Java Lambda Expressions

Lambda expressions in Java, introduced in Java SE 8. It represents the instances of functional interfaces (interfaces with a single abstract method). They provide a concise way to express instances of single-method interfaces using a block of code.

Key Functionalities of Lambda Expression

Lambda Expressions implement the only abstract function and therefore implement functional interfaces. Lambda expressions are added in Java 8 and provide the following functionalities.

• Functional Interfaces: A functional interface is an interface that contains only one abstract method.
• Code as Data: Treat functionality as a method argument.
• Class Independence: Create functions without defining a class.
• Pass and Execute: Pass lambda expressions as objects and execute on demand.

Example: Implementing a Functional Interface with Lambda

The below Java program demonstrates how a lambda expression can be used to implement a user-defined functional interface.

interface FuncInterface
{
    // An abstract function
    void abstractFun(int x);

    // A non-abstract (or default) function
    default void normalFun()
    {
       System.out.println("Hello");
    }
}

class Test
{
    public static void main(String args[])
    {
        // lambda expression to implement above functional interface. 
        FuncInterface fobj = (int x)->System.out.println(2*x);

        // This calls above lambda expression and prints 10.
        fobj.abstractFun(5);
    }
}

10

Structure of Lambda Expression

Below diagram demonstrates the structure of Lambda Expression:

Syntax of Lambda Expressions

Java Lambda Expression has the following syntax:

(argument list) -> { body of the expression }

Components:

• Argument List: Parameters for the lambda expression
• Arrow Token (->): Separates the parameter list and the body
• Body: Logic to be executed.

Types of Lambda Parameters

There are three Lambda Expression Parameters are mentioned below:

1. Lambda with Zero Parameter
2. Lambda with Single Parameter
3. Lambda with Multiple Parameters

1. Lambda with Zero Parameter 

Syntax:

() -> System.out.println("Zero parameter lambda");

Example: The below Java program demonstrates a Lambda expression with zero parameter.

@FunctionalInterface
interface ZeroParameter {
    void display();
}

public class Geeks {
    public static void main(String[] args)
    {
        // Lambda expression with zero parameters
        ZeroParameter zeroParamLambda = ()
            -> System.out.println(
                "This is a zero-parameter lambda expression!");

        // Invoke the method
        zeroParamLambda.display();
    }
}

This is a zero-parameter lambda expression!

2. Lambda with a Single Parameter

Syntax:

(p) -> System.out.println("One parameter: " + p);

It is not mandatory to use parentheses if the type of that variable can be inferred from the context.

Example: The below Java program demonstrates the use of lambda expression in two different scenarios with an ArrayList.

• We are using lambda expression to iterate through and print all elements of an ArrayList.
• We are using lambda expression with a condition to selectively print even number of elements from an ArrayList.

import java.util.ArrayList;

class Test {

    public static void main(String args[])
    {
        // Creating an ArrayList with elements {1, 2, 3, 4}
        ArrayList<Integer> al = new ArrayList<Integer>();
        al.add(1);
        al.add(2);
        al.add(3);
        al.add(4);

        // Using lambda expression to print all elements of al
        System.out.println("Elements of the ArrayList: ");
        al.forEach(n -> System.out.println(n));

        // Using lambda expression to print even elements of al
        System.out.println(
            "Even elements of the ArrayList: ");
        al.forEach(n -> {
            if (n % 2 == 0)
                System.out.println(n);
        });
    }
}

Elements of the ArrayList: 
1
2
3
4
Even elements of the ArrayList: 
2
4

Note: In the above example, we are using lambda expression with the foreach() method and it internally works with the consumer functional interface. The Consumer interface takes a single paramter and perform an action on it.

3. Lambda Expression with Multiple Parameters

Syntax:

(p1, p2) -> System.out.println("Multiple parameters: " + p1 + ", " + p2);

Example: The below Java program demonstrates the use of lambda expression to implement functional interface to perform basic arithmetic operations.

@FunctionalInterface
interface Functional {
    int operation(int a, int b);
}

public class Test {

    public static void main(String[] args) {
        
        // Using lambda expressions to define the operations
        Functional add = (a, b) -> a + b;
        Functional multiply = (a, b) -> a * b;

        // Using the operations
        System.out.println(add.operation(6, 3));  
        System.out.println(multiply.operation(4, 5));  
    }
}

9
20

Note: Lambda expressions are just like functions and they accept parameters just like functions. 

Common Built-in Functional Interfaces

• Comparable<T>: int compareTo(T o);
• Comparator<T>: int compare(T o1, T o2);

These are commonly used in sorting and comparisons.

Note: Other commonly used interface include Predicate<T>, it is used to test conditions, Function<T, R>, it represent a function that take the argument of type T and return a result of type R and Supplier<T>, it represent a function that supplies results.

Based on the syntax rules just shown, which of the following is/are NOT valid lambda expressions?

1. () : {};
2. () : "geeksforgeeks";
3. () : { return "geeksforgeeks";};
4. (Integer i) : {return "geeksforgeeks" + i;}
5. (String s) : {return "geeksforgeeks";};
6. () : { return "Hello" }
7. x : { return x + 1; }
8. (int x, y) : x + y

Explanation of above lambda expressions:

1. () -> {}: (valid). No parameters, no return value (empty body). It is valid
2. () -> "geeksforgeeks": (valid). This lambda takes no parameters and returns a String without using a return keyword or braces. This is allowed for single-expression lambdas.
3. () -> { return "geeksforgeeks"; }: (valid). This lambda takes no parameters and returns a value using a code block. Since it uses braces {}, the return keyword is required. It is valid.
4. (Integer i) -> {return "geeksforgeeks" + i;}: (valid). This lambda takes one parameter i of type Integer and returns a concatenated string. It uses a code block with return, which is correct. It is valid.
5. (String s) -> {return "geeksforgeeks";}: (valid). This lambda takes one parameter s of type String, but does not use it. That's still perfectly valid.
6. () -> { return "Hello" }: (invalid). Missing semicolon after the return statement inside the block.
7. x -> { return x + 1; }: (invalid). This is missing the type of parameter if used in a context where type inference is not possible
8. (int x, y) -> x + y: (invalid). If you specify the type of one parameter, you must specify the type of all parameters.

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