Java 8 一元运算符UnaryOperator and BinaryOperator Example

Java 8介绍了UnaryOperator和BinaryOperator可以用于指定作为lambda表达式。 UnaryOperator与BinaryOperator是函数式接口。二者根据传入的参数决定了UnaryOperator继承了Function并且BinaryOperator继承了BiFunction. UnaryOperator接受一个操作符并返回相同类型的值。BinaryOperator接收两个操作数返回一个相同类型的值。

UnaryOperator

java.util.function.UnaryOperator是java 8的函数式接口，继承于java.util.function.Function。UnaryOperator接收一个参数，并返回类型的参数。 UnaryOperator作为lambda表达式传递一个参数。使用UnaryOperator时，需要定义Function.apply(Object)，实例会是UnaryOperator的实例。例子如下 
UnaryOperatorDemo.java

package com.concretepage.util.stream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.function.UnaryOperator;
public class UnaryOperatorDemo {
public static void main(String[] args) {
List<Integer> list = Arrays.asList(10,20,30,40,50);
UnaryOperator<Integer> unaryOpt = i->i*i;
unaryOperatorFun(unaryOpt, list).forEach(x->System.out.println(x));
}
private static List<Integer> unaryOperatorFun(UnaryOperator<Integer> unaryOpt, List<Integer> list){
List<Integer> uniList = new ArrayList<>();
list.forEach(i->uniList.add(unaryOpt.apply(i)));
return uniList;
}
}

Find the output.

100
400
900
1600
2500

BinaryOperator

BinaryOperator

java.util.function.BinaryOperator是函数式接口，并且是lambda表达式。BinaryOperator继承自 java.util.function.BiFunction。接收两个参数返回一个相同类型的值。例子如下

BinaryOperatorDemo.java

package com.concretepage.util.stream;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.BinaryOperator;
public class BinaryOperatorDemo {
public static void main(String[] args) {
Map<String,String> map = new HashMap<>();
map.put("X", "A");
map.put("Y", "B");
map.put("Z", "C");
BinaryOperator<String> binaryOpt = (s1,s2)-> s1+"-"+s2;
binaryOperatorFun(binaryOpt, map).forEach(x->System.out.println(x));
}
private static List<String> binaryOperatorFun(BinaryOperator<String> binaryOpt, Map<String,String> map){
List<String> biList = new ArrayList<>();
map.forEach((s1,s2)->biList.add(binaryOpt.apply(s1,s2)));
return biList;
}
}

Find the output.

X-A
Y-B
Z-C

BinaryOperator.maxBy and BinaryOperator.minBy

BinaryOperator.maxBy accepts a Compotator and returns BinaryOperator which will return maximum between two elements. In the same way, BinaryOperator.minBy is used
to get minimum between two elements. 
MaxByMinBy.java

package com.concretepage.util;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;
public class MaxByMinBy {
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);
//Using BinaryOperator.maxBy
System.out.println("---BinaryOperator.maxBy---");
Map<String, Optional<Student>> eldestByClass = list.stream().collect(Collectors.groupingBy(Student::getClassName,
Collectors.reducing(BinaryOperator.maxBy(ageComparator))));
eldestByClass.forEach((k,v)->System.out.println("Class:"+k+" Age:"+
((Optional<Student>)v).get().getAge()+" Name:"+((Optional<Student>)v).get().getName()));

//Using BinaryOperator.minBy
System.out.println("---BinaryOperator.minBy---");
Map<String, Optional<Student>> youngestByClass = list.stream().collect(Collectors.groupingBy(Student::getClassName,
Collectors.reducing(BinaryOperator.minBy(ageComparator))));
youngestByClass.forEach((k,v)->System.out.println("Class:"+k+" Age:"+
((Optional<Student>)v).get().getAge()+" Name:"+((Optional<Student>)v).get().getName()));
}
}

Find the Student class used in Example. 
Student.java

package com.concretepage.util;
public class Student {
private String name;
private Integer age;
private String className;
public Student(String name,Integer age, String className){
this.name=name;
this.age=age;
this.className = className;
}
public String getName() {
return name;
}
public Integer getAge() {
return age;
}
public String getClassName() {
return className;
}
}

Find the output.

---BinaryOperator.maxBy---
Class:A Age:23 Name:Ram
Class:B Age:22 Name:Mohan
---BinaryOperator.minBy---
Class:A Age:22 Name:Shyam
Class:B Age:21 Name:Ramesh