Java - Use Collectors class to compute summary statistics

Introduction

The methods are named summarizingDouble(), summarizingLong(), and summarizingInt().

They take a function to be applied on the elements of the stream and return a DoubleSummaryStatistics, a LongSummaryStatistics, and an IntSummaryStatistics, respectively.

```DoubleSummaryStatistics incomeStats =
Person.persons()
.stream()
.collect(Collectors.summarizingDouble(Person::getIncome));
```

Demo

```import java.time.LocalDate;
import java.time.Month;
import java.util.Arrays;
import java.util.DoubleSummaryStatistics;
import java.util.List;
import java.util.stream.Collectors;

public class Main {
public static void main(String[] args) {
DoubleSummaryStatistics stats =
Person.persons()//w ww  .ja  va 2  s.  co  m
.stream()
.collect(Collectors.summarizingDouble(Person::getIncome));

long count = stats.getCount();
double sum = stats.getSum();
double min = stats.getMin();
double avg = stats.getAverage();
double max = stats.getMax();

System.out.printf("count=%d, sum=%.2f, min=%.2f, average=%.2f, max=%.2f%n", count, sum, min, max, avg);

}
}
class Person {
// An enum to represent the gender of a person
public static enum Gender {
MALE, FEMALE
}

private long id;
private String name;
private Gender gender;
private LocalDate dob;
private double income;

public Person(long id, String name, Gender gender, LocalDate dob, double income) {
this.id = id;
this.name = name;
this.gender = gender;
this.dob = dob;
this.income = income;
}

public long getId() {
return id;
}

public void setId(long id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public Gender getGender() {
return gender;
}

public boolean isMale() {
return this.gender == Gender.MALE;
}

public boolean isFemale() {
return this.gender == Gender.FEMALE;
}

public void setGender(Gender gender) {
this.gender = gender;
}

public LocalDate getDob() {
return dob;
}

public void setDob(LocalDate dob) {
this.dob = dob;
}

public double getIncome() {
return income;
}

public void setIncome(double income) {
this.income = income;
}

public static List<Person> persons() {
Person ken = new Person(1, "Java", Gender.MALE, LocalDate.of(1970, Month.MAY, 4), 6020.0);
Person jeff = new Person(2, "Jack", Gender.MALE, LocalDate.of(1970, Month.JULY, 15), 7320.0);
Person donna = new Person(3, "Javascript", Gender.FEMALE, LocalDate.of(1972, Month.JULY, 29), 8720.0);
Person chris = new Person(4, "XML", Gender.MALE, LocalDate.of(1993, Month.DECEMBER, 16), 5800.0);
Person laynie = new Person(5, "Json", Gender.FEMALE, LocalDate.of(2002, Month.DECEMBER, 13), 0.0);
Person lee = new Person(6, "Database", Gender.MALE, LocalDate.of(2011, Month.MAY, 9), 2400.0);

// Create a list of persons
List<Person> persons = Arrays.asList(ken, jeff, donna, chris, laynie, lee);

return persons;
}

@Override
public String toString() {
String str = String.format("(%s, %s, %s, %s, %.2f)", id, name, gender, dob, income);
return str;
}
}
```