java - 无法通过选择检索房间数据库中单行的数据

Question

主要 Activity

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();

MyDao接口(interface)代码:-

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);

我的消息类:-

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;
}

尝试从给定日期和时间的数据库的一行中检索数据:-

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();

我的房间数据库类:-

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()
}

依赖关系-

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'
}

如何消除错误？哪里出了问题？如何从日期和时间字段中搜索读取单行数据？

我已经添加了我正在使用的整个代码。请告诉我哪里出了问题。我在这上面停留了 2-3 天。一定是出了什么问题。这是什么？为什么查询返回空值？

Answer 1

您的查询未返回任何内容。所以消息为空。您需要确保您的查询返回某些内容或更好的结果，添加代码来处理未找到任何内容的情况，因为这似乎是一种合理的可能状态。

调试器可以比 SO 更快地给出相同的答案。

这将避免 NullPointerException:

String text = (message == null) ? “No data found.” : message.getMsg();