[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

• \ : T is an abbreviation for Type. It is a generalized expression that any type can be used.

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

Example)

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 ()

Example)

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

Basic

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
  System.out.println(foo.get());
}

** [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
       System.out.println(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.