Android由：java.lang.NullPointerException

Question

Actually, I am developing an app which can open another app according what user said to the cell phone. For example, when I said 'Facebook', the app will open Facebook automatically. But I face a problem that the debugger showed Caused by: java.lang.NullPointerException .

Here's my source code.

Test.java

package com.example.Voice_Recognizer;
import java.util.ArrayList;
import java.util.List;
import android.app.Activity;
import android.app.Application;
import android.content.Intent;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.graphics.drawable.Drawable;
import android.os.Bundle;
import android.speech.RecognizerIntent;
import android.view.View;
import android.widget.EditText;

public class Test extends Activity
{
    private EditText editText;

    private List<String> label = new ArrayList<String>();
    private List<Drawable> icon = new ArrayList<Drawable>();
    private List<ApplicationInfo> installedApps = new ArrayList<ApplicationInfo>();
    private List<Class> clsName = new ArrayList<Class>();

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        editText = (EditText) findViewById(R.id.edittext);
        getApp();
    }

    public void getApp() {
        PackageManager pm = getPackageManager();
        List<ApplicationInfo> apps = pm.getInstalledApplications(0);
        for(ApplicationInfo app : apps) {
            //checks for flags; if flagged, check if updated system app
            if((app.flags & ApplicationInfo.FLAG_UPDATED_SYSTEM_APP) == 1) {
                installedApps.add(app);
                clsName.add(pm.getClass());
                label.add((String)pm.getApplicationLabel(app));
                icon.add(pm.getApplicationIcon(app));
                //it's a system app, not interested
            } else if ((app.flags & ApplicationInfo.FLAG_SYSTEM) == 1) {
                //Discard this one
                //in this case, it should be a user-installed app
            } else {
                installedApps.add(app);
                clsName.add(pm.getClass());
                label.add((String)pm.getApplicationLabel(app));
                icon.add(pm.getApplicationIcon(app));
            }
        }
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data)
    {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode == 1)
        {
            if (resultCode == Activity.RESULT_OK)
            {
                ArrayList<String> matches = data
                        .getStringArrayListExtra(RecognizerIntent.EXTRA_RESULTS);

                Calculation cal=new Calculation(label, matches, installedApps, clsName);
                /*if (matches.size() > 0)
                {
                    editText.setText(matches.get(0));
                }*/
            }
        }
    }

    public void onClick_Voice_Recognizer(View view)
    {

        Intent intent = new Intent(RecognizerIntent.ACTION_RECOGNIZE_SPEECH);
        intent.putExtra(RecognizerIntent.EXTRA_LANGUAGE_MODEL,
                RecognizerIntent.LANGUAGE_MODEL_FREE_FORM);

        intent.putExtra(RecognizerIntent.EXTRA_PROMPT, "");
        startActivityForResult(intent, 1);
    }
}

Calculation.java

package com.example.Voice_Recognizer;
import android.app.Activity;
import android.content.ComponentName;
import android.content.Intent;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.widget.Toast;

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

public class Calculation extends Activity {
    private List<String> label;
    private List<String> input;
    private List<ApplicationInfo> pkgName;
    private List<Class> clsName;
    private String outputCls;
    private String outputPkg;

    public Calculation (List<String> label, List<String> input, List<ApplicationInfo>pkgName, List<Class> clsName) {
        this.label = new ArrayList<String>(label);
        this.input = new ArrayList<String>(input);
        this.pkgName = new ArrayList<ApplicationInfo>(pkgName);
        this.clsName=new ArrayList<Class>(clsName);
        outputCls="";
        outputPkg="";
        comparison();
    }

    private void comparison() {
        int count = 0;
        for(int i = 0; i < input.size(); i++)
            for(int k = 0; k < label.size(); k++){
                    if(input.get(i).equalsIgnoreCase(label.get(k))) {
                        try {
                            Intent intent = new Intent(Intent.ACTION_MAIN);
                            intent.addCategory(Intent.CATEGORY_LAUNCHER);
                            intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                            intent.setComponent(new ComponentName(pkgName.get(k).packageName, clsName.get(k).getSimpleName()));
                            startActivity(intent);
                        } catch (NullPointerException e) {
                            Toast.makeText(this, "Error", Toast.LENGTH_LONG).show();
                        }

                    }
            }
        //this.count.add(count);
    }

    private static ComponentName unflattenFromString(String str) {
        int sep = str.indexOf('/');
        if (sep < 0 || (sep+1) >= str.length()) {
            return null;
        }
        String pkg = str.substring(0, sep);
        String cls = str.substring(sep+1);
        if (cls.length() > 0 && cls.charAt(0) == '.') {
            cls = pkg + cls;
        }
        return new ComponentName(pkg, cls);
    }
}

The Error Message

Caused by: java.lang.NullPointerException
        at com.example.Voice_Recognizer.Calculation.comparison(Calculation.java:46)
        at com.example.Voice_Recognizer.Calculation.<init>(Calculation.java:31)
        at com.example.Voice_Recognizer.Test.onActivityResult(Test.java:68)

Answer 1

Calculation.java is a Activity class and you have

  Calculation cal=new Calculation(label, matches, installedApps, clsName)

which is wrong. Activity is not a normal java class it has a lifecycle and it is declared in manifest.

You should never instantiate Activity class. You should probably make it normal java class and pass the Activity context to the constructor.

Note : startActivity() is a method of Activity. You will need context.