Question

我一直试图在点击按钮时显示已安装的应用程序列表，但是包管理器抛出空指针异常

这是我在点击

时调用的java类

import android.app.ListActivity;
import android.content.Context;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.graphics.drawable.Drawable;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;

import java.util.ArrayList;
import java.util.List;


public class AppList extends AppCompatActivity {

PackageManager packman = getPackageManager();
List<ApplicationInfo> apps = packman.getInstalledApplications(0);
ListView applist;
List<ApplicationInfo> installedApps = new ArrayList<ApplicationInfo>();
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_app_list);

    applist = (ListView)findViewById(R.id.listView);

    for(ApplicationInfo app : apps) {
        //checks for flags; if flagged, check if updated system app
        if((app.flags & ApplicationInfo.FLAG_UPDATED_SYSTEM_APP) != 0) {
            installedApps.add(app);
            //it's a system app, not interested
        } else if ((app.flags & ApplicationInfo.FLAG_SYSTEM) != 0) {
            //Discard this one
            //in this case, it should be a user-installed app
        } else {
            installedApps.add(app);
        }
    }


    CustomAdapter custom = new CustomAdapter();

    applist.setAdapter(custom);


}

class CustomAdapter extends BaseAdapter
{

    @Override
    public int getCount() {
        return installedApps.size();
    }

    @Override
    public Object getItem(int position) {
        return null;
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        convertView = getLayoutInflater().inflate(R.layout.single_row,null);
        ImageView app_face = (ImageView)convertView.findViewById(R.id.app_face);
        TextView app_name = (TextView)convertView.findViewById(R.id.app_name);

        Drawable face = packman.getApplicationIcon(apps.get(position));
        String label = (String)packman.getApplicationLabel(apps.get(position));

        app_face.setImageDrawable(face);
        app_name.setText(label);

        return convertView;
    }
}
}

这是我在运行应用程序时遇到的错误

enter code here

FATAL EXCEPTION: main

Process: com.example.limat.applocker, PID: 17438

java.lang.RuntimeException: Unable to instantiate activity 

Unable to instantiate activity ComponentInfo{com.example.limat.applocker/com.example.limat.applocker.AppList}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference


at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)

at android.app.ActivityThread.-wrap12(ActivityThread.java)

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)

at android.os.Handler.dispatchMessage(Handler.java:102)

at android.os.Looper.loop(Looper.java:154)

at android.app.ActivityThread.main(ActivityThread.java:6119)

at java.lang.reflect.Method.invoke(Native Method)

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference

at com.example.limat.applocker.AppList.<init>(AppList.java:23)

at java.lang.Class.newInstance(Native Method)

at android.app.Instrumentation.newActivity(Instrumentation.java:1078)

at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557)

at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 

at android.app.ActivityThread.-wrap12(ActivityThread.java) 

at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 

at android.os.Handler.dispatchMessage(Handler.java:102) 

at android.os.Looper.loop(Looper.java:154) 

at android.app.ActivityThread.main(ActivityThread.java:6119) 

at java.lang.reflect.Method.invoke(Native Method) 

at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) "

Answer 1

在活动生命周期内初始化包管理器，因为上下文可用。由于始终在上下文中调用getPackageManager，因此早期代码将导致空上下文

您可以在onCreate（）方法

中调用此语句

packman = getPackageManager();

Answer 2

将以下代码行放在onCreate方法

中

PackageManager packman = getPackageManager();
List<ApplicationInfo> apps = packman.getInstalledApplications(0);
ListView applist;
List<ApplicationInfo> installedApps = new ArrayList<ApplicationInfo>();

您正在尝试在onCreate方法之前初始化这些内容。阅读活动生命周期：https://developer.android.com/guide/components/activities/activity-lifecycle.html

包管理器初始化的空指针异常

2 个答案: