Java8 streams and lambda expression GroupBy MaxBy MinBy Example

	package com.company.samples;
	import java.util.*;
	import java.util.function.BinaryOperator;
	import java.util.function.Function;
	import java.util.stream.Collectors;
	import static java.util.stream.Collectors.*;
	public class Java8GroupByMaxByMinByExample {
	public static void main (String[] args) {
	Student s1 = new Student ("Shyam", 22, "A");
	Student s2 = new Student ("Ram", 23, "A");
	Student s3 = new Student ("Mohan", 22, "B");
	Student s4 = new Student ("Ramesh", 21, "B");
	List<Student> list = Arrays.asList (s1, s2, s3, s4);
	Comparator<Student> ageComparator = Comparator.comparing (Student::getAge);
	//Find eldest student in each class and group by class
	Map<String, Optional<Student>> eldestByClass = list.stream ().
	collect (groupingBy (Student::getClassName, Collectors.reducing (BinaryOperator.maxBy (ageComparator))));
	//Group students by classname
	Map<String, List<Student>> studentsByClass = list.stream ()
	.collect (groupingBy (s -> s.getClassName ()));
	//Group students by age for each class
	Map<String, List<Integer>> studentAgesByClass = list.stream ()
	.collect (groupingBy (st -> st.getClassName (), mapping ((Student s) -> s.getAge (), toList ())));
	//Group students by classname
	Map<String, List<String>> studentNamesByClass = list.stream ()
	.collect (groupingBy (s -> s.getClassName (), mapping ((Student s) -> s.getName (), toList ())));
	//another syntax -1
	Map<String, List<String>> studentNamesByClass2 = list.stream ()
	.collect (groupingBy (s -> s.getClassName (), mapping (s -> s.getClassName (), toList ())));
	//another syntax -2
	Map<String, List<String>> studentNamesByClass3 = list.stream ()
	.collect (groupingBy (s -> s.getClassName (), mapping (Student::getClassName, toList ())));
	//another syntax -3 use set instead of List to eliminate duplications
	Map<String, Set<String>> studentNamesByClass4 = list.stream ()
	.collect (groupingBy (s -> s.getClassName (), mapping (Student::getClassName, toSet ())));
	//Find youngest student in each class and group by class
	Map<String, Optional<Student>> youngestByClass = list.stream ().collect (groupingBy (Student::getClassName,
	Collectors.reducing (BinaryOperator.minBy (ageComparator))));
	}
	}
	class Student {
	private String name;
	private int age;
	private String className;
	public Student (String name, int age, String className) {
	this.name = name;
	this.age = age;
	this.className = className;
	}
	public String getName () {
	return name;
	}
	public void setName (String name) {
	this.name = name;
	}
	public int getAge () {
	return age;
	}
	public void setAge (int age) {
	this.age = age;
	}
	public String getClassName () {
	return className;
	}
	public void setClassName (String className) {
	this.className = className;
	}
	}

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