Question

我正在尝试开始在Android上创建我的第一个真正的练习应用程序，但我遇到了一些麻烦。这是一个应用程序，将包含一种待办事项列表样式格式的“想法”。我完成了将要显示的活动以及将添加想法的活动，但是在单击按钮时应用程序停止工作。调试后我发现原因在于onClick方法，特别是：

部分

06-26 22:44:26.590    1136-1136/com.ivywire.ideastem           E/AndroidRuntime: FATAL EXCEPTION: main
        java.lang.NullPointerException
        at com.ivywire.ideastem.IdeasDbAdapter.createIdea(IdeasDbAdapter.java:78)
        at com.ivywire.ideastem.IdeaAddActivity$1.onClick(IdeaAddActivity.java:44)
        at android.view.View.performClick(View.java:4204)
        at android.view.View$PerformClick.run(View.java:17355)
        at android.os.Handler.handleCallback(Handler.java:725)
        at android.os.Handler.dispatchMessage(Handler.java:92)
        at android.os.Looper.loop(Looper.java:137)
        at android.app.ActivityThread.main(ActivityThread.java:5041)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:511)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
        at dalvik.system.NativeStart.main(Native Method)

所以我试着拿出一些零件

Idea idea = new Idea(name, summary);
databaseHandler.createIdea(idea);

取出时有效。这是完整的活动代码。

import android.content.Intent;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.view.View.OnClickListener;


public class IdeaAddActivity extends Activity {
    IdeasDbAdapter databaseHandler;

    private EditText ideaName;
    private EditText ideaSummary;
    private Button addButton;

    String name;
    String summary;

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

        ideaName = (EditText) findViewById(R.id.ideaName);
        ideaSummary = (EditText) findViewById(R.id.ideaSummary);
        addButton = (Button) findViewById(R.id.addIdeaButton);

        name = ideaName.getText().toString();
        summary = ideaSummary.getText().toString();

        databaseHandler = new IdeasDbAdapter(this);

        addButton.setOnClickListener(addListener);
    }

    private OnClickListener addListener = new OnClickListener(){
        public void onClick(View view){
            Idea idea = new Idea(name, summary);
            databaseHandler.createIdea(idea);
            sendToMain(view);
        }
    };

    public void sendToMain(View view){
        Intent intent = new Intent(this, MainActivity.class);
        startActivity(intent);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.idea_add, menu);
        return true;
    }

}

这是IdeasDbAdapter类：

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class IdeasDbAdapter {

    public static final String KEY_ROWID = "_id";
    public static final String KEY_NAME = "name";
    public static final String KEY_SUMMARY = "summary";

    private static final String TAG = "IdeasDbAdapter";
    private DatabaseHelper mDbHelper;
    private SQLiteDatabase mDb;

    private static final String DATABASE_NAME = "Brainstorm";
    private static final String SQLITE_TABLE = "Ideas";
    private static final int DATABASE_VERSION = 1;

    private final Context mCtx;

    private static final String DATABASE_CREATE =
            "CREATE TABLE if not exists " + SQLITE_TABLE + " (" +
                    KEY_ROWID + " integer PRIMARY KEY autoincrement," +
                    KEY_NAME + "," +
                    KEY_SUMMARY + ");";

    private static class DatabaseHelper extends SQLiteOpenHelper {

        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }


        @Override
        public void onCreate(SQLiteDatabase db) {
            Log.w(TAG, DATABASE_CREATE);
            db.execSQL(DATABASE_CREATE);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                    + newVersion + ", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXISTS " + SQLITE_TABLE);
            onCreate(db);
        }
    }

    public IdeasDbAdapter(Context ctx) {
        this.mCtx = ctx;
    }

    public IdeasDbAdapter open() throws SQLException {
        mDbHelper = new DatabaseHelper(mCtx);
        mDb = mDbHelper.getWritableDatabase();
        return this;
    }

    public void close() {
        if (mDbHelper != null) {
            mDbHelper.close();
        }
    }

    public long createIdea(Idea idea) {
        String name = idea.getName();
        String summary = idea.getSummary();
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_NAME, name);
        initialValues.put(KEY_SUMMARY, summary);

        return mDb.insert(SQLITE_TABLE, null, initialValues);
    }

    public boolean deleteAllIdeas() {

        int doneDelete = 0;
        doneDelete = mDb.delete(SQLITE_TABLE, null , null);
        Log.w(TAG, Integer.toString(doneDelete));
        return doneDelete > 0;

    }

    public Cursor fetchIdeasByName(String inputText) throws SQLException {
        Log.w(TAG, inputText);
        Cursor mCursor = null;
        if (inputText == null  ||  inputText.length () == 0)  {
            mCursor = mDb.query(SQLITE_TABLE, new String[] {KEY_ROWID,
                    KEY_NAME, KEY_SUMMARY},
                    null, null, null, null, null);

        }
        else {
            mCursor = mDb.query(true, SQLITE_TABLE, new String[] {KEY_ROWID,
                    KEY_NAME, KEY_SUMMARY},
                    KEY_NAME + " like '%" + inputText + "%'", null,
                    null, null, null, null);
        }
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;

    }

    public Cursor fetchAllIdeas() {

        Cursor mCursor = mDb.query(SQLITE_TABLE, new String[] {KEY_ROWID,
                KEY_NAME, KEY_SUMMARY},
                null, null, null, null, null);

        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }
}

nullpointer异常中引用的第78行是

return mDb.insert(SQLITE_TABLE, null, initialValues);

如果有人能给我提示，因为我很困惑并尽可能地搜索，我会很感激。

Answer 1

我不知道你的其他代码上的字符串但看看我的代码，看看你要做什么

addactivity：

 saveBtn.setOnClickListener(new OnClickListener() {
        @Override
        public void onClick(View v) {

           memo = new Memo();
            memo.setHeadLine(headLineTxt.getText().toString());
            memo.setFullMemo(fullMemoTxt.getText().toString());
                              memo = datasource.createMemo(memo);
                  finish();
        }
    });

IdeasDbAdapter：

public  Memo createMemo(Memo memo) {
    //Used to store data like : COLUMN_NAME , VALUE.
    ContentValues values = new ContentValues();
    values.put(COLUMN_HEAD_LINE, memo.getHeadLine());
    values.put(COLUMN_FULL_MEMO, memo.getFullMemo());
    //Insert new memo and get the id of it.
    long insertId = database.insert(TABLE_NAME, null,
            values);

我希望它对你有所帮助。

Answer 2

我正在寻找一个很好的教程，解释如何为它创建一个好的SQLite数据库和内容提供程序。我找到了这个，我认为如果你遵循它，你将拥有一个良好的架构，访问，删除或升级你的数据没有问题：

http://www.vogella.com/articles/AndroidSQLite/article.html#todo

尝试向SQLite数据库添加条目会阻止应用程序正常工作

2 个答案: