Record Patterns and Pattern Matching in Java

YatinAugust 13th, 2023Last Updated: August 13th, 2023

0 269 6 minutes read

Hello. In this tutorial, we will explore Java Records, Record Patterns, and Pattern Matching.

1. Recap of Java Records

Java Records is a new feature introduced in Java 16 (JEP 395) to simplify the creation of classes that are mainly used to store data and have no additional behavior other than accessing and manipulating the data. They are often referred to as “data classes” or “immutable value classes.” Records provide a concise and more readable way to declare and define such classes by automatically generating certain methods and constructors.

Here are the key characteristics and benefits of Java Records:

Concise Syntax: Records provide a more concise syntax compared to traditional Java classes. You can declare a record using the record keyword, followed by the class name and its components (fields). Here’s an example:
```
public record Person(String firstName, String lastName, int age) {

}
```
Automatic Methods: When you define a record, Java automatically generates several methods for you, including:
- Constructor: A constructor that takes parameters for each component field.
- Getter Methods: Implicit getter methods for each component field.
- equals(): An implementation of the equals() method that compares record instances based on their components.
- hashCode(): An implementation of the hashCode() method based on the record’s components.
- toString(): A human-readable string representation of the record’s components.
Immutability: By default, record components are final, making the record immutable. This means that once a record is created, its state cannot be changed. This immutability is useful for ensuring consistent and predictable behavior in your code.
Value-Based Equality: Records automatically implement value-based equality, meaning that two record instances are considered equal if their components are the same, even if they are different instances.
Inheritance: Records can extend other classes (except final classes) and implement interfaces, just like regular classes. However, the automatic generation of methods (like equals(), hashCode(), and toString()) remains consistent regardless of inheritance.
Customization: While the automatic methods are provided by default, you can still customize them if needed. For instance, you can provide your implementations for equals(), hashCode(), and toString() if the default behavior isn’t suitable for your use case.

Records are particularly useful when you need to create classes solely for storing data, reducing the amount of boilerplate code you would need to write with traditional Java classes. They enhance code readability and maintainability while ensuring good practices like immutability and value-based equality.

2. Pattern Matching

Pattern matching is a programming language feature that allows you to match the structure of data and execute code based on that structure. It’s a way to write more concise and readable code when dealing with complex conditional logic.

Switch expressions in Java allow you to use patterns as case labels, which makes your code more expressive and readable when performing multiple conditional checks. Here’s a simple example of how switch expressions work with pattern matching:

PatternMatchingExample.java

import com.jcg.example;

public class PatternMatchingExample {
    public static void main(String[] args) {
        Object data = 42;

        String result = switchExample(data);

        System.out.println(result);
    }

    public static String switchExample(Object data) {
        return switch (data) {
            case String s -> "It's a string: " + s;
            case Integer i -> "It's an integer: " + i;
            case Double d -> "It's a double: " + d;
            default -> "Unknown data";
        };
    }
}

2.1 Code Explanation

We start by defining a PatternMatchingExample class with a main method where we create an object named data and call the switchExample method with this object.
The switchExample method takes an Object parameter and returns a String. It uses a switch expression to match the given data against different cases.
Inside the switch expression, we have several cases using the arrow (->) notation. These cases utilize pattern matching to match the structure of the data object:

case String s ->: If the data is a String, the value is bound to the variable s, and we return a string that indicates it’s a string.
case Integer i ->: If the data is an Integer, the value is bound to the variable i, and we return a string that indicates it’s an integer.
case Double d ->: If the data is a Double, the value is bound to the variable d, and we return a string that indicates it’s a double.
default ->: If none of the above cases match, we return a default message indicating unknown data.

In the main method, we create an object data with the value 42 and then call the switchExample method, passing this object as an argument.
The result of the switchExample method is stored in the result variable, and we print the value of result to the console.

2.2 Output

In this example, the input data is an integer (42). The switch expression matches the integer case (case Integer i) and binds the value to the variable i. Since the input is an integer, the output indicates that it’s an integer with the value 42. This demonstrates how pattern matching in Java’s switch expressions can lead to more concise and readable code when dealing with complex conditional logic.

Output

It's an integer: 42

3 Record Patterns

A record pattern is a framework enabling us to compare values with a specific record type and link variables to the related components within the record. Furthermore, there’s an option to assign an identifier to the record pattern, effectively creating a named record pattern. This naming allows for convenient referencing of the associated record pattern variable.

Let’s consider a scenario where you have a Person record:

Person.java

public record Person(String firstName, String lastName, int age) {

}

Now, let’s say you want to perform pattern matching on a Person record instance to extract its components. Here’s how it might look in a hypothetical future version of Java that supports record patterns:

RecordPatternExample.java

import com.jcg.example;

public class RecordPatternExample {
    public static void main(String[] args) {
        Person person = new Person("John", "Doe");

        String result = matchPerson(person);

        System.out.println(result);
    }

    public static String matchPerson(Person person) {
        return switch (person) {
            case Person(String firstName, String lastName) -> "First Name: " + firstName + ", Last Name: " + lastName;
            default -> "Unknown person";
        };
    }
}

In the example above, the matchPerson method takes a Person record instance and uses a switch expression to perform pattern matching on it. The case Person(String firstName, String lastName) pattern matches the Person record’s components and binds them to the firstName and lastName variables. The code then returns a formatted string containing the extracted components.

4. Conclusion

In the dynamic landscape of software development, Java continues to evolve, integrating new features that enhance code clarity, conciseness, and readability. Among these, the introduction of Java Records, along with the potential concepts of Record Patterns and Pattern Matching, demonstrates the language’s commitment to adapting to modern programming paradigms.

Java Records: Java Records, introduced in Java 16, have brought a new level of simplicity and efficiency to the creation of data-centric classes. These data classes automatically generate essential methods such as constructors, getters, equals(), hashCode(), and toString(). This automation drastically reduces boilerplate code, promoting cleaner and more focused class definitions. By default, record components are immutable, aligning with the principles of functional programming and ensuring data consistency. Value-based equality and structural pattern matching are inherent features of records, making them powerful tools for representing data structures.
Record Patterns: Record patterns enable the extraction of values from a record and their assignment to variables through the utilization of pattern matching.
Pattern Matching: Pattern matching, introduced with Java 12’s switch expressions and expanded in later versions, empowers developers to write expressive and concise code when dealing with complex conditional logic. With pattern matching, developers can match specific patterns in data structures and perform appropriate actions. The integration of pattern matching with records, as in the example of deconstructing record instances, would be a natural extension, enhancing Java’s expressiveness.

In conclusion, Java Records, although relatively new, offer a promising approach to designing immutable and data-centric classes with minimal effort. The theoretical concepts of Record Patterns and the continued expansion of Pattern Matching showcase Java’s commitment to providing modern tools that facilitate better code organization, maintainability, and readability. As the Java ecosystem evolves, these features could significantly contribute to the language’s ability to meet the demands of contemporary software development practices. Developers are encouraged to stay informed about updates in Java and its evolving features to harness the full potential of these modern programming constructs.