尝试检索PackageInfo时获取空上下文

Question

Spent the entire day on this but can't get it to work.

I'm trying to create an app drawer. I want get a list of currently installed applications and then put it in an GridLayout.

I'm getting a nullPointerException when I try to call getPackageManager from a class that extends my MainActivity.

InstalledApp class:

public class InstalledApp extends MainActivity {
Context context = this;

public InstalledApp(Context c) {
    this.context = c;
}

class PInfo {
    public String appname = "";
    public String pname = "";
    public String versionName = "";
    public int versionCode = 0;
    public Drawable icon;
    public void prettyPrint() {
        System.out.println(appname + "\t" + pname + "\t" + versionName + "\t" + versionCode);
    }
}

public ArrayList<PInfo> getPackages() {
    ArrayList<PInfo> apps = getInstalledApps(false); /* false = no system packages */ //Line 29
    final int max = apps.size();
    for (int i=0; i<max; i++) {
        apps.get(i).prettyPrint();
    }
    return apps;
}

private ArrayList<PInfo> getInstalledApps(boolean getSysPackages) {
    ArrayList<PInfo> res = new ArrayList<>();
    List<PackageInfo> packs = getPackageManager().getInstalledPackages(0); //line 39
    for( int i = 0; i < packs.size(); i++ ) {
        PackageInfo p = packs.get(i);
        if ((!getSysPackages) && (p.versionName == null)) {
            continue ;
        }
        InstalledApp.PInfo newInfo = new PInfo();
        newInfo.appname = p.applicationInfo.loadLabel(getPackageManager()).toString();
        newInfo.pname = p.packageName;
        newInfo.versionName = p.versionName;
        newInfo.versionCode = p.versionCode;
        newInfo.icon = p.applicationInfo.loadIcon(getPackageManager());
        res.add(newInfo);
    }
    return res;
}

}

In MainActivity:

InstalledApp installedApp = new InstalledApp(this);
    ArrayList<InstalledApp.PInfo> pInfoArrayList = installedApp.getPackages(); //Line 73
    GridAdapter adapter = new GridAdapter(this, pInfoArrayList);
    gridLayout.setAdapter(adapter);

    gridLayout.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) {
            Toast.makeText(MainActivity.this, "" + position, Toast.LENGTH_SHORT).show();
        }
    });

Log cat:

     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference
        at android.content.ContextWrapper.getPackageManager(ContextWrapper.java:90)
        at squaredem.materiallauncher.InstalledApp.getInstalledApps(InstalledApp.java:39)
        at squaredem.materiallauncher.InstalledApp.getPackages(InstalledApp.java:29)
        at squaredem.materiallauncher.MainActivity.onCreate(MainActivity.java:73)

Thanks in advance! :)

Answer 1

You are creating InstalledApp installedApp = new InstalledApp(this); installedApp object which is extending Activity, but this activity is not created by the system but by you. So this InstalledApp object does not have Context object bounded to system.

Invoke of getPackageManager().getInstalledPackages(0); is possible because MainActivity extending Context, but its null...

Better does not extend MainActivity with InstalledApp. And use context. getPackageManager().getInstalledPackages(0); context. getPackageManager().getInstalledPackages(0); context field which you pass in InstalledApp constructor

Answer 2

You cannot just create a subclass of Activity , create an instance of it manually, and expect it to work.

Move the getPackages() and getInstalledApps() methods into MainActivity , and you will have better luck.