无法启动活动 ComponentInfo NPE [重复]

Question

我正在尝试使用 Kotlin 和 Java 在 Android 上制作应用程序。

我的灵感来自这个教程：

https://www.youtube.com/watch?v=Qg3L_B9--zY

我稍微改了一下，现在出现了一个漂亮的错误：

“原因：java.lang.NullPointerException：尝试在空对象引用上调用虚拟方法 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()'”

这是错误日志：

11-21 10:46:03.006 27125-27125/? E/libprocessgroup: failed to make and chown /acct/uid_10058: Read- 
only file system
11-21 10:46:03.006 27125-27125/? W/Zygote: createProcessGroup failed, kernel missing CONFIG_CGROUP_CPUACCT?

11-21 10:46:03.006 27125-27125/? I/art: Not late-enabling -Xcheck:jni (already on)
11-21 10:46:03.079 27125-27125/com.example.budgetapp I/System.out: Util Constructor
11-21 10:46:03.143 27125-27125/com.example.budgetapp E/MainActivity: before onCreate()
11-21 10:46:03.150 27125-27125/com.example.budgetapp W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter androidx.vectordrawable.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
11-21 10:46:03.183 27125-27125/com.example.budgetapp E/MainActivity: after onCreate()

11-21 10:46:03.206 27125-27125/com.example.budgetapp I/art: Rejecting re-init on previously-failed class java.lang.Class<androidx.core.view.ViewCompat$2>
11-21 10:46:03.206 27125-27125/com.example.budgetapp I/art: Rejecting re-init on previously-failed class java.lang.Class<androidx.core.view.ViewCompat$2>
11-21 10:46:03.275 27125-27125/com.example.budgetapp E/MainActivity: after setContentView()
11-21 10:46:03.275 27125-27125/com.example.budgetapp D/AndroidRuntime: Shutting down VM


    --------- beginning of crash
11-21 10:46:03.276 27125-27125/com.example.budgetapp E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.budgetapp, PID: 27125
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.budgetapp/com.example.budgetapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
        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:5254)
        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 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
        at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:149)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:99)
        at android.content.Context.obtainStyledAttributes(Context.java:437)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
        at Util.setupUIViews(Util.java:43)
        at com.example.budgetapp.MainActivity.onCreate(MainActivity.kt:27)
        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:2387) 
        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:5254) 
        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) 

这是我的课程：

MainActivity.kt：

package com.example.budgetapp

import Util
import android.app.DatePickerDialog
import android.os.Bundle
import android.util.Log
import androidx.appcompat.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*
import java.util.*

class MainActivity : AppCompatActivity() {
    private val tag = "MainActivity"
    var util = Util()


    override fun onCreate(savedInstanceState: Bundle?) {
        Log.e(tag, "before onCreate()")

        super.onCreate(savedInstanceState)
        Log.e(tag, "after onCreate()")

        setContentView(R.layout.activity_main)
        Log.e(tag, "after setContentView()")



        util.setupUIViews()
        Log.e(tag, "after setupUIView()")

        util.initToolbar()
        Log.e(tag, "after initToolbar()")

        util.setupListView()
        Log.e(tag, "after setupListView()")



        //Calendars:
        val c = Calendar.getInstance()
        val year = c.get(Calendar.YEAR)
        val month = c.get(Calendar.MONTH)
        val day = c.get(Calendar.DAY_OF_MONTH)

        val c2 = Calendar.getInstance()
        val year2 = c2.get(Calendar.YEAR)
        val month2 = c2.get(Calendar.MONTH)
        val day2 = c2.get(Calendar.DAY_OF_MONTH)

        Log.e(tag, "year1: $year")
        Log.e(tag, "month1: $month")
        Log.e(tag, "month2: $month2")
        Log.e(tag, "day1: $day")

        //button click to show DatePickerDialog
        pickDateBtn.setOnClickListener {
            val dpd = DatePickerDialog(
                this,
                DatePickerDialog.OnDateSetListener {_, mYear, mMonth, mDay ->
                    var monthSel=mMonth+1
                    dateTv2.text = "$mDay/$monthSel/$mYear"
                },
                year,
                month,
                day
            )
            dpd.show()

            val dpd2 = DatePickerDialog(
                this,
                DatePickerDialog.OnDateSetListener {_, mYear2, mMonth2, mDay2 ->
                    var monthSel2=mMonth2+1
                    dateTv.text = "$mDay2/$monthSel2/$mYear2"
                },
                year2,
                month2,
                day2
            )
            dpd2.show()
        }
    }
}

SimpleAdapter.java：

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;

import com.example.budgetapp.R;

public class SimpleAdapter extends BaseAdapter {
    public Context mContext;
    private LayoutInflater layoutInflater;
    private TextView title, description;
    private String[] titleArray, descriptionArray;
    private ImageView imageView;


   public SimpleAdapter(Context context, String[] title, String [] description){
            mContext= context;
            titleArray=title;
            descriptionArray= description;
            layoutInflater= LayoutInflater.from(context);
        }



    @Override
    public int getCount(){
        return titleArray.length;
    }

    @Override
    public Object getItem(int position) {
        return  titleArray[position];
    }

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

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        if(convertView == null) {
            layoutInflater.inflate(R.layout.main_activity_single_item, null);
        }
        title= convertView.findViewById(R.id.tvMain);
        description = convertView.findViewById(R.id.tvDescription);
        imageView = convertView.findViewById(R.id.ivMain);

        title.setText(titleArray[position]);
        description.setText(descriptionArray[position]);

        if(titleArray[position].equalsIgnoreCase("Timetable")){
            imageView.setImageResource(R.drawable.timetable);
        }else if(titleArray[position].equalsIgnoreCase("Subjects")){
            imageView.setImageResource(R.drawable.book);
        }else if(titleArray[position].equalsIgnoreCase("Faculty")) {
            imageView.setImageResource(R.drawable.play);
        }else{
             imageView.setImageResource(R.drawable.settings);
            }
    return convertView;
    }
}

和 Util.java：

import android.widget.ListView;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import com.example.budgetapp.R;

import java.util.logging.Logger;

public class Util extends AppCompatActivity {

   public Util(){
       System.out.println("Util Constructor");
   }

    public Toolbar getToolbar() {
        return toolbar;
    }

    public void setToolbar(Toolbar toolbar) {
        this.toolbar = toolbar;
    }

    public ListView getListView() {
        return listView;
    }

    public void setListView(ListView listView) {
        this.listView = listView;
    }

    @Override
    public String toString() {
        return "Util{" +
                "toolbar=" + toolbar +
                ", listView=" + listView +
                '}';
    }

    private Toolbar toolbar;
    private ListView listView;
    Logger logger;

        public void setupUIViews(){
            toolbar = findViewById(R.id.ToolbarMain);
            logger.info("inside Util class, setupUiViews, before findViewById(lvMain)");
            listView = findViewById(R.id.lvMain);
        }

        public void initToolbar(){
            setSupportActionBar(toolbar);
            getSupportActionBar().setTitle(" App");
        }

        public void setupListView(){
            String[] title = getResources().getStringArray(R.array.Main);
            String[] description=getResources().getStringArray(R.array.Description);

            SimpleAdapter simpleAdapter = new SimpleAdapter(this, title, description);
            listView.setAdapter(simpleAdapter);
        }
}

代码在从 Util 类进入 setupUIViews() 方法之前就崩溃了。

请你看看，告诉我如何解决这个问题？

我调试了，但不明白问题出在哪里。

另外，查看了其他类似的问题，但每个示例都是特定的，所以我无法推断我的情况。

非常感谢您的宝贵时间和帮助！

Answer 1

你调用 util. setupUIViews()，在内部方法中，它调用了findViewById(R.id.ToolbarMain)。该方法属于Activity，我们看源码

return getWindow().findViewById(id);

//in the window, (PhoneWindow)
@Nullable
    public <T extends View> T findViewById(@IdRes int id) {
        return getDecorView().findViewById(id);
    }
// the DecorView is the basic view, it set when you call setContentView
public void setContentView(@LayoutRes int layoutResID) {
        getWindow().setContentView(layoutResID);
        initWindowDecorActionBar();
    }
//PhoneWindow setContentView
@Override
    public void setContentView(View view) {
        setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));
    }
@Override
    public void setContentView(View view, ViewGroup.LayoutParams params) {
  // decor, when theme attributes and the like are crystalized. Do not 
  // check the feature
        // before this happens.
        if (mContentParent == null) {
            installDecor(); // this method init DecorView
       } else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
            mContentParent.removeAllViews();
        }
...
}

但你不调用 util setContenView 来设置它的视图。所以它会报告错误。

而且您的代码编写方式是错误的。我们在活动或片段自己的类中设置视图； 我们不能使用另一个活动来设置其他活动。

在 android 中，每个活动都有自己的窗口。尝试将 Utils 方法移入 MainActiviy。

Answer 2

util 类是从 AppCompatActivity 扩展的，但未正确初始化。您不能以这种方式使用 AppCompatActivity。相反，您需要执行以下操作：

class MainActivity : AppCompatActivity() {
    private val tag = "MainActivity"
    lateinit var util: Util

    override fun onCreate(savedInstanceState: Bundle?) {
        util  = Util(this)
    }
}

还有 Util（在 kotlin 中）类：

class Util(private delegate:MainActivity) {
    public void setupUIViews(){
        toolbar = delegate.findViewById(R.id.ToolbarMain);
        listView = delegate.findViewById(R.id.lvMain);
    }
}

另外，我认为使用这种模式并将所有与视图相关的内容移动到“Util”类并不是最好的。使用 MVP 或 MVVM 等其他模式将获得更好的结果。