[英]Not able to retrieve data of a single row in room database by selecting it
主要活動
myRoomDatabase3 = Room.databaseBuilder(getApplicationContext(),MyRoomDatabase.class,"messagedb3").fallbackToDestructiveMigration().allowMainThreadQueries().build();
Message message4 = new Message();
message4.setTime("14:10");
message4.setContactName("Deepika");
message4.setDate("02-12-2018");
message4.setMsg("HI");
MainActivity.myRoomDatabase3.myDao().addMessage(message4);
Toast.makeText(MainActivity.this,"Message added",Toast.LENGTH_SHORT).show();
我的MyDao界面代碼:-
import android.arch.persistence.room.Dao;
import android.arch.persistence.room.Insert;
import android.arch.persistence.room.Query;
@Dao
public interface MyDao {
@Insert
public void addMessage(Message message);
@Query("SELECT * FROM messages3 WHERE message_date LIKE :givenDate AND " + "message_time LIKE :givenTime LIMIT 1")
public Message getMessageInfo(String givenDate,String givenTime);
}
我的留言課:-
import android.arch.persistence.room.ColumnInfo;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.PrimaryKey;
@Entity(tableName = "messages3")
public class Message {
@PrimaryKey(autoGenerate = true)
private int id;
@ColumnInfo(name = "message_date")
private String date;
@ColumnInfo(name = "message_time")
private String time;
@ColumnInfo(name = "message_contactName")
private String contactName;
@ColumnInfo(name = "message_msg")
private String msg;
public String getDate() {
return date;
}
public void setDate(String date) {
this.date = date;
}
public String getTime() {
return time;
}
public void setTime(String time) {
this.time = time;
}
public String getContactName() {
return contactName;
}
public void setContactName(String contactName) {
this.contactName = contactName;
}
public String getMsg() {
return msg;
}
public void setMsg(String msg) {
this.msg= msg;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}
嘗試從日期和時間如給定的數據庫的一行中檢索數據:
Calendar calender = Calendar.getInstance();
calender.set(Calendar.YEAR,2018);
calender.set(Calendar.MONTH, 11);
calender.set(Calendar.DAY_OF_MONTH, 02);
SimpleDateFormat simpledateformat = new SimpleDateFormat("dd-MM-yyyy");
String Date = simpledateformat.format(calender.getTime());
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 14);
cal.set(Calendar.MINUTE, 10);
cal.set(Calendar.SECOND,00);
SimpleDateFormat simpletimeformat = new SimpleDateFormat("hh:mm");
String Time = simpletimeformat.format(cal.getTime());
Message message = MainActivity.myRoomDatabase3.myDao().getMessageInfo(Date,Time);
String contactName = (message==null) ? "No data found" : message.getContactName();
String text = (message == null) ? "No data found" : message.getMsg();
我的房間數據庫課程:-
import android.arch.persistence.room.Database;
import android.arch.persistence.room.RoomDatabase;
@Database(entities = {Message.class},version = 12,exportSchema = false)
public abstract class MyRoomDatabase extends RoomDatabase {
public abstract MyDao myDao();
}
我的build.gradle文件:-
allprojects {
repositories {
google()
jcenter()
}
}
依賴關系
dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:28.0.0'
implementation 'com.android.support:appcompat-v7:28.0.0-rc02'
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
implementation 'android.arch.persistence.room:runtime:1.0.0'
annotationProcessor 'android.arch.persistence.room:compiler:1.0.0'
}
如何清除錯誤?出什么問題?如何通過在日期和時間字段中進行搜索來讀取單行數據?
我已經添加了整個代碼,請告訴我出了點問題,我堅持了2-3天,這一定是錯誤的,這是什么? 為什么查詢返回null?
您的查詢未返回任何內容。 因此消息為空。 您需要確保查詢返回的結果更好,或者添加代碼以處理未發現任何情況的情況,因為這似乎是一個合理的可能狀態。
調試器可以比SO更快地給出相同的答案。
這將避免NullPointerException:
String text = (message == null) ? “No data found.” : message.getMsg();
如果要將結果僅作為Message的單個記錄,則使用以下查詢:
SELECT * FROM messages WHERE message_date= :givenDate AND message_time= :givenTime LIMIT 1
現在,它將查詢結果綁定到提供的消息對象。
我發現了,這是一個完全愚蠢的錯誤。 當我在數據庫中輸入信息時,我將時間輸入為14:10,但是當我嘗試訪問它時,我使用的是時間格式“ hh:mm”而不是“ HH:mm”,所以我試圖找到時間為02:10而不是14:10的列。 這是兩個不同的值,因此返回null。
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