How to Use Java Lambda Expressions (Anonymous Functions)?

Java Lambda expressions, aka anonymous functions, allow you to pass a block of code as a parameter.
In this way, you can write shorter, cleaner and more functional code.

🧠 Technical Summary

Main topic: Lambda expressions (anonymous functions) in Java
Purpose: Simplify code and increase readability by reducing unnecessary class definitions
Steps:

1. Learning the lambda expression structure
2. Experiment with examples on JShell
3. Implementing built-in functional interfaces (Predicate, Consumer, Function...)

💡 Lambda expressions support Java's functional programming approach and work integrated with the java.util.function package.

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⚙️ 1. Understanding Lambda Structure

Lambda general syntax:

(argümanlar) -> { gövde }

Arguments: Can be written in parentheses, without specifying the type (Java automatically detects it).

Arrow Sign (->): Connects the parameters to the code block.

Body: Contains the block of code to be executed.

💬 To test the examples using JShell, type jshell in the terminal and use the /exit command to exit.

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🐾 2. Simple Lambda Example

Let's create a list:

List<String> evcilHayvanlar = Arrays.asList("Köpek", "Kedi");

Let's print each item to the screen:

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

Simple version of the same code (method reference):

evcilHayvanlar.forEach(System.out::println);

💡 System.out::println is short for x -> System.out.println(x).

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📜 3. Multi-Line Lambda Body

Used if the lambda body contains more than one line:

evcilHayvanlar.forEach(x -> {
    System.out.println("Adı: " + x);
    System.out.println("Sıra No: " + evcilHayvanlar.indexOf(x));
});

Output:

Adı: Köpek
Sıra No: 0
Adı: Kedi
Sıra No: 1

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🧩 4. Built-in Functional Interfaces

The java.util.function package that comes with Java 8 provides ready-made interfaces for frequently used Lambda scenarios. These contain a single method and are directly compatible with Lambda expressions.

✅ Predicate (Condition Check)

Tests a condition, returns boolean.

Predicate<String> filtreleK = x -> x.startsWith("K");
evcilHayvanlar.stream().filter(filtreleK).forEach(System.out::println);

Output: Dog, Cat

📦 Consumer

It uses the value but does not return it (void is returned).

Consumer<String> hayvanYazdir = x -> System.out.println(x);
evcilHayvanlar.forEach(hayvanYazdir);

Output: Dog, Cat

🔄 Function (Value Conversion)

Converts one value to another value.

Function<String, String> buyukHarfYap = x -> x.toUpperCase();
evcilHayvanlar.stream().map(buyukHarfYap).forEach(System.out::println);

Output: DOG, CAT

⏰ Supplier (Value Provider)

It doesn't take any parameters, it produces a value when called.

Supplier<java.time.LocalTime> saatVer = () -> java.time.LocalTime.now();
System.out.println(saatVer.get());

Output: instant time like 21:52:38.384278

🔁 UnaryOperator (Single Operation)

Input and output type are the same, usually used in conversion.

UnaryOperator<String> buyukHarf = x -> x.toUpperCase();
System.out.println(buyukHarf.apply("kopek"));

Output: COPEK

➕ BinaryOperator (Binary Operation)

It combines two values, input and output type are the same.

BinaryOperator<Integer> toplama = (x, y) -> x + y;
System.out.println(toplama.apply(5, 3));

Output: 8

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❓ Frequently Asked Questions (FAQ)

1. Why are Lambda expressions anonymous?

Because they don't have names. They are written for one-time transactions.

2. What does method reference (::) do?

If Lambda calls only one method, this statement shortens the code: System.out::println.

3. Can all interfaces be Lambda?

No. There can be functional interfaces (@FunctionalInterface) that contain only a single abstract method.

4. What is the difference between Predicate and Function?

Predicate returns only boolean; Function releases the return type.

5. Where are Lambda expressions used?

Stream API is common in scenarios such as event management, filtering, and data processing.

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🏁 Conclusion

In this guide, you learned how to create Lambda expressions in Java, use method references and built-in interfaces (Predicate, Consumer, Function, etc.) in the java.util.function package.

Now you can write your codes with a shorter, readable and functional approach. 💡 You can test your own examples in the Java environment on GenixNode and run your codes instantly.