public class Student
{
private String studentName;
private String teacherName;
private int level;
private String className;
public Student(String studentName, String teacherName, int level,
String className)
{
super();
this.studentName = studentName;
this.teacherName = teacherName;
this.level = level;
this.className = className;
}
public String getStudentName()
{
return studentName;
}
public void setStudentName(String studentName)
{
this.studentName = studentName;
}
public String getTeacherName()
{
return teacherName;
}
public void setTeacherName(String teacherName)
{
this.teacherName = teacherName;
}
public int getLevel()
{
return level;
}
public void setLevel(int level)
{
this.level = level;
}
public String getClassName()
{
return className;
}
public void setClassName(String className)
{
this.className = className;
}
@Override
public String toString()
{
return "Student [studentName=" + studentName + ", teacherName="
+ teacherName + ", level=" + level + ", className=" + className
+ "]";
}
}
StreamGroupByDemo1.java
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
/**
* Group by teacher name:
*
* SELECT column_name, count(column_name)
* FROM table
* GROUP BY column_name;
*
* This example will show how to "group classes by a teacher's name". Here you
* will pass the groupingBy method a function in the form of a method reference
* extracting each teacher name to the corresponding Student which will return 1
* key to many elements. This is similar to guava's Multimap collection which
* allows for easy mapping of a single key to multiple values.
*
*/
public class StreamGroupByDemo1
{
public static void main(String[] args)
{
List<Student> studentList = getStudentList();
Map<String, List<Student>> groupByTeachersMap = studentList.stream()
.collect(Collectors.groupingBy(Student::getTeacherName));
for (Map.Entry<String, List<Student>> entry : groupByTeachersMap
.entrySet())
{
System.out.println("Teacher Name : " + entry.getKey());
List<Student> list = entry.getValue();
for (Student student : list)
{
System.out.println(student);
}
System.out.println("-------------------------------------");
}
}
private static List<Student> getStudentList()
{
List<Student> studentList = new ArrayList<Student>();
studentList.add(new Student("Peter", "Mr.John", 1, "Java Basics"));
studentList.add(new Student("Ram", "Mr.John", 1, "Webservice Basics"));
studentList.add(new Student("Juli", "Mr.John", 2, "Advance Java"));
studentList.add(new Student("Dave", "Mr.Kumar", 1, "Ruby basics"));
studentList.add(new Student("Raja", "Mr.Kumar", 2, "Advance Ruby"));
return studentList;
}
}
Output
Teacher Name : Mr.John
Student [studentName=Peter, teacherName=Mr.John, level=1, className=Java Basics]
Student [studentName=Ram, teacherName=Mr.John, level=1, className=Webservice Basics]
Student [studentName=Juli, teacherName=Mr.John, level=2, className=Advance Java]
-------------------------------------
Teacher Name : Mr.Kumar
Student [studentName=Dave, teacherName=Mr.Kumar, level=1, className=Ruby basics]
Student [studentName=Raja, teacherName=Mr.Kumar, level=2, className=Advance Ruby]
-------------------------------------
StreamGroupByDemo2.java
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
/**
* Group by class level:
*
* SELECT column_name, count(column_name)
* FROM table
* GROUP BY column_name;
*
* This example will show "show all classes by level".
*
*/
public class StreamGroupByDemo2
{
public static void main(String[] args)
{
List<Student> studentList = getStudentList();
Map<Integer, List<Student>> groupByLevelMap = studentList.stream()
.collect(Collectors.groupingBy(Student::getLevel));
for (Map.Entry<Integer, List<Student>> entry : groupByLevelMap
.entrySet())
{
System.out.println("Level : " + entry.getKey());
List<Student> list = entry.getValue();
for (Student student : list)
{
System.out.println(student);
}
System.out.println("----------------------------------");
}
}
private static List<Student> getStudentList()
{
List<Student> studentList = new ArrayList<Student>();
studentList.add(new Student("Peter", "Mr.John", 1, "Java Basics"));
studentList.add(new Student("Ram", "Mr.John", 1, "Webservice Basics"));
studentList.add(new Student("Juli", "Mr.John", 2, "Advance Java"));
studentList.add(new Student("Dave", "Mr.Kumar", 1, "Ruby basics"));
studentList.add(new Student("Raja", "Mr.Kumar", 2, "Advance Ruby"));
return studentList;
}
}
Output
Level : 1
Student [studentName=Peter, teacherName=Mr.John, level=1, className=Java Basics]
Student [studentName=Ram, teacherName=Mr.John, level=1, className=Webservice Basics]
Student [studentName=Dave, teacherName=Mr.Kumar, level=1, className=Ruby basics]
----------------------------------
Level : 2
Student [studentName=Juli, teacherName=Mr.John, level=2, className=Advance Java]
Student [studentName=Raja, teacherName=Mr.Kumar, level=2, className=Advance Ruby]
----------------------------------
StreamGroupByDemo3.java
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
/**
* GroupBy aggregate:
*
* SELECT LEVEL, count(LEVEL)
* FROM STUDENT
* GROUP BY LEVEL;
*
* This example will "count the number of classes per level":
*
*/
public class StreamGroupByDemo3
{
public static void main(String[] args)
{
List<Student> studentList = getStudentList();
/*
* In an overloaded groupingBy method, you can pass a second collector.
* Collectors have various reduce operations which can be passed, in
* this case Collectors.counting which will count the number of classes
* in each level.
*/
Map<Integer, Long> groupByLevelMap = studentList.stream()
.collect(Collectors.groupingBy(Student::getLevel,Collectors.counting()));
for (Map.Entry<Integer, Long> entry : groupByLevelMap.entrySet())
{
System.out.println("Level = " + entry.getKey() + ", Count = "
+ entry.getValue());
}
}
private static List<Student> getStudentList()
{
List<Student> studentList = new ArrayList<Student>();
studentList.add(new Student("Peter", "Mr.John", 1, "Java Basics"));
studentList.add(new Student("Ram", "Mr.John", 1, "Webservice Basics"));
studentList.add(new Student("Juli", "Mr.John", 2, "Advance Java"));
studentList.add(new Student("Dave", "Mr.Kumar", 1, "Ruby basics"));
studentList.add(new Student("Raja", "Mr.Kumar", 2, "Advance Ruby"));
return studentList;
}
}
Output
Level = 1, Count = 3
Level = 2, Count = 2
Click the below link to download the code:
https://sites.google.com/site/ramj2eev1/home/javabasics/StreamDemo_groupBy_level.zip?attredirects=0&d=1
Github Link:
https://github.com/ramram43210/Java/tree/master/BasicJava/StreamDemo_groupBy_level
Bitbucket Link:
https://bitbucket.org/ramram43210/java/src/4c6dd26ede26896338d54f8923608ecad05bf799/BasicJava/StreamDemo_groupBy_level/?at=master
See also:
All JavaEE Viedos Playlist
All JavaEE Viedos
All JAVA EE Links
Servlets Tutorial
All Design Patterns Links
JDBC Tutorial
Java Collection Framework Tutorial
JAVA Tutorial
Kids Tutorial
