[英]Convert a ResultSet into List of HashMap in java
我有一個結果集(由存儲過程返回),例如:
我需要一個HashMap列表,並且HashMap將由單個(由FirstName,LastName和Id組合而成的)學生記錄組成。 下圖將描述HashMap的結構。
因此,基本上在此示例中,我需要3 HashMap的列表。
您能建議一些有效的方法嗎? 謝謝!
為什么我需要一個哈希表列表?
我需要根據此ResultSet創建一個XML文件。 HashMap列表是我可以想到的所需XML結構的最佳方法。
XML結構:
如果我的方法在編碼標准或效率方面不正確,請建議我使用其他方法。
我在想什么:
我還沒試過 但是除了迭代ResultSet並臨時存儲FirstName,LastName和Id以檢查其是否與先前的值相同之外,別無其他選擇。 如果相同,則將marks數組添加到MarksList,否則將其視為另一個學生的記錄。 我希望可能會有不同的方式。
[更新]:
我想我應該使用對象列表,而不是HashMap列表。 但是問題仍然是,如何解釋結果集中的值並將值設置到對象中。
UPDATE
提供的代碼可將ResultSet
的數據正確提取到JAXB
使用的對象中。
您不需要HashMap
和/或HashMap
List
即可創建xml
文件。只要您創建正確的數據對象,就可以使用JAXB輕松實現。
首先,創建classes
以匹配xml
所需的結構。
ScoreRecord
類,用於保存課程信息,例如課程名稱和標記。 Student
類,其中包含學生信息,例如名字,姓氏和ScoreRecords
對象列表。 StudentGroup
班級,該班級包含所有屬於同一系的學生 ScoreRecord
類:
import java.math.BigDecimal;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
@XmlRootElement(name = "Mark")
@XmlType(propOrder = { "sub", "percent" })
public class ScoreRecord {
private String sub;
private String percent;
public void setSub(String sub) {
this.sub = sub;
}
public String getSub() {
return sub;
}
public void setPercent(String percent) {
this.percent = percent;
}
public String getPercent() {
return percent;
}
}
Student
班:
import java.util.List;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlRootElement;
import javax.xml.bind.annotation.XmlType;
@XmlRootElement(name = "Student")
@XmlType(propOrder = { "firstName", "lastName", "id", "scoreRecords" })
public class Student {
private Integer id;
private String firstName;
private String lastName;
private List<ScoreRecord> scoreRecords;
public void setId(Integer id) {
this.id = id;
}
@XmlElement(name = "Id")
public Integer getId() {
return id;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
@XmlElement(name = "FirstName")
public String getFirstName() {
return firstName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
@XmlElement(name = "LastName")
public String getLastName() {
return lastName;
}
public void setScoreRecords(List<ScoreRecord> scoreRecords) {
this.scoreRecords = scoreRecords;
}
@XmlElementWrapper(name = "MarksList")
@XmlElement(name = "Mark")
public List<ScoreRecord> getScoreRecords() {
return scoreRecords;
}
}
StudentGroup
類:
import java.util.List;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlElementWrapper;
import javax.xml.bind.annotation.XmlRootElement;
@XmlRootElement(name = "Records")
public class StudentGroup {
private List<Student> students;
public void setStudents(List<Student> students) {
this.students = students;
}
@XmlElement(name = "StudentRecord")
public List<Student> getStudents() {
return students;
}
}
現在,創建一個類以“按原樣”保存數據庫中的數據
public class DbStudent {
private String firstName;
private String lastName;
private Integer id;
private String sub;
private String percent;
public DbStudent(String firstName, String lastName, int id, String sub, String percent) {
this.firstName = firstName;
this.lastName = lastName;
this.id = id;
this.sub = sub;
this.percent = percent;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getFirstName() {
return firstName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getLastName() {
return lastName;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getId() {
return id;
}
public void setSub(String sub) {
this.sub = sub;
}
public String getSub() {
return sub;
}
public void setPercent(String percent) {
this.percent = percent;
}
public String getPercent() {
return percent;
}
}
創建一個方法來檢索數據作為此對象類型的List
。 我假設您已經擁有一些獲取ResultSet
,對其進行迭代並將.add
到DbStudent
對象列表中的東西。
就像是:
public List<DbStudent> getStudents() throws ClassNotFoundException, SQLException {
List<DbStudent> entries = new ArrayList<DbStudent>();
Class.forName(databaseDriver);
this.connection = DriverManager.getConnection(connectionString);
Statement sttm = this.connection.createStatement();
ResultSet rs = sttm.executeQuery("select * from TMP_STUDENT"); //in your case procedure call
if (rs != null) {
while (rs.next()) { //add the results into the list
entries.add(new DbStudent(rs.getString("FIRSTNAME"), rs.getString("LASTNAME"), rs.getInt("ID"),
rs.getString("SUB"), rs.getString("PERCENT")));
}
rs.close();
}
return entries;
}
現在,主要方法。 它包含從數據庫對象列表中提取所有信息的邏輯。 基本上,我們按ID對其進行排序,然后遍歷它,以檢查是否找到新學生。 如果找到新學生,則將前一個學生添加到“ Student
對象列表中。 這個學生已經設定好分數了。
public static void main(String[] args) throws JAXBException {
//get the data from the database as is
OracleConnection myOracleConnection = new OracleConnection(ORACLE_DRIVER, ORACLE_CONN);
List<DbStudent> dbStudentList = null;
try {
dbStudentList = myOracleConnection.getStudents(); //get the list of students from the procedure or query
myOracleConnection.CloseConnection();
} catch (ClassNotFoundException e) {
e.printStackTrace();
System.out.println("Stopping execution and exiting...");
System.exit(-1);
} catch (SQLException e) {
e.printStackTrace();
System.out.println("Stopping execution and exiting...");
System.exit(-1);
}
//sort the list on Id, so we can know when we find a new student
Collections.sort(dbStudentList, new Comparator<DbStudent>() {
public int compare(DbStudent s1, DbStudent s2) {
return s1.getId().compareTo(s2.getId());
}
});
List<Student> studentList=new ArrayList<Student>(); //list which will hold all the student objects
Integer previousId = 0; //control variable
List<ScoreRecord> marksList = new ArrayList<ScoreRecord>(); //list to store the marks for each student
Student s = null;
for (int i=0;i<dbStudentList.size();i++) {
if(i==dbStudentList.size()-1){ //if we reached the end, no more students after this record, set the marks and add the student to the list
s.setScoreRecords(marksList);
studentList.add(s);
}
if (dbStudentList.get(i).getId().compareTo(previousId) != 0) {
//new student found
if(s!=null){
//if new student found add the previous one to the list after setting the marks
s.setScoreRecords(marksList);
studentList.add(s);
}
s = new Student(); //create a new student
s.setFirstName(dbStudentList.get(i).getFirstName());
s.setId(dbStudentList.get(i).getId());
s.setLastName(dbStudentList.get(i).getLastName());
ScoreRecord sr = new ScoreRecord();
sr.setSub(dbStudentList.get(i).getSub());
sr.setPercent(dbStudentList.get(i).getPercent());
marksList = new ArrayList<ScoreRecord>(); //reset marks list
marksList.add(sr);
} else {
//same student
ScoreRecord sr = new ScoreRecord();
sr.setSub(dbStudentList.get(i).getSub());
sr.setPercent(dbStudentList.get(i).getPercent());
marksList.add(sr); //add mark to existing marks list
}
previousId=dbStudentList.get(i).getId(); //set the control variable to the new id
}
StudentGroup sg=new StudentGroup(); //create the student wrapper
sg.setStudents(studentList); //add the student list to the wrapper
//create xml with JAXB
JAXBContext context = JAXBContext.newInstance(StudentGroup.class);
Marshaller m = context.createMarshaller();
m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
m.marshal(sg, new File(STUDENT_XML));
}
xml
的輸出正是您想要的:
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<Records>
<StudentRecord>
<FirstName>AA1</FirstName>
<LastName>BB1</LastName>
<Id>1</Id>
<MarksList>
<Mark>
<sub>Math</sub>
<percent>51%</percent>
</Mark>
<Mark>
<sub>Phy</sub>
<percent>61%</percent>
</Mark>
<Mark>
<sub>Bio</sub>
<percent>61%</percent>
</Mark>
</MarksList>
</StudentRecord>
<StudentRecord>
<FirstName>AA2</FirstName>
<LastName>BB2</LastName>
<Id>2</Id>
<MarksList>
<Mark>
<sub>Bio</sub>
<percent>62%</percent>
</Mark>
</MarksList>
</StudentRecord>
<StudentRecord>
<FirstName>AA3</FirstName>
<LastName>BB3</LastName>
<Id>3</Id>
<MarksList>
<Mark>
<sub>Math</sub>
<percent>53%</percent>
</Mark>
<Mark>
<sub>Phy</sub>
<percent>63%</percent>
</Mark>
<Mark>
<sub>Chem</sub>
<percent>63%</percent>
</Mark>
</MarksList>
</StudentRecord>
</Records>
您這里需要的是正確的數據對象。 在這里它應該是學生記錄,在內部應保存分數記錄的列表,該記錄將具有id,FirstName,LastName,scoreRecords(應該是包含主題名稱,分數,%等的列表)。
然后只需使用JAXB將其轉換為XML
如果按LastName和FirstName對sql語句/存儲過程進行排序,則無需在內存中收集整個數據。 只需迭代結果集並收集數據,直到FirstName和LastName更改為止,然后將您的子數據和百分比數據匯總並直接將其流式傳輸到文件或dom中即可。 等等....
或更改存儲過程或以已經為您聚合數據的方式創建一個新過程。
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