Java Android-应用程序无法启动活动组件信息

我目前正在学习Android编程。我知道这个错误在这个论坛上出现了数百万次，但请帮我找到我错过的

问题是，当我在android emulator上运行我的应用程序时，应用程序崩溃并显示弹出窗口：“不幸的是，数据Mahasiswa已停止。”

以下是整个logcat堆栈跟踪：

06-11 17:50:52.203 1700-1700/com.example.user.tugasss I/art: Late-enabling -Xcheck:jni
06-11 17:50:52.203 1700-1700/com.example.user.tugasss W/art: Unexpected CPU variant for X86 using defaults: x86
06-11 17:50:52.783 1700-1700/com.example.user.tugasss W/System: ClassLoader referenced unknown path: /data/app/com.example.user.tugasss-1/lib/x86
06-11 17:50:53.324 1700-1700/com.example.user.tugasss D/AndroidRuntime: Shutting down VM
06-11 17:50:53.325 1700-1700/com.example.user.tugasss E/AndroidRuntime: FATAL EXCEPTION: main
                                                                        Process: com.example.user.tugasss, PID: 1700
                                                                        java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.user.tugasss/com.example.user.tugasss.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setTitle(int)' on a null object reference
                                                                            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                                                                            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                                                                            at android.app.ActivityThread.-wrap12(ActivityThread.java)
                                                                            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                                                                            at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                            at android.os.Looper.loop(Looper.java:154)
                                                                            at android.app.ActivityThread.main(ActivityThread.java:6077)
                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
                                                                         Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setTitle(int)' on a null object reference
                                                                            at com.example.user.tugasss.MainActivity.setFragmentTitle(MainActivity.java:111)
                                                                            at com.example.user.tugasss.MainActivity.onCreate(MainActivity.java:52)
                                                                            at android.app.Activity.performCreate(Activity.java:6664)
                                                                            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
                                                                            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
                                                                            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                                                                            at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                                                                            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                                                                            at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                            at android.os.Looper.loop(Looper.java:154) 
                                                                            at android.app.ActivityThread.main(ActivityThread.java:6077) 
                                                                            at java.lang.reflect.Method.invoke(Native Method) 
                                                                            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755) 

DataBaseHelper.java

   package com.example.user.tugasss;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class DataBaseHelper extends SQLiteOpenHelper {

    private static final String DATABASE_NAME = "mahasiswadb";
    private static final int DATABASE_VERSION = 1;

    public static final String TABEL_MAHASISWA = "mahasiswa";
    public static final String TABEL_ANGKATAN = "angkatan";

    public static final String ID            = "id";
    public static final String NAMA          = "nama";
    public static final String TANGGAL_LAHIR = "tgllahir";
    public static final String JURUSAN       = "jurusan";
    public static final String ANGKATAN      = "agt_id";


    public static final String CREATE_TABEL_MAHASISWA = "CREATE TABLE IF NOT EXISTS "
            + TABEL_MAHASISWA + "(" + ID + " INTEGER PRIMARY KEY, "
            + NAMA + " TEXT, " + JURUSAN + " TEXT, "
            + TANGGAL_LAHIR + " DATE, " + ANGKATAN + "INT, " + "FOREIGN KEY(" + ANGKATAN + ") REFERENCES "
            + TABEL_ANGKATAN + " (id) " + ");";

    public static final String CREATE_TABEL_ANGKATAN = "CREATE TABLE IF NOT EXISTS "
            + TABEL_ANGKATAN + "(" + ID + " INTEGER PRIMARY KEY,"
            + NAMA + " TEXT"+");";

    private static DataBaseHelper instance;

    public static synchronized DataBaseHelper getHelper(Context context) {
        if (instance == null)
            instance = new DataBaseHelper(context);
        return instance;
    }

    private DataBaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }
    @Override
    public void onOpen(SQLiteDatabase db)
    {
        super.onOpen(db);
        if (!db.isReadOnly()){
            //enable foreign key constraints
            db.execSQL("PRAGMA foreign_keys=ON;");
        }
    }

    @Override
    public void onCreate(SQLiteDatabase db) {

        db.execSQL(CREATE_TABEL_ANGKATAN);
        db.execSQL(CREATE_TABEL_MAHASISWA);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    }
}

MainActivity.java

package com.example.user.tugasss;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.Menu;
import android.view.MenuItem;

import com.example.user.tugasss.CustomDialogFragment.DialogFragmentListener;

public class MainActivity extends FragmentActivity implements DialogFragmentListener {

    private Fragment contentFragment;
    private ListFragment mahasiswaListFragment;
    private AddFragment mahasiswaAddFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        FragmentManager fragmentManager = getSupportFragmentManager();
        AngkatanDAO agtDAO =  new AngkatanDAO(this);

        //angkatans
        if(agtDAO.getAngkatans().size() <= 0)
            agtDAO.loadAngkatans();
        /*
         * This is called when orientation is changed.
         */
        if (savedInstanceState != null) {
            if (savedInstanceState.containsKey("content")) {
                String content = savedInstanceState.getString("content");
                if (content.equals(AddFragment.ARG_ITEM_ID)) {
                    if (fragmentManager
                            .findFragmentByTag(AddFragment.ARG_ITEM_ID) != null) {
                        setFragmentTitle(R.string.add_mhs);
                        contentFragment = fragmentManager
                                .findFragmentByTag(AddFragment.ARG_ITEM_ID);
                    }
                }
            }
            if (fragmentManager.findFragmentByTag(ListFragment.ARG_ITEM_ID) != null) {
                mahasiswaListFragment = (ListFragment) fragmentManager
                        .findFragmentByTag(ListFragment.ARG_ITEM_ID);
                contentFragment = mahasiswaListFragment;
            }
        } else {
            mahasiswaListFragment = new ListFragment();
            setFragmentTitle(R.string.app_name);
            switchContent(mahasiswaListFragment, ListFragment.ARG_ITEM_ID);
        }
    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        if (contentFragment instanceof AddFragment) {
            outState.putString("content", AddFragment.ARG_ITEM_ID);
        } else {
            outState.putString("content", ListFragment.ARG_ITEM_ID);
        }
        super.onSaveInstanceState(outState);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.action_add:
                setFragmentTitle(R.string.add_mhs);
                mahasiswaAddFragment = new AddFragment();
                switchContent(mahasiswaAddFragment, AddFragment.ARG_ITEM_ID);

                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /*
     * We consider EmpListFragment as the home fragment and it is not added to
     * the back stack.
     */
    public void switchContent(Fragment fragment, String tag) {
        FragmentManager fragmentManager = getSupportFragmentManager();
        while (fragmentManager.popBackStackImmediate())
            ;

        if (fragment != null) {
            FragmentTransaction transaction = fragmentManager
                    .beginTransaction();
            transaction.replace(R.id.content_frame, fragment, tag);

            if (!(fragment instanceof ListFragment)) {
                transaction.addToBackStack(tag);
            }
            transaction.commit();
            contentFragment = fragment;
        }
    }

    protected void setFragmentTitle(int resourseId) {
        setTitle(resourseId);
        getActionBar().setTitle(resourseId);

    }

    /*
     * We call super.onBackPressed(); when the stack entry count is > 0. if it
     * is instanceof EmpListFragment or if the stack entry count is == 0, then
     * we prompt the user whether to quit the app or not by displaying dialog.
     * In other words, from EmpListFragment on back press it quits the app.
     */
    @Override
    public void onBackPressed() {
        FragmentManager fm = getSupportFragmentManager();
        if (fm.getBackStackEntryCount() > 0) {
            super.onBackPressed();
        } else if (contentFragment instanceof ListFragment
                || fm.getBackStackEntryCount() == 0) {
            //tampilkan dialog ketika keluar
            onShowQuitDialog();
        }
    }

    public void onShowQuitDialog() {
        AlertDialog.Builder builder = new AlertDialog.Builder(this);
        builder.setCancelable(false);

        builder.setMessage("Anda Ingin Keluar?");
        builder.setPositiveButton(android.R.string.yes,
                new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int id) {
                        finish();
                    }
                });
        builder.setNegativeButton(android.R.string.no,
                new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int id) {
                        dialog.cancel();
                    }
                });
        builder.create().show();
    }

    @Override
    public void onFinishDialog() {
        if (mahasiswaListFragment != null) {
            mahasiswaListFragment.updateView();
        }
    }
}

package com.example.user.tugass；
导入android.app.AlertDialog；
导入android.content.DialogInterface；
导入android.os.Bundle；
导入android.support.v4.app.Fragment；
导入android.support.v4.app.FragmentActivity；
导入android.support.v4.app.FragmentManager；
导入android.support.v4.app.FragmentTransaction；
导入android.view.Menu；
导入android.view.MenuItem；
导入com.example.user.tugasss.CustomDialogFragment.DialogFragmentListener；
公共类MainActivity扩展FragmentActivity实现DialogFragmentListener{
私有片段内容片段；
私有ListFragment-mahasiswaListFragment；
私有AddFragment-mahasiswaAddFragment；
@凌驾
创建时受保护的void（Bundle savedInstanceState）{
super.onCreate（savedInstanceState）；
setContentView（R.layout.activity_main）；
FragmentManager FragmentManager=getSupportFragmentManager（）；
AngkatanDAO agtDAO=新的AngkatanDAO（本）；
//安加达人
如果（agtDAO.getAngkatans（）.size（）0
*是instanceof EmpListFragment，或者如果堆栈项计数==0，则
*我们通过显示对话框提示用户是否退出应用程序。
*换句话说，从背面的EmpListFragment按它退出应用程序。
*/
@凌驾
public void onBackPressed（）{
FragmentManager fm=getSupportFragmentManager（）；
if（fm.getBackbackEntryCount（）>0）{
super.onBackPressed（）；
}else if（ListFragment的contentFragment实例
||fm.getBackbackEntryCount（）==0）{
//坦皮尔坎对话
onShowQuitDialog（）；
}
}
公共void onShowQuitDialog（）{
AlertDialog.Builder=新建AlertDialog.Builder（此）；
builder.setCancelable（false）；
builder.setMessage（“Anda Ingin Keluar？”）；
builder.setPositiveButton（android.R.string.yes，
新建DialogInterface.OnClickListener（）{
@凌驾
public void onClick（DialogInterface对话框，int-id）{
完成（）；
}
});
builder.setNegativeButton（android.R.string.no，
新建DialogInterface.OnClickListener（）{
public void onClick（DialogInterface对话框，int-id）{
dialog.cancel（）；
}
});
builder.create（）.show（）；
}
@凌驾
public void onFinishDialog（）{
if（mahasiswaListFragment！=null）{
mahasiswaListFragment.updateView（）；
}
}
}

---

有人能帮我解决这个问题吗？

这里使用这些字符串创建您的表：

我不确定您上一个参数TABEL_ANGKATAN想要什么，所以如果您想要其他相应的更改，我现在将其作为文本

public static final String CREATE_TABEL_MAHASISWA = "CREATE TABLE IF NOT EXISTS "
            + TABEL_MAHASISWA + "(" + ID + " INTEGER PRIMARY KEY, "
            + NAMA + " TEXT, " + JURUSAN + " TEXT, "
            + TANGGAL_LAHIR + " DATE" + ")"
            + TABEL_ANGKATAN + " TEXT"+");";

//corrected the second query as it was incomplete
    public static final String CREATE_TABEL_ANGKATAN = "CREATE TABLE IF NOT EXISTS "
            + TABEL_ANGKATAN + "(" + ID + " INTEGER PRIMARY KEY,"
            + NAMA + " TEXT"+");";

---

使用这些将删除您当前遇到的错误

了解如何调试和读取错误消息。查看错误消息中的create table SQL语句，您将看到您编写的create table字符串错误。您的SQL查询无效。您必须更正它。嘿，谢谢您的帮助。但是我遇到了另一个错误。您能帮助我吗又是我吗？请阅读错误，这是一个空指针异常，您正在设置操作栏标题。您的操作栏为空，请先初始化它，然后为操作栏设置标题