Oppiskele Functional Interfaces Explained | Getting Started with Lambda Expressions

To work effectively with lambda expressions in Java, you must understand the concept of functional interfaces. A functional interface is an interface that contains exactly one abstract method. This single abstract method defines the contract for the lambda expression, allowing you to provide behavior in a concise way.

Java provides the @FunctionalInterface annotation to indicate that an interface is intended to be a functional interface. This annotation is not required, but it helps the compiler and other developers by enforcing the rule that only one abstract method is present. If you mistakenly add a second abstract method, the compiler will generate an error if the annotation is present.

Main.java


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package com.example;

@FunctionalInterface
interface StringProcessor {
    String process(String input);
}

public class Main {
    public static void main(String[] args) {
        StringProcessor toUpperCase = (s) -> s.toUpperCase();
        StringProcessor addExclamation = (s) -> s + "!";

        System.out.println(toUpperCase.process("hello")); // Output: HELLO
        System.out.println(addExclamation.process("hello")); // Output: hello!
    }
}

Functional interfaces are the foundation for lambda expressions in Java. When you write a lambda expression, you are essentially providing an implementation for the single abstract method of a functional interface. This is why lambda expressions can only be assigned to variables of functional interface types.

Java's standard library provides many built-in functional interfaces in the java.util.function package. Some common examples include:

Predicate<T>: represents a boolean-valued function of one argument;
Function<T, R>: represents a function that accepts one argument and produces a result;
Consumer<T>: represents an operation that accepts a single input argument and returns no result;
Supplier<T>: represents a supplier of results.

These interfaces make it easy to use lambda expressions for common programming tasks, such as filtering, mapping, and consuming data.

Main.java


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package com.example;

import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<String> words = List.of("apple", "banana", "pear", "kiwi");
        Predicate<String> isShortWord = word -> word.length() <= 4;

        List<String> shortWords = words.stream()
            .filter(isShortWord)
            .collect(Collectors.toList());

        System.out.println(shortWords); // Output: [pear, kiwi]
    }
}

1. What makes an interface a functional interface?

2. Which annotation is used to indicate a functional interface?

3. Given the following interface, can it be used as a target for a lambda expression?

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Main.java


              1234567891011121314151617
            
package com.example;

@FunctionalInterface
interface StringProcessor {
    String process(String input);
}

public class Main {
    public static void main(String[] args) {
        StringProcessor toUpperCase = (s) -> s.toUpperCase();
        StringProcessor addExclamation = (s) -> s + "!";

        System.out.println(toUpperCase.process("hello")); // Output: HELLO
        System.out.println(addExclamation.process("hello")); // Output: hello!
    }
}

Java's standard library provides many built-in functional interfaces in the java.util.function package. Some common examples include:

Predicate<T>: represents a boolean-valued function of one argument;
Function<T, R>: represents a function that accepts one argument and produces a result;
Consumer<T>: represents an operation that accepts a single input argument and returns no result;
Supplier<T>: represents a supplier of results.

These interfaces make it easy to use lambda expressions for common programming tasks, such as filtering, mapping, and consuming data.

Main.java


              12345678910111213141516171819
            
package com.example;

import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<String> words = List.of("apple", "banana", "pear", "kiwi");
        Predicate<String> isShortWord = word -> word.length() <= 4;

        List<String> shortWords = words.stream()
            .filter(isShortWord)
            .collect(Collectors.toList());

        System.out.println(shortWords); // Output: [pear, kiwi]
    }
}

1. What makes an interface a functional interface?

2. Which annotation is used to indicate a functional interface?

3. Given the following interface, can it be used as a target for a lambda expression?

Oliko kaikki selvää?

Kiitos palautteestasi!

Osio 1. Luku 2