Question

我正在尝试保存并在listView上显示条目，当我点击saveBtn时，会出现以下错误。

尝试在空对象引用上调用虚方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）'“

任何人都可以帮助我吗？

MainActivity.java

import android.content.Context;
import android.content.Intent;
import android.database.Cursor;
import android.media.Image;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.TabHost;
import android.widget.TextView;
import android.widget.Toast;

import org.w3c.dom.Text;

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

public class MainActivity extends Activity_Write {


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

        Button newBtn = (Button) findViewById(R.id.btnNewEntry);

        newBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this, Activity_Write.class));
            }
        });
    }

Activity_Write.java

import android.content.EntityIterator;
import android.content.Intent;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.Menu;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TabHost;
import android.widget.TextView;
import android.widget.Toast;

import org.w3c.dom.Text;

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

/**
 * Created by Koh on 11/11/2015.
 */
public class Activity_Write extends AppCompatActivity {

EditText TitleTxt;
EditText WriteTxt;

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

    TitleTxt = (EditText) findViewById(R.id.txtTitle);
    WriteTxt = (EditText) findViewById(R.id.txtWrite);
    entryListView = (ListView) findViewById(R.id.listView);

    final Button SaveBtn = (Button) findViewById(R.id.btnSave);
    SaveBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            addEntry(TitleTxt.getText().toString(), WriteTxt.getText().toString());
            populateList();
            finish();
            Toast.makeText(getApplicationContext(), "The post has been saved", Toast.LENGTH_SHORT).show();
        }
    });

    TitleTxt.addTextChangedListener(new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            SaveBtn.setEnabled(!TitleTxt.getText().toString().trim().isEmpty());
        }

        @Override
        public void afterTextChanged(Editable s) {

        }
    });
}

private void populateList() {
    ArrayAdapter<Entry> adapter = new EntryListAdaptor();
    entryListView.setAdapter(adapter);
}

private void addEntry(String title, String passage) {
    Entries.add(new Entry(title, passage));
}

public class EntryListAdaptor extends ArrayAdapter<Entry> {
    public EntryListAdaptor() {
        super (Activity_Write.this, R.layout.listview_item, Entries);
    }

    @Override
    public View getView(int position, View view, ViewGroup parent) {
        if (view == null)
            view = getLayoutInflater().inflate(R.layout.listview_item, parent, false);

        Entry currentEntry = Entries.get(position);

        TextView title = (TextView) view.findViewById(R.id.entryTitle);
        title.setText(currentEntry.getTitle());
        TextView passage = (TextView) view.findViewById(R.id.entryPassage);
        passage.setText(currentEntry.getPassage());

        return view;
    }
}

Entry.java

import android.net.Uri;

/**
* Created by Koh on 12/11/2015.
*/
public class Entry {

public String _title, _passage;

public Entry (String title, String passage) {
    _title = title;
    _passage = passage;
}

public String getTitle() {
    return _title;
}
public String getPassage() {
    return _passage;
}

的AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="unipersonalinc.mypersonalediary" >

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/AppTheme"
    android:windowSoftInputMode="stateAlwaysHidden">
    <activity android:name=".MainActivity" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".Activity_Write"></activity>
</application>

</manifest>

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"  android:layout_width="match_parent"
android:layout_height="match_parent"        android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"       tools:context=".MainActivity">

<LinearLayout
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:weightSum="1">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="71dp"
        android:layout_weight="0.35">

        <LinearLayout
            android:orientation="horizontal"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:weightSum="1"
            android:focusableInTouchMode="true">

            <EditText
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/txtSearch"
                android:layout_weight="0.98"
                android:hint="Search..." />

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Search"
                android:id="@+id/btnSearch" />
        </LinearLayout>

        <LinearLayout
            android:orientation="horizontal"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:weightSum="1">

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Sort By Date"
                android:id="@+id/btnSortDate"
                android:layout_gravity="left"
                android:layout_weight="0.90" />

            <Button
                android:layout_width="174dp"
                android:layout_height="wrap_content"
                android:text="Sorty By Title"
                android:id="@+id/btnSortytitle"
                android:layout_gravity="right" />

        </LinearLayout>

    </LinearLayout>

    <ListView
        android:layout_width="wrap_content"
        android:layout_height="252dp"
        android:id="@+id/listView"
        android:layout_weight="0.56" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="New Entry"
        android:id="@+id/btnNewEntry"
        android:layout_gravity="center_horizontal"
        android:layout_marginTop="10dp" />

    </LinearLayout>

</RelativeLayout>

activity_Write.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:id="@+id/Passagetxt">

<ImageView
    android:layout_width="300dp"
    android:layout_height="200dp"
    android:id="@+id/imEntryImage"
    android:layout_gravity="center"
    android:src="@drawable/download"
    android:layout_marginTop="10dp"
    android:layout_marginBottom="0dp" />

<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/txtTitle"
    android:hint="Title..."
    android:layout_marginBottom="2dp" />

<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:inputType="textMultiLine"
    android:ems="10"
    android:id="@+id/txtWrite"
    android:hint="Write Here..."
    android:background="#00000000"
    android:layout_marginBottom="10dp" />

<LinearLayout
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <Button
        style="?android:attr/buttonStyleSmall"
        android:layout_width="100dp"
        android:layout_height="wrap_content"
        android:text="Save"
        android:id="@+id/btnSave"
        android:layout_gravity="right"
        android:enabled="false" />

    <Button
        android:layout_width="100dp"
        android:layout_height="wrap_content"
        android:text="Cancel"
        android:id="@+id/btnCancel"
        android:layout_gravity="right" />
    </LinearLayout>

</LinearLayout>

listView_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal" android:layout_width="match_parent"
android:layout_height="match_parent">

<ImageView
    android:layout_width="100dp"
    android:layout_height="80dp"
    android:id="@+id/ivEntryImage" />

<LinearLayout
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:id="@+id/entryTitle"
        android:layout_marginTop="10dp"
        android:text="Title" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/entryPassage"
        android:layout_marginTop="10dp"
        android:text="Passage" />

</LinearLayout>

DatabaseHandler.Java

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.net.Uri;

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

/**
 * Created by Koh on 13/11/2015.
 */
public class DatabaseHandler extends SQLiteOpenHelper {

private static final int DATABASE_VERSION = 1;

private static final String DATABASE_NAME = "entriesManager",
TABLE_ENTRIES = "entries",
KEY_ID = "id",
KEY_TITLE = "title",
KEY_PASSAGE = "passage",
KEY_IMAGEURI = "imageUri";

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

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("CREATE TABLE " + TABLE_ENTRIES + "(" + KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + KEY_TITLE + " TEXT," + KEY_PASSAGE + " TEXT," + KEY_IMAGEURI + " TEXT)");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_ENTRIES);

    onCreate(db);
}

public void createEntry(Entry entry) {
    SQLiteDatabase db = getWritableDatabase();

    ContentValues values = new ContentValues();


    values.put(KEY_ID,entry.getId());
    values.put(KEY_TITLE, entry.getTitle());
    values.put(KEY_PASSAGE, entry.getPassage());
    values.put(KEY_IMAGEURI, entry.getImageUri().toString());

    db.insert(TABLE_ENTRIES, null, values);
    db.close();
}

public Entry getEntry(int id) {
    SQLiteDatabase db = getReadableDatabase();
    Cursor cursor = db.query(TABLE_ENTRIES, new String[] {KEY_ID, KEY_TITLE, KEY_PASSAGE, KEY_IMAGEURI}, KEY_ID + "=?", new String[] { String.valueOf(id) }, null, null, null, null);

    if (cursor != null)
        cursor.moveToFirst();

    Entry entry = new Entry(Integer.parseInt(cursor.getString(0)), cursor.getString(1), cursor.getString(2), Uri.parse(cursor.getString(3)));
    db.close();
    cursor.close();
    return entry;
}

public void deleteEntry(Entry entry) {
    SQLiteDatabase db = getWritableDatabase();
    db.delete(TABLE_ENTRIES, KEY_ID + "=?", new String[]{String.valueOf(entry.getId())});
    db.close();
}

public int getEntriesCount() {
    SQLiteDatabase db;

    db = getReadableDatabase();
    Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_ENTRIES, null);
    int count = cursor.getCount();
    db.close();
    cursor.close();

    return count;
}

public int updataEntry(Entry entry) {
    SQLiteDatabase db = getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_TITLE, entry.getTitle());
    values.put(KEY_PASSAGE, entry.getPassage());
    values.put(KEY_IMAGEURI, entry.getImageUri().toString());

    return db.update(TABLE_ENTRIES, values, KEY_ID + "=?", new String[]{String.valueOf(entry.getId())});
}

public List<Entry> getAllEntries() {
    List<Entry> entries = new ArrayList<Entry>();

    SQLiteDatabase db = getWritableDatabase();
    Cursor cursor = db.rawQuery("SELECT * FROM " + TABLE_ENTRIES, null);

    if (cursor.moveToFirst()) {
        do {
            Entry entry = new Entry(Integer.parseInt(cursor.getString(0)), cursor.getString(1), cursor.getString(2), Uri.parse(cursor.getString(3)));
            entries.add(entry);
        }
        while (cursor.moveToNext());
    }
    return entries;
}

}

Answer 1

修改MainActivity和Activity_Write的代码，如下所示

<强> MainActivity.java

public class MainActivity extends AppCompatActivity {

    private static final int REQUEST_CODE = 100;

    private List<Entry> entryList;

    private ListView lsvtEntries;
    private EntryListAdaptor adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button newBtn = (Button) findViewById(R.id.btnNewEntry);
        lsvtEntries = (ListView) findViewById(R.id.listView);

        entryList = new ArrayList<>();
        adapter = new EntryListAdaptor();
        lsvtEntries.setAdapter(adapter);


        newBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivityForResult(new Intent(MainActivity.this, Activity_Write.class), REQUEST_CODE);
            }
        });
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == Activity.RESULT_OK) {
            switch (requestCode) {
                case REQUEST_CODE:
                    if (data != null) {
                        String title = data.getStringExtra("Title");
                        String passage = data.getStringExtra("Passage");
                        Entry newEntry = new Entry(title, passage);
                        entryList.add(newEntry);
                        adapter.notifyDataSetChanged();
                    }
                    break;
            }
        }
    }

    private class EntryListAdaptor extends ArrayAdapter<Entry> {

        private TextView tvTitle, tvMessage;

        public EntryListAdaptor() {
            super(MainActivity.this, R.layout.layout_item, entryList);
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            if (convertView == null)
                convertView = LayoutInflater.from(MainActivity.this).inflate(R.layout.layout_item, null);

            tvTitle = (TextView) convertView.findViewById(R.id.entryTitle);
            tvMessage = (TextView) convertView.findViewById(R.id.entryPassage);

            tvTitle.setText(entryList.get(position).getTitle());
            tvMessage.setText(entryList.get(position).getPassage());

            return convertView;
        }
    }
}

<强> Activity_Write.java

public class Activity_Write extends AppCompatActivity {

    EditText TitleTxt;
    EditText WriteTxt;


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

        TitleTxt = (EditText) findViewById(R.id.txtTitle);
        WriteTxt = (EditText) findViewById(R.id.txtWrite);

        final Button SaveBtn = (Button) findViewById(R.id.btnSave);
        SaveBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent data = new Intent();
                data.putExtra("Title", TitleTxt.getText().toString());
                data.putExtra("Passage", WriteTxt.getText().toString());
                setResult(RESULT_OK, data);
                finish();
                Toast.makeText(getApplicationContext(), "The post has been saved", Toast.LENGTH_SHORT).show();
            }
        });

        TitleTxt.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {
                SaveBtn.setEnabled(!TitleTxt.getText().toString().trim().isEmpty());
            }

            @Override
            public void afterTextChanged(Editable s) {

            }
        });
    }

}

需要帮助尝试在空对象引用上调用虚方法'void android.widget.ListView.setAdapter（android.widget.ListAdapter）'

1 个答案: