[Java Silver] Summary of points related to lambda expressions

What is a lambda expression?

A mechanism to assign the code you want to execute to an interface type variable that has only one method that needs to be implemented.

What is a functional interface?

A functional interface is an interface that has only ** one method ** that needs to be implemented. It is called "functional" because there is only one output for each input.

Types of functional interfaces

Interface type Methods that need to be implemented Feature
Consumer<T> void accept(T) Receives and processes arguments. Arguments that do not return resultsconsumer
Supplier<T> T get() Return only the result without receiving anythingSupplier
Predicate<T> boolean test(T) Take an argument and evaluate itAssertive
Function<T, R> R apply(T) Receives an argument and the specified type(R)Returns the result ofProcessor

Functional interface example

Reference) Class for Java8 lambda expression

**-The forEach method of the java.util.List interface can receive a Consumer type lambda expression ** reference) Official Document Interface List Official Document forEach


public static void main(String[] args){
   List<Integer> list = List.of(1,2,3);
   list.forEach((x) ->{System.out.println(x)}); //Receives and processes arguments and does not return values

** · toUpperCase method ** of java.lang.String class can receive Function type lambda expression

reference Official document toUpperCase ()


public static void main(String[] args){
   Function<String, String> func = (x) -> {return x.toUpperCase();}; //Takes an argument and returns the result of the specified type
   System.out.println(func.apply("hello world"));

How to write a lambda expression


Function type interface type variable name = (argument)-> {// processing ** return **};

  Function<String, String> func = (x) -> { return x.toUpperCase(); }; 

<When there is one argument, () is omitted>

Functional interface type Variable name = Arguments-> {// Processing ** return **};

 Function<String, String> func = x -> { return x.toUpperCase(); }; 

<When there is only one process, {} is omitted>

Functional interface type Variable name = Arguments-> // Processing; ** * I can't write a return statement! !! ** **

list.forEach( x -> System.out.println(x) ); 

Lambda-style notes

** [The formal argument of a lambda expression is a constant or a substantially constant] **

-Compilation error example (from Java Silver Kuromoto)

void sample(){
  int i = 0;
  Supplier<Integer> foo = () -> i;    
 i++;   //i needs to be substantially constant

** [Note the data type of the argument] ** Variable declarations in lambda expressions can ** omit specifying data types **. Because the type of the argument can be inferred from the type of the functional interface you are trying to assign.

Even if you write a lambda expression variable with its arguments omitted and the type omitted, it is a newly defined variable. → ** Check if it overlaps with an existing variable in the scope of the lambda expression! !! ** **

-Example of compilation error

final String val = "Hello World";
Consumer<String> func = (val) -> System.out.println(val); //Duplicate name caused by val of lambda expression and val of local variable, compilation error

** [Note the scope] ** The ** scope ** of a lambda expression extends to the entire block where the lambda expression is written.

-Example of compilation error

public class Main{
   public static void main(String[] args){
     Srting val = "A";  //Local variable val in scope of lambda expression
     Function f = (val) -> {  //Lambda expression variable val

Future tasks

To be honest, I still don't understand the Lambda style at all ... I would like to understand the following site about the mechanism of lambda expression.

Computer Science and Magic Y Combinator

Also, I don't really understand when to actually use a lambda expression. What is the method that can accept a lambda expression? Is it always available when writing simple processes that are not reused? Can it be used only by a method of a functional interface? How do you distinguish it from a method of a functional interface?

There are many things I don't understand.

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