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##1. Introduction to Lambda expressions

Lambda expression is a new feature of Java8 and one of the most worth learning new features in Java8. (Another new feature is flow programming.)

• A Lambda expression is, essentially, an

  anonymous method. You can use this anonymous method to implement the methods in the interface.

• Function: Lambda expressions are usually used to

  simplify interface implementation. There are many ways to implement interfaces, such as: ① Design the implementation class of the interface, ② Use anonymous inner classes. But ③ using lambda expression is simpler than these two methods.

• Requirements: lambda expressions,

  can only implement functional interfaces: That is, in an interface, there is only one abstract method that the implementation class must implement.

@FunctionalInterface annotation, used before the interface, is used to determine whether the interface is a functional interface. If it is not a functional interface, an error will be reported. The functionality is similar to @Override.

2. Lambda expression syntax

lambda expression is essentially an anonymous method, so when writing lambda expressions, no need You don't care what the method name is, and you don't need to care about the return value type. You only need to care about two parts:

Parameter list, Method body.

() Parameter part: The parameter list of the method must be consistent with the method parameter part in the implemented interface, including the number and type of parameters.
• {}Method body part: The implementation part of the method. If the method defined in the interface has a return value, pay attention to the return value when implementing it.
• -> : Separates the parameter part and the method body part.

Lambda表達(dá)式基礎(chǔ)語法:(參數(shù))?->{
	方法體}

The following defines 6 types of

functional interfaces with different parameters and return values, and uses lambda expressions to implement the methods in the interface:

#The following is the lambda expression implementation for the above six functional interfaces.

/**
?*?@Description:
?*?@author?Guoqianliang
?*?@date?19:50?-?2021/2/15
?*/public?class?BasicSyntax?{
????public?static?void?main(String[]?args)?{
????????//?1.實(shí)現(xiàn)無參數(shù),無返回值的函數(shù)式接口
????????NoneReturnNoneParameter?lambda1?=?()?->?{
????????????System.out.println("這是無參,無返回值的方法");
????????};
????????lambda1.test();

????????//?2.實(shí)現(xiàn)一個(gè)參數(shù),無返回值的函數(shù)式接口
????????NoneReturnSingleParameter?lambda2?=?(int?a)?->?{
????????????System.out.println("這是一個(gè)參數(shù),無返回值的方法,參數(shù)a:"?+?a);
????????};
????????lambda2.test(10);

????????//?3.實(shí)現(xiàn)多個(gè)參數(shù),無返回值的函數(shù)式接口
????????NoneReturnMutipleParameter?lambda3?=?(int?a,?int?b)?->?{
????????????System.out.println("這是多個(gè)參數(shù),無返回值的方法,參數(shù)a="?+?a?+?",b="?+?b);
????????};
????????lambda3.test(10,?20);

????????//?4.實(shí)現(xiàn)無參數(shù),有返回值有返回值的函數(shù)式接口
????????SingleReturnNoneParameter?lambda4?=?()?->?{
????????????System.out.println("這是無參數(shù),有返回值的方法,返回值是:");
????????????return?10;
????????};
????????System.out.println(lambda4.test());

????????//?5.實(shí)現(xiàn)一個(gè)參數(shù),有返回值的函數(shù)式接口
????????SingleReturnSingleParameter?lambda5?=?(int?a)?->?{
????????????System.out.println("這是一個(gè)參數(shù),有返回值的方法,返回值是:");
????????????return?a;
????????};
????????System.out.println(lambda5.test(10));

????????//?6.實(shí)現(xiàn)多個(gè)參數(shù),有返回值的函數(shù)式接口
????????SingleReturnMutipleParameter?lambda6?=?(int?a,?int?b)?->?{
????????????System.out.println("這是多個(gè)參數(shù),有返回值的方法,返回值是:");
????????????return?a?+?b;
????????};
????????System.out.println(lambda6.test(1,?2));
????}}

The parameter type of the parameter list can be omitted.
• If there is and is only one parameter in the parameter list, the parentheses can be omitted.
• If there is only one statement in the method body, the curly braces can be omitted. (Note: If this statement is a return statement, the return keyword must also be omitted after omitting the braces)

3. Function reference

Lambda expressions are to simplify the interface. In lambda expressions, more complex logic should not appear. If the logic that needs to be processed is relatively complex, a separate method will generally be written. Just reference this method directly in the lambda expression. That is,

refers to an existing method so that it can replace the lambda expression to complete the implementation of the interface.

1. Static method reference

Syntax:

Class::Static method

Do not add parentheses after the quoted method.
• The referenced method, parameters (number, type) and return value must be consistent with those defined in the interface.

/**
?*?@Description:?方法引用
?*?@author?Guoqianliang
?*?@date?0:26?-?2021/2/16
?*/public?class?Lambda1?{

????private?static?interface?Calculate?{
????????int?calculate(int?a,?int?b);
????}

????private?static?int?calculate(int?x,?int?y)?{
????????if?(x?>?y)?{
????????????return?x?-?y;
????????}?else?if?(x?

2. Non-static method reference

Syntax:

Object::Non-static method

Do not add parentheses after the quoted method.
• The referenced method, parameters (number, type) and return value must be consistent with those defined in the interface.

/**
?*?@Description:?方法引用
?*?@author?Guoqianliang
?*?@date?0:26?-?2021/2/16
?*/public?class?Lambda1?{

????private?static?interface?Calculate?{
????????int?calculate(int?a,?int?b);
????}

????//?非靜態(tài)方法
????private?int?calculate2(int?a,?int?b)?{
????????if?(a?!=?b)?{
????????????return?a?-?b;
????????}
????????return?a?+?b;
????}

????public?static?void?main(String[]?args)?{
????????//?非靜態(tài)方法引用
????????Calculate?calculate2?=?new?Lambda1()::calculate2;
????????System.out.println(calculate.calculate(10,?20));
????}}

3. Constructor method reference

Syntax:

Class name::new

You can distinguish between different constructor methods through the parameters of the methods in the interface.
• If a method defined in a functional interface is just to get an object of a class. At this point, you can use the reference to the constructor method to simplify the implementation of this method.

/**
?*?@Description:?構(gòu)造方法引用
?*?@author?Guoqianliang
?*?@date?11:20?-?2021/2/16
?*/public?class?Lambda2?{

????@FunctionalInterface
????private?interface?GetPersonWithNoneParameter?{
????????Person?get();
????}

????@FunctionalInterface
????private?interface?GetPersonWithSingleParameter?{
????????Person?get(String?name);
????}

????@FunctionalInterface
????private?interface?GetPersonWithMutipleParameter?{
????????Person?get(String?name,?int?age);
????}

????private?static?class?Person?{
????????String?name;
????????int?age;

????????public?Person()?{
????????????System.out.println("Person類的無參構(gòu)造方法執(zhí)行了");
????????}

????????public?Person(String?name)?{
????????????this.name?=?name;
????????????System.out.println("Person類的有參構(gòu)造方法執(zhí)行了");
????????}

????????public?Person(String?name,?int?age)?{
????????????this.name?=?name;
????????????this.age?=?age;
????????????System.out.println("Person類的兩個(gè)參數(shù)的構(gòu)造方法執(zhí)行了");
????????}
????}

????public?static?void?main(String[]?args)?{
????????//?1.使用lambda表達(dá)式,實(shí)現(xiàn)GetPersonWithNoneParameter接口
????????GetPersonWithNoneParameter?getPerson?=?Person::new;
????????//?2.使用lambda表達(dá)式,實(shí)現(xiàn)GetPersonWithSingleParameter接口
????????GetPersonWithSingleParameter?getPerson2?=?Person::new;
????????//?3.使用lambda表達(dá)式,實(shí)現(xiàn)GetPersonWithMutipleParameter接口
????????GetPersonWithMutipleParameter?getPerson3?=?Person::new;

????????System.out.println(getPerson.get());
????????System.out.println(getPerson2.get("樹先生"));
????????System.out.println(getPerson3.get("你好",?23));
????}}

4. Special references to object methods

When using lambda expressions to implement certain interfaces, if in the lambda expression Contains an object. In the method body, directly using this object to call one of its methods can complete the overall logic.

/**
?*?@Description:?對象方法的特殊應(yīng)用
?*?@author?Guoqianliang
?*?@date?11:54?-?2021/2/16
?*/public?class?Lambda3?{

????@FunctionalInterface
????private?interface?MyInterface?{
????????//?String?get(Person?person);
????????void?set(Person?person,?String?name);
????}

????private?static?class?Person?{
????????private?String?name;

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

????????public?String?getName()?{
????????????return?name;
????????}
????}

????public?static?void?main(String[]?args)?{
????????Person?p1?=?new?Person();
????????p1.setName("小明");//????????邏輯實(shí)現(xiàn)只是為了獲取到對象的名字//????????MyInterface?lambda2?=?Person::getName;//????????System.out.println(lambda2.get(p1));
????????
????????//?邏輯實(shí)現(xiàn)只是為了給對象的某些屬性進(jìn)行賦值
????????MyInterface?lambda1?=?(x,?n)?->?x.setName(n);
????????MyInterface?lambda2?=?Person::setName;
????????lambda2.set(p1,?"李華");
????????System.out.println(p1.getName());
????}}

4. Issues to note with Lambda expressions

If

local variables are used, they will be declared by default As a constant, the value cannot change.

/**
?*?@Description:
?*?@author?Guoqianliang
?*?@date?13:05?-?2021/2/16
?*/public?class?Lambda4?{
????public?static?void?main(String[]?args)?{
????????//?1.定義一個(gè)局部變量
????????int?x?=?10;
????????//?2.使用lambda表達(dá)式實(shí)現(xiàn)接口
????????LambdaTest?lambda?=?()?->?{
????????????System.out.println("x="?+?x);
????????};
????????//?3.?無法修改常量x
????????//?x=20;
????}}@FunctionalInterfaceinterface?LambdaTest?{
????void?test();}

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