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.mainmenu);
mAdapter = new MyAdapter(getSupportFragmentManager());
mPager = (ViewPager) findViewById(R.id.mainmenu_pager);
mPager.setAdapter(mAdapter);
mPager.setOnPageChangeListener(this);
mPager.setCurrentItem(1);
home = (TextView) findViewById(R.id.mmenu_home);
settings = (TextView) findViewById(R.id.mmenu_settings);
friends = (TextView) findViewById(R.id.mmenu_friends);
// init starting position color.
//home.setTextColor(Color.WHITE);
home.setText("HOME");
settings.setText("SETTINGS");
friends.setText("FRIENDS")
}
public static class MyAdapter extends FragmentPagerAdapter {
public MyAdapter(FragmentManager fm) {
super(fm);
}
@Override
public int getCount() {
return 3;
}
@Override
public Fragment getItem(int position) {
switch (position) {
case 0:
return new Mfragment_settings();
case 1:
return new Mfragment_main();
case 2:
return new Mfragment_friends();
default:
return null;
}
}
}
public void onPageScrollStateChanged(int arg0) {
}
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
public void onPageSelected(int arg0) {
switch (arg0) {
case 0:
onSettingsScreen();
break;
case 1:
onHomeScreen();
break;
case 2:
onFriendsScreen();
break;
}
}
public void onHomeScreen() {
home.setTextColor(Color.WHITE);
System.out.println("2");
friends.setTextColor(Color.GRAY);
settings.setTextColor(Color.GRAY);
}
public void onSettingsScreen() {
settings.setTextColor(Color.WHITE);
friends.setTextColor(Color.GRAY);
home.setTextColor(Color.GRAY);
System.out.println("PAGE 1");
}
public void onFriendsScreen() {
friends.setTextColor(Color.WHITE);
System.out.println("3");
home.setText(Color.GRAY);
settings.setTextColor(Color.GRAY);
}
}
而且我mainmenu.xml文件..
<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:background="@drawable/mfrag_home" >
<android.support.v4.view.ViewPager
android:id="@+id/mainmenu_pager"
android:layout_width="match_parent"
android:layout_height="match_parent" />
<LinearLayout
android:id="@+id/linearLayout1"
android:layout_width="wrap_content"
android:layout_height="27dp"
android:layout_alignParentLeft="true"
android:layout_alignParentRight="true"
android:layout_marginTop="45dp" >
<TextView
android:id="@+id/mmenu_settings"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingLeft="15dp"
android:paddingTop="2dp"
android:text="TextView"
android:textColor="@color/White" />
<TextView
android:id="@+id/mmenu_home"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingLeft="45dp"
android:paddingTop="2dp"
android:text="TextView"
android:textColor="@color/White" />
<TextView
android:id="@+id/mmenu_friends"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:paddingLeft="49dp"
android:paddingTop="2dp"
android:text="TextView"
android:textColor="@color/White" />
</LinearLayout>
，最後我logcat的錯誤。
06-19 00:52:51.346: E/AndroidRuntime(21457): FATAL EXCEPTION: main
06-19 00:52:51.346: E/AndroidRuntime(21457): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.nordoid.tracephone/com.nordoid.tracephone.MainMenu}: java.lang.NullPointerException
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2071)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2096)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.ActivityThread.access$600(ActivityThread.java:138)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1207)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.os.Handler.dispatchMessage(Handler.java:99)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.os.Looper.loop(Looper.java:213)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.ActivityThread.main(ActivityThread.java:4788)
06-19 00:52:51.346: E/AndroidRuntime(21457): at java.lang.reflect.Method.invokeNative(Native Method)
06-19 00:52:51.346: E/AndroidRuntime(21457): at java.lang.reflect.Method.invoke(Method.java:511)
06-19 00:52:51.346: E/AndroidRuntime(21457): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
06-19 00:52:51.346: E/AndroidRuntime(21457): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
06-19 00:52:51.346: E/AndroidRuntime(21457): at dalvik.system.NativeStart.main(Native Method)
06-19 00:52:51.346: E/AndroidRuntime(21457): Caused by: java.lang.NullPointerException
06-19 00:52:51.346: E/AndroidRuntime(21457): at com.nordoid.tracephone.MainMenu.onHomeScreen(MainMenu.java:116)
06-19 00:52:51.346: E/AndroidRuntime(21457): at com.nordoid.tracephone.MainMenu.onPageSelected(MainMenu.java:103)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:541)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:507)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.support.v4.view.ViewPager.setCurrentItem(ViewPager.java:488)
06-19 00:52:51.346: E/AndroidRuntime(21457): at com.nordoid.tracephone.MainMenu.onCreate(MainMenu.java:41)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.Activity.performCreate(Activity.java:5008)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
06-19 00:52:51.346: E/AndroidRuntime(21457): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2035)
06-19 00:52:51.346: E/AndroidRuntime(21457): ... 11 more