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Parameterized lambda

Learn parameterized lambdas with (x) -> ....

Parameterized lambda

A lambda can take parameters: (String s) -> s.length(). The type is usually inferred; (s) -> s.length() also works. With a single parameter the parentheses are optional: s -> s.length().

Multiple parameters are separated by commas: (a, b) -> a + b. When the body is a single expression, no return is needed; Java returns the result automatically. With a block body return is required: (a, b) -> { return a + b; }.

Block lambda and return

When a lambda body needs multiple statements, you write a block with curly braces. Inside the block, a return statement returns the result. Example: (int a, int b) -> { int sum = a + b; return sum * 2; }.

Even with a single-line block body, return is required: (a, b) -> { return a + b; }. This differs from an expression body, where return is omitted. Mixing the two styles by mistake causes a compile error.

Practice A two-number adder lambda

Define java.util.function.BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;. Call it with int result = add.apply(3, 4);. Print the result with System.out.println(result);.

Starter code

Main.java
import java.util.function.BiFunction;

public class Main {
    public static void main(String[] args) {
        // Define a BiFunction<Integer, Integer, Integer> lambda, call apply, print

    }
}

Sample solution

Main.java
import java.util.function.BiFunction;

public class Main {
    public static void main(String[] args) {
        BiFunction<Integer, Integer, Integer> add = (a, b) -> a + b;
        int result = add.apply(3, 4);
        System.out.println(result);
    }
}

Expected output

7

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the single-parameter lambda

Question 
java.util.function.Function<String, Integer> len = ____ -> s.length();
Answer 
java.util.function.Function<String, Integer> len = s -> s.length();

Explanation: With one parameter parentheses are optional.

2 What does (a, b) -> a + b return?

  • A Runnable instance
  • Throws an error
  • The sum of a and b Correct answer
  • Prints a and b

Explanation: An expression body returns the result automatically; the sum is returned.

3 Match the lambda body types

Term Meaning
() -> ... No parameters
s -> ... Single parameter
(a, b) -> ... Two parameters
{ return ...; } Block body

All lessons in Intermediate

1 Classes

  1. class
  2. Attributes
  3. Methods
  4. new
  5. toString
  6. Summary - Classes

2 Constructor

  1. Constructor basics
  2. Parameterized constructor
  3. Constructor overloading
  4. this()
  5. Default constructor
  6. Summary - Constructor

3 Inheritance

  1. extends
  2. super
  3. Method overriding
  4. Object class
  5. final class and method
  6. Summary - Inheritance

4 Project: Library

  1. Class design
  2. Book model
  3. Member management
  4. Borrow flow
  5. Putting it together

5 Encapsulation

  1. private fields
  2. Getter
  3. Setter
  4. Package-private
  5. Record
  6. Summary - Encapsulation

6 Static

  1. static fields
  2. static methods
  3. static block
  4. Class variables
  5. instanceof
  6. Summary - Static

7 Constants

  1. static final
  2. Uppercase convention
  3. Constants with enum
  4. Interface constants
  5. Math constants
  6. Summary - Constants

8 Project: Quiz app

  1. Quiz model
  2. Adding questions
  3. Running the quiz
  4. Score calculation
  5. Putting it together

9 Interface

  1. interface
  2. implements
  3. default methods
  4. static methods
  5. Multiple interfaces
  6. Summary - Interface

10 Abstract class

  1. abstract class
  2. abstract methods
  3. Abstract and concrete
  4. Abstract vs interface
  5. Sealed class
  6. Summary - Abstract class

11 Polymorphism

  1. Polymorphism basics
  2. Dynamic binding
  3. Override rules
  4. Abstract reference
  5. Pattern matching
  6. Summary - Polymorphism

12 Project: Blog app

  1. Model design
  2. Blog class
  3. Adding comments
  4. Search and filter
  5. Putting it together

13 File I/O

  1. File class
  2. FileReader and FileWriter
  3. BufferedReader
  4. PrintWriter
  5. try-with-resources
  6. Summary - File I/O

14 Serialization

  1. Jackson ObjectMapper
  2. readValue
  3. writeValueAsString
  4. JsonProperty
  5. List and Map serialization
  6. Summary - Serialization

15 Generics

  1. Generic class
  2. Generic method
  3. Bounded type
  4. Wildcard
  5. Type erasure
  6. Summary - Generics

16 Generic collections

  1. List<E>
  2. Map<K,V>
  3. Set<E>
  4. Queue and Deque
  5. Collections class
  6. Summary - Generic collections

17 Iterator

  1. Iterator
  2. remove
  3. ListIterator
  4. Iterable
  5. Spliterator
  6. Summary - Iterator

18 Lambda expressions

  1. Lambda basics
  2. Parameterized lambda
  3. Method reference
  4. forEach with lambda
  5. Comparator with lambda
  6. Summary - Lambda expressions

19 Stream API basics

  1. stream()
  2. filter
  3. map
  4. forEach and count
  5. collect toList
  6. Summary - Stream API basics

20 Intermediate final

  1. Project design
  2. OOP model
  3. Stream layer
  4. File I/O
  5. Putting it together