Android由：java.lang.NullPointerException

Question

实际上，我正在开发一个可以根据用户对手机说的内容打开另一个应用程序的应用程序。 例如，当我说“ Facebook”时，该应用程序将自动打开Facebook。 但是我遇到了调试器显示的问题， Caused by: java.lang.NullPointerException 。

这是我的源代码。

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);
    }
}

错误讯息

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是一个Activity类，您拥有

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

这是错误的。 Activity不是普通的Java类，它具有生命周期，并在清单中声明。

您永远不要实例化Activity类。 您可能应该使它成为普通的Java类，并将Activity上下文传递给构造函数。

注意： startActivity()是Activity的一种方法。 您将需要上下文。