Lambda expressions in Java

With the release of Java 8, lambda expressions have become one of the most concerned and discussed topics among Java developers. Lambda expressions can simplify Java programmers' tedious writing methods, while also improving program readability and maintainability. In this article, we will take a deep dive into lambda expressions in Java and how they provide a simpler and more intuitive programming experience in Java code.

What is a Lambda expression?

Lambda expression is an anonymous function that can be passed as a parameter to a method or returned from a method. Lambda expressions in Java were mainly introduced to implement functional programming to simplify the writing of anonymous inner classes. In Java, the writing of anonymous inner classes is verbose and unintuitive in some cases. The introduction of lambda expressions makes the writing of code simpler and easier to read.

The syntax of Lambda expression

The lambda expression in Java mainly consists of arrow symbols, parameter list and function body. The following is the basic syntax of lambda expression:

(parameter1, parameter2, ..., parameterN) -> {function body}

The above syntax is explained as follows:

• parameter1, parameter2, ..., parameterN: Represents the parameters of lambda expression.
• ->: Arrow symbol, use arrow symbol to separate parameter list and method body.
• {function body}: Represents the method body of the lambda expression.

The sample code is as follows:

// Lambda expression example
public class LambdaExample {

public static void main(String[] args) {
    // 傳遞 lambda 表達式給 Thread 類
    Thread t1 = new Thread(() -> System.out.println("Lambda 表達式"));
    t1.start();
}

}

In the above code, lambda expression() -> System.out.println("Lambda expression") is an anonymous function that will be passed as a parameter to the Thread class in Java.

Type inference for Lambda expressions

In some cases, the Java compiler can automatically infer the type of a lambda expression through the context. For example, when you pass a lambda expression as a parameter to a functional interface, the compiler automatically infers the type of the lambda expression. The sample code is as follows:

// Lambda expression type inference example
public class LambdaTypeInferenceExample {

public static void main(String[] args) {
    // 在 lambda 表達式作為參數時可以自動推斷類型
    List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
    numbers.forEach((Integer value) -> System.out.println(value));
}

}

In the above code, Lambda expression (Integer value) -> The type of System.out.println(value) can be automatically inferred without explicitly specifying the type.

Variable scope of Lambda expressions

Unlike anonymous inner classes, lambda expressions can access local variables of the method in which they are located. These local variables are called the "free variables" of the lambda expression. In a lambda expression, free variables can be accessed, but the free variables must be implicitly final or de facto final. The following is a sample code:

// Free variable example of Lambda expression
public class LambdaFreeVariableExample {

public static void main(String[] args) {
    String message = "Hello";
    Runnable runnable = () -> {
        System.out.println(message);
    };
    runnable.run();
}

}

In the above code, the lambda expression () -> System.out.println(message) accesses the free variable message because the message variable is implicitly final.

Lambda Expressions and Stream API

Java 8 also introduces a new Stream API that is closely related to lambda expressions. The Stream API provides a way to operate collections, allowing developers to process collection data more conveniently. Many methods in the Stream API accept lambda expressions as parameters. The following is a sample code:

// Stream API Example
public class StreamExample {

public static void main(String[] args) {
    List<String> list = Arrays.asList("Java", "Spring", "Hibernate", "Struts");
    long count = list.stream()
                    .filter(s -> s.startsWith("S"))
                    .count();
    System.out.println(count);
}

}

In the above code, lambda expressions -> s.startsWith("S") is passed to the Stream API as a parameter of the filter method filter(), which is used to filter strings starting with S.

Summary

The lambda expression in Java is a very important feature, which provides Java programmers with a concise, efficient, easy-to-read, and easy-to-maintain coding method. Not only that, lambda expressions are also closely related to other new features such as Stream API, providing a more convenient and efficient way for Java development. If you are a Java developer, you must actively master and use lambda expressions, which will bring great help to your development work.

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