获取NullPointer异常

时间：2015-09-12 16:29:39

标签： java android nullpointerexception

此代码用于制作标签：

import android.app.ActionBar;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.media.Image;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;

import android.os.Bundle;
import android.widget.FrameLayout;

public class MainActivity extends FragmentActivity implements ActionBar.TabListener {              

ActionBar actionBar;

ViewPager viewPager;

protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
viewPager= (ViewPager) findViewById(R.id.pager);
viewPager.setAdapter(new MyAdapter(getSupportFragmentManager()));


actionBar=getActionBar();//Here is error Null Pointer
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
actionBar.setDisplayShowTitleEnabled(true);
ActionBar.Tab Tab1=actionBar.newTab();
Tab1.setText("Text");
Tab1.setTabListener(this);
ActionBar.Tab Tab2=actionBar.newTab();
Tab2.setText("Image");
Tab2.setTabListener(this);
ActionBar.Tab Tab3=actionBar.newTab();
Tab3.setText("About Us");
Tab3.setTabListener(this);
actionBar.addTab(Tab1);
actionBar.addTab(Tab2);
actionBar.addTab(Tab3);

}


@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {

}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {

}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {

}
}

class MyAdapter extends FragmentPagerAdapter
{

public MyAdapter(android.support.v4.app.FragmentManager fm) {
super(fm);
}

@Override
public Fragment getItem(int position) {
Fragment fragment=null;
if(position==0)
{
fragment=new FragmentAFragment();
}
if(position==1)
{
fragment=new FragmentBFragment();
}
if(position==2)
{
fragment=new FragmentCFragment();
}
return fragment;
}

@Override
public int getCount() {
return 3;
}
}

我还使用了ActionBarActivity和getSupportActivity，但还有另一个不兼容错误。

这是 LogCat 输出：

   09-12 22:23:10.286    3822-3822/? I/art﹕ Not late-enabling -Xcheck:jni (already on)
    09-12 22:23:10.325    3822-3829/? I/art﹕ Debugger is no longer active
    09-12 22:23:10.470    3822-3822/? D/AndroidRuntime﹕ Shutting down VM
    09-12 22:23:10.471    3822-3822/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
        Process: com.example.santosh.encode, PID: 3822
        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.santosh.encode/com.example.santosh.encode.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' on a null object reference
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
                at android.app.ActivityThread.access$800(ActivityThread.java:151)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
                at android.os.Handler.dispatchMessage(Handler.java:102)
                at android.os.Looper.loop(Looper.java:135)
                at android.app.ActivityThread.main(ActivityThread.java:5257)
                at java.lang.reflect.Method.invoke(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:372)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setNavigationMode(int)' on a null object reference
                at com.example.santosh.encode.MainActivity.onCreate(MainActivity.java:34)
                at android.app.Activity.performCreate(Activity.java:5990)
                at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
                at android.app.ActivityThread.access$800(ActivityThread.java:151)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
                at android.os.Handler.dispatchMessage(Handler.java:102)
                at android.os.Looper.loop(Looper.java:135)
                at android.app.ActivityThread.main(ActivityThread.java:5257)
                at java.lang.reflect.Method.invoke(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:372)
                at 

com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
                at com.android.internal.os.Zyg

oteInit.main(ZygoteInit.java:698)
        09-12 22:23:11.142    3822-3834/? I/art﹕ Background partial concurrent mark sweep GC freed 369(79KB) AllocSpace objects, 0(0B) LOS objects, 53% free, 442KB/954KB, paused 8.882ms total 35.959ms

这是我的android manifest.xml ：

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/app"
        android:label="@string/app_name"
        android:theme="@style/Theme.AppCompat" >
        <activity
            android:name=".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>
        <activity
            android:name=".FragmentA"
            android:label="@string/title_activity_fragment" >
        </activity>
        <activity
            android:name=".FragmentB"
            android:label="@string/title_activity_fragment_b" >
        </activity>
        <activity
            android:name=".FragmentC"
            android:label="@string/title_activity_fragment_c" >
        </activity>
    </application>

</manifest>

现在有些编辑已完成，但仍面临问题。

package com.example.santosh.encode;
import com.example.santosh.encode.Base64;

import android.app.ActionBar;
import android.app.ActionBar;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.content.Intent;
import android.media.Image;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;



import android.os.Bundle;
import android.support.v7.app.ActionBarActivity;
import android.view.Window;



public class MainActivity extends FragmentActivity implements  ActionBar.TabListener {


    ActionBar actionBar;
    ViewPager viewPager;

    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);



         actionBar=getActionBar();
        getWindow().requestFeature(Window.FEATURE_ACTION_BAR);


        setContentView(R.layout.activity_main);
        actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        viewPager= (ViewPager) findViewById(R.id.pager);

        viewPager.setAdapter(new MyAdapter(getSupportFragmentManager()));


        actionBar.setDisplayShowTitleEnabled(true);
        ActionBar.Tab Tab1=actionBar.newTab();
        Tab1.setText("Text");
        Tab1.setTabListener(this);
        ActionBar.Tab Tab2=actionBar.newTab();
        Tab2.setText("Image");
        Tab2.setTabListener(this);
        ActionBar.Tab Tab3=actionBar.newTab();
        Tab3.setText("About Us");
        Tab3.setTabListener(this);
        actionBar.addTab(Tab1);
        actionBar.addTab(Tab2);
        actionBar.addTab(Tab3);

    }


    @Override
    public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {

    }

    @Override
    public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {

    }

    @Override
    public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {

    }
}

class MyAdapter extends FragmentPagerAdapter
{

    public MyAdapter(android.support.v4.app.FragmentManager fm) {
        super(fm);
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment=null;
        if(position==0)
        {
            fragment=new FragmentAFragment();
        }
        if(position==1)
        {
            fragment=new FragmentBFragment();
        }
        if(position==2)
        {
            fragment=new FragmentCFragment();
        }
        return fragment;
    }

    @Override
    public int getCount() {
        return 3;
    }
}

现在LogCat显示..................... 

09-13 01:06:13.967    5526-5526/com.example.santosh.encode I/art﹕ Not late-enabling -Xcheck:jni (already on)
09-13 01:06:14.086    5526-5526/com.example.santosh.encode D/AndroidRuntime﹕ Shutting down VM
09-13 01:06:14.087    5526-5526/com.example.santosh.encode E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.santosh.encode, PID: 5526
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.santosh.encode/com.example.santosh.encode.MainActivity}: android.util.AndroidRuntimeException: requestFeature() must be called before adding content
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: android.util.AndroidRuntimeException: requestFeature() must be called before adding content
            at com.android.internal.policy.impl.PhoneWindow.requestFeature(PhoneWindow.java:302)
            at com.example.santosh.encode.MainActivity.onCreate(MainActivity.java:36)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
09-13 01:06:14.112    5526-5533/com.example.santosh.encode I/art﹕ Debugger is no longer active
09-13 01:06:14.124    5526-5533/com.example.santosh.encode W/art﹕ Suspending all threads took: 12.092ms

2 个答案:

答案 0 :(得分：0)

我认为这是问题所在.. 如果你使用SUPPORTLIBRARY►android.support.v7.app.ActionBar

你必须得到ActionBar的支持。

所以，改变它

actionBar=getActionBar();

到

actionBar = getSupportActionBar();

答案 1 :(得分：0)

将导入更改为：

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBarActivity;

并且做：

actionBar = getSupportActionBar();
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