Android 房間數據庫問題

Question

我試圖在 class 練習中完成這個小練習，我們必須使用房間數據庫保存數據，然后在 ListView 中顯示該信息。

這個應用程序是關於一個玩家的。 玩家有 4 個字段（Id、Name、Position、進球數）。

• 第一個保存的玩家的 ID 應該是 #1，第二個玩家的 ID 應該是 #2，依此類推。

• 用戶必須通過 EditText 字段輸入播放器的名稱。

• 然后有三個單選按鈕，用戶必須選擇 select 一個才能選擇他們的 position（守門員、防守、前鋒）。 最后，用戶必須使用 EditText 字段輸入該球員的進球數。

• 最后，用戶必須使用 EditText 字段輸入該球員的進球數。

一旦用戶單擊“保存”按鈕，之前選擇的所有字段都將被清除，它們將是一條顯示玩家 ID# 的快速吐司消息。

為了查看所有保存的數據，用戶必須單擊“查看全部”按鈕，這會將他們帶到第二個活動並顯示球員的姓名，Position，在下一個活動中得分的進球數。

我對房間不太熟悉，所以每當我按下“保存”保存按鈕時，我的應用程序就會崩潰。 任何幫助將不勝感激，謝謝！

MainActivity.Java

public class MainActivity extends AppCompatActivity {

private EditText playerEdt, goalsEdt;
private int id = 0;

private RadioGroup groupRad;
private RadioButton radioButton;

private MyDatabase myDb;

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

    playerEdt = findViewById(R.id.edtPlayer);
    goalsEdt = findViewById(R.id.edtGoals);

    myDb = MyDatabase.getInstance(MainActivity.this);


}

public void saveData(View view) {
    id++;
    String name = playerEdt.getText().toString();

    groupRad = findViewById(R.id.radGroup);

    int selectedId = groupRad.getCheckedRadioButtonId();

    radioButton  = findViewById(selectedId);

    String position = radioButton.getText().toString();

    String goalsString = goalsEdt.getText().toString();

    int goals = Integer.valueOf(goalsString);

    Player player = new Player(id, name, position, goals);

    myDb.playerDao().insert(player);

    playerEdt.setText("");
    goalsEdt.setText("");
    groupRad.clearCheck();



}

public void viewData(View view) {
    Intent intent = new Intent(MainActivity.this, SecondActivity.class);

    startActivity(intent);
}
}

第二活動.Java

public class SecondActivity extends AppCompatActivity {

ListView listView;
MyDatabase database;

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

    listView = findViewById(R.id.listView);

    database = MyDatabase.getInstance(SecondActivity.this);

    List<Player> players = database.playerDao().getAll();

    ArrayAdapter adapter = new ArrayAdapter(SecondActivity.this, android.R.layout.simple_list_item_1, players);

    listView.setAdapter(adapter);
}
}

播放器.Java

@Entity
public class Player {
@PrimaryKey
private int id;

@ColumnInfo(name = "player_name")
private String name;

@ColumnInfo(name = "player_position")
private String position;

@ColumnInfo(name = "player_goals")
private int goals;

public Player(int id, String name, String position, int goals) {
    this.id = id;
    this.name = name;
    this.position = position;
    this.goals = goals;
}

public int getId() {
    return id;
}

public void setId(int id) {
    this.id = id;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getPosition() {
    return position;
}

public void setPosition(String position) {
    this.position = position;
}

public int getGoals() {
    return goals;
}

public void setGoals(int goals) {
    this.goals = goals;
}
}

PlayerDao.java

@Dao
public abstract class  PlayerDao {
@Insert
public abstract void insert (Player player);

@Delete
public abstract void delete (Player player);

@Update
public abstract void update (Player player);


@Query("select * from Player")
public abstract List<Player> getAll();


}

MyDatabase.Java

@Database(entities = Player.class, version = 1)
public abstract class MyDatabase extends RoomDatabase {

public abstract PlayerDao playerDao();

private static MyDatabase instance;

public  static MyDatabase getInstance(Context context){

    if( instance == null){
        instance = Room.databaseBuilder(context, MyDatabase.class, "PlayerDb")
                .allowMainThreadQueries()
                .build();

    }

    return instance;
}
}

activity_main.xml： https://www.codepile.net/pile/d5rq8mx2

activity_second.xml： https://www.codepile.net/pile/2vbYzXq3

Answer 1

看來您有許多問題：-

您需要為 Player class 的所有成員設置吸氣劑。

添加了這些（和設置器）：-

public int getGoals() {
    return goals;
}

public void setGoals(int goals) {
    this.goals = goals;
}

public String getPosition() {
    return position;
}

public void setPosition(String position) {
    this.position = position;
}

核心問題是在saveData中查找視圖的嘗試返回空值，因此您會收到 null 指針異常 (NPE)，因為視圖不存在於 SAVE 按鈕中。

解決方案是查找視圖（即在 onCreate 方法中使用 findViewById）。

缺乏數據驗證也會導致問題。 以下版本的 MainActivity.java 處理問題：-

public class MainActivity extends AppCompatActivity {

    private EditText playerEdt, goalsEdt;
    private int id = 0;

    private RadioGroup groupRad;
    private RadioButton radioButton;
    private MyDatabase myDb;

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

        playerEdt = findViewById(R.id.edtPlayer);
        goalsEdt = findViewById(R.id.edtGoals);
        groupRad = findViewById(R.id.radGroup);
        myDb = MyDatabase.getInstance(MainActivity.this);
    }

    public void saveData(View view) {
        id++;
        String name = playerEdt.getText().toString();
        if (name.length() < 1) {
            Toast.makeText(view.getContext(),"Player Name is blank. Try again.",Toast.LENGTH_SHORT).show();
            playerEdt.requestFocus();
        }
        radioButton = findViewById(groupRad.getCheckedRadioButtonId());
        if (radioButton == null) {
            Toast.makeText(view.getContext(),"You must select Goalie Defence or Forward. Try again",Toast.LENGTH_SHORT).show();
            return;
        }
        String position = radioButton.getText().toString();

        String goalsString = goalsEdt.getText().toString();
        int goals = 0;
        try {
            goals = Integer.valueOf(goalsString);
        } catch (Exception e) {
            Toast.makeText(view.getContext(),"You must give the number of Goals. try again.",Toast.LENGTH_SHORT).show();
            goalsEdt.requestFocus();
        }

        Player player = new Player(id, name, position, goals);
        if (myDb.playerDao().insert(player) < 1) {
            Toast.makeText(view.getContext(),"Player not Added (duplicate)",Toast.LENGTH_SHORT).show();
            return;
        }
        playerEdt.setText("");
        goalsEdt.setText("");
        groupRad.clearCheck();
        Toast.makeText(view.getContext(),"Player Added. Name is " + name + " Position is " + position + " Goals = " + goals,Toast.LENGTH_SHORT).show();
    }

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

此外，PlayerDao.java 已更改為在播放器名稱重復時不會失敗，並且在插入、刪除或更新時也返回有用的值。 這是：-

@Dao
public abstract class  PlayerDao {
    @Insert(onConflict = OnConflictStrategy.IGNORE)
    public abstract long insert (Player player);

    @Delete
    public abstract int delete (Player player);

    @Update
    public abstract int update (Player player);


    @Query("select * from Player")
    public abstract List<Player> getAll();

}

Answer 2

首先，如果您在 Player.java id中使用autoGenerate = true會更好：

@Entity
public class Player {
@PrimaryKey(autoGenerate = true)
private int id;  

通過這樣做，您不必給您的玩家 ID，Room 會為您完成這項工作。

並且對於保存時應用崩潰，您必須檢查日志並查看導致應用崩潰的原因。 更新您的問題，以便用戶可以幫助您。