Java Function Design Pattern in Object-Oriented Programming

Java functional design pattern can improve code reusability and maintainability through functional interfaces, lambda expressions and function combinations: define functional interfaces and clarify function types. Write anonymous functions using lambda expressions. Use function composition to link functions to create new functionality. Efficiently process lists via filter(), sort(), and function references.

Java Functional Design Pattern: Improve code reusability and maintainability

Introduction
Functional programming is a powerful paradigm in Java that focuses on creating reusable, easy-to-maintain code. This article will explore functional design patterns and demonstrate how to apply them to real-world cases to improve code quality.

Functional interface
The functional interface defines a function type, which has only one abstract method. Functional interfaces in Java are marked with the @FunctionalInterface annotation, for example:

@FunctionalInterface
interface MathOperation {
    int operate(int a, int b);
}

lambda expression
lambda expression allows us to simplify the code using anonymous functions. They are defined using the -> operator syntax:

MathOperation addition = (a, b) -> a + b;

Function composition
Function composition is the process of chaining functions together to form a new function. This can be achieved by using the andThen() and compose() methods:

MathOperation subtractThenAdd =
        MathOperation.add.andThen(MathOperation.subtract);

Practical case

Filtering List
Consider such a list, we want to filter out numbers less than 10:

List<Integer> numbers = List.of(1, 2, 3, 4, 8, 12, 15);

You can use the filter() method combined with lambda expression to implement filtering:

List<Integer> filteredNumbers =
        numbers.stream()
                .filter(number -> number >= 10)
                .toList();

Sort List
Now, we want to sort the filtered list in descending order:

List<Integer> sortedNumbers =
        filteredNumbers.stream()
                .sorted(Comparator.reverseOrder())
                .toList();

Function Reference
Function Reference Provided A more concise way to reference existing methods or constructors. They are defined using ClassName::methodName syntax:

MathOperation multiply = Math::multiplyExact;

Conclusion
The Java Functional Design Pattern provides powerful tools that enable us to create efficient, reusable and easy-to-maintain code. By leveraging functional interfaces, lambda expressions, function composition, and function references, we can greatly improve the quality of our code and simplify complex tasks.

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