java 尝试检索 PackageInfo 时获取空上下文
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Getting Null Context when trying to retrieve PackageInfo
提问by Matteo
Spent the entire day on this but can't get it to work.
花了一整天的时间，但无法让它发挥作用。
I'm trying to create an app drawer. I want get a list of currently installed applications and then put it in an GridLayout.
我正在尝试创建一个应用程序抽屉。我想获取当前安装的应用程序列表，然后将其放入 GridLayout。
I'm getting a nullPointerException when I try to call getPackageManagerfrom a class that extends my MainActivity.
当我尝试getPackageManager从扩展我的 MainActivity 的类调用时，我收到 nullPointerException 。
InstalledApp class:
InstalledApp 类：
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:
在主活动中：
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:
日志猫：
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! :)
提前致谢！:)
回答by michal.luszczuk
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 Contextobject bounded to system.
您正在创建InstalledApp installedApp = new InstalledApp(this); 扩展 Activity 的 installedApp 对象，但此 Activity 不是由系统创建的，而是由您创建的。所以这个 InstalledApp 对象没有绑定到系统的Context对象。
Invoke of getPackageManager().getInstalledPackages(0);is possible because MainActivity extending Context, but its null...
调用 ofgetPackageManager().getInstalledPackages(0);是可能的，因为 MainActivity 扩展了 Context，但它的 null...
Better does not extend MainActivity with InstalledApp. And use context. getPackageManager().getInstalledPackages(0);context field which you pass in InstalledApp constructor
Better 不使用 InstalledApp 扩展 MainActivity。并使用 context. getPackageManager().getInstalledPackages(0);您在 InstalledApp 构造函数中传递的上下文字段
回答by CommonsWare
You cannot just create a subclass of Activity, create an instance of it manually, and expect it to work.
您不能只是创建 的子类Activity，手动创建它的实例，然后期望它工作。
Move the getPackages()and getInstalledApps()methods into MainActivity, and you will have better luck.
将getPackages()和getInstalledApps()方法移到 中MainActivity，您将获得更好的运气。
