Lambda expressions in Java

What is a lambda expression?

• Lambda expression is a function without any signature.
• It doesn't have a name, return type, and modifiers.
• It can have any number of arguments.
• For one argument lambda expressions, parenthesis are optional.
• Compiler can understand the data type.
• Curly braces are mandatory for many arguments.

Advantages of Lambda expressions

• It helps to iterate, filter, and extract data from a collection in a very efficient way.
• It saves a lot of code.
• Code reuse
• Reduce class files

Functional interface

• An interface that has only one abstract method.
• @FunctionalInterface annotation is used to declare an interface as a functional interface.
• Ex: Runnable interface

Lambda expression signature

• (argument-list) -> {body}
• Ex:

( ) -> { }        
(p1) -> { }        
(p1, p2) -> { }

Rules

public void m1(int a, int b){    
    System.out.println(a+b);
}
    //lambda expressions
( a,b ) -> System.out.println(a+b);

• Lambda expression doesn't have a name, a modifier and a return type.
• If the code contains only one line. curly braces are optional.
• Compiler can guess the return type.

public int square(int n){
    return n*n;
}

//lambda expressions
(int n) -> {return n*n;}
(n) -> {return n*n;}
n -> n*n;

• If only one parameter is there, parentheses are optional.
• Since it has only one line, curly braces are optional.
• Without curly braces, the return type is optional.

Valid statements

Interf i = n -> n*n;
Interf i = (n) -> n*n;
Interf i = n -> {return n*n;};

Invalid statements

Interf i = n -> return n*n;
Interf i = n -> {return n*n};
Interf i = n -> {return n*n;}
Interf i = n -> {n*n;};

Example 01

public void m1(){
    System.out.println("Hello");
}

//lambda expressions
( ) -> System.out.println("Hello");

Example 02

public void add(int a, int b){
    System.out.println(a+b);
}

//lambda expressions
(int a, int b) -> System.out.println(a+b);
(a, b) -> System.out.println(a+b);

Example 03

public int square(int n){
    return n*n;
}

//lambda expressions
(int n) -> {return n*n;}
(n) -> {return n*n;}
n -> n*n;