StreamsExample.java

package com.learn.streams;

import com.learn.data.Student;
import com.learn.data.StudentDataBase;

import java.util.List;
import java.util.Map;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class StreamsExample {

 public static void main(String[] args) {

 // we want student name and their activities in a Map
 Map<String, List<String>> studentActivitiesMap = StudentDataBase.getAllStudents()
 .stream()
 .collect(Collectors.toMap(Student::getName, Student::getActivities));

 /**
 * collect method starts the whole process.
 */
 System.out.println(studentActivitiesMap);

 // with filter
 Map<String, List<String>> studentActivitiesMapWithFilter = StudentDataBase.getAllStudents()
 .stream()
 .filter(student -> student.getGradeLevel() >= 3)
 .collect(Collectors.toMap(Student::getName, Student::getActivities));
 System.out.println(studentActivitiesMapWithFilter);

 Predicate<Student> predicate1 = student -> student.getGradeLevel() >= 3;
 Predicate<Student> predicate2 = student -> student.getGpa() >= 3.9;
 Map<String, List<String>> studentActivitiesMapWithTwoFilter = StudentDataBase.getAllStudents()
 .stream()
 .filter(predicate1)
 .filter(predicate2)
 .collect(Collectors.toMap(Student::getName, Student::getActivities));
 System.out.println(studentActivitiesMapWithTwoFilter);

 Predicate<Student> predicate4 = (student -> {
 System.out.println(student);
 return student.getGradeLevel() >= 3;
 });

 /**
 * collect method starts the whole process.
 * if we don't have collect then this won't even start.
 * Streams are Lazy.
 * i.e. No Intermediate Operations will be invoked until the terminal operation is invoked.
 */
 StudentDataBase.getAllStudents()
 .stream()
 .filter(predicate4);
 }
}