Java 函数式编程在 Java 8th 版本之后出现。函数式编程意味着解决问题的方法发生了根本性的变化。函数式编程允许使用表达式（声明函数）、将函数作为参数传递以及使用函数作为语句进行编程。
函数式编程的类型
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在讨论函数式编程概念之前，让我们先看看函数式编程和结构化编程之间的区别。结构化编程强调逻辑结构或过程，而函数式编程主要关注数据。结构化编程遵循自上而下的方法，而函数式编程遵循自下而上的方法。
函数式编程被分为称为对象的微小运行时实体，而结构化编程则分为小单元或函数。结构化编程的安全性较低，而函数式编程的安全性很高。结构化编程无法处理复杂问题，而函数式编程可以处理任何级别的复杂问题。
下面给出了提到的示例：
语法：
objectName.stream();
代码：
动物.java
package com.streams;
public class Animal {
String name;
String color;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getColor() {
return color;
}
public void setColor(String color) {
this.color = color;
}
public Animal(String name, String color) {
super();
this.name = name;
this.color = color;
}
}AnimalNames.java
package com.streams;//creating package
//importing required packages to the code
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
public class AnimalNames {
public static void main(String[] args) {
getMyDogNames();//calling the method
}
private static void getMyDogNames() {
Animal a1=new Animal("Puppy","black"); //creating a object for animal class
Animal a2=new Animal("Tommy","brown");//creating a object for animal class
Animal a3=new Animal("Zimmy","white");//creating a object for animal class
List<Animal> persons = Arrays.asList(a1,a2,a3); //passing object references to list
String dogName = persons.stream() //covert all elements into stream
.filter(name -> "Puppy".equals(name.getName()))//filtering given elements from persons class
.map(Animal::getName) //persons elements iterate over map
.findAny()//find out the given element passed from filter
.orElse("");//
System.out.println("Dog Name :" + dogName); //print the dog name
List<String> collect = persons.stream()
.map(Animal::getName)
.collect(Collectors.toList());//collecting all names from list
System.out.println("All Dog names");
collect.forEach(System.out::println);//iterating collection with for each loop
}
}输出：
说明：
语法：
(arguments) ->
{
//code for implementation
}
Arguments: argument-list can be have values or no values
Example: arguments1, arguments2, arguments3,……
->: Joins code implementation and arguments.语法：
(argument1) ->
{
//code for implementation
}代码：
package com.lambda;//creating a package
interface Square{ //creating interface for quare
public int getArea(int side); //create a method for get area
}
public class AreaOfSquare{ //As we are working with Lambda expression so no need to implement interface of square
public static void main(String[] args) {
Square area=(side)->{ // Lambda expression with only one argument.
Return side*side; //returning area
};
System.out.println(“Area of Square=>”+area.getArea(10)); //printing area by calling interface getArea method
}
}输出：
说明：
语法：
() ->
{
//code for implementation
}代码：
package com.lambda;//creating a package
interface Name{ //creating interface for Name
public String getMyName(); //create a method for get name
}
public class MyNameString{ //As we are working with Lambda expression so no need to implement interface of Name
public static void main(String[] args) {
Name name=()->{ // Lambda expression with out argument.
return "Hi, Amardeep!"; //returning name
};
System.out.println(name.getMyName()); //printing name by calling interface getMyName method
}
}输出：
说明：
语法：
Class-Name:: static method name
代码：
package method.reference;//creating package
interface RectangleArea{ //creating RectangleArea interface
public int getArea(int l,int b); //creating method getArea in interface
}
public class StaticMethodReference { //creating a classs
public static int showRectangleArea(int length, int breadth){ //creating method for getting rectangle area
return length*breadth;
}
public static void main(String[] args) {
// Referring static method
RectangleArea area = StaticMethodReference::showRectangleArea; //calling class name with method name
// Calling interface method
int outArea=area.getArea(10,20);
System.out.println("Area of rectangle :"+outArea);//printing area
}
}输出：
说明：
函数式编程是通过流、lambda 表达式和方法引用来实现的。它减少了代码行并提高了性能。
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