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。
UnaryOperatorDemo.java
Find the output.
BinaryOperatorDemo.java
Find the output.
MaxByMinBy.java
Find the Student class used in Example.
Student.java
Find the output.
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; } }
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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; } }
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())); } }
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; } }
---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