1.4-Software-Development-Principles

Records in Java

Introduction

Records were introduced in Java for situation in which you need to store a set of data. That data is always final and cannot be changed. A record can be easily written when you don’t want to design an entire class. Afterward you can use these instances to run your program.

A specific purpose is temporarily storing data from a csv file, with the goal of storing it in another format. Certainly when no additional methods are needed for that data.

Note: Records are always Serialisable , but you cannot influence the way in which they store their data.

Records by example

public record Employee(String id, String firstName, String lastName, String email, int age) { }

This declaration provides you with an object what can be instantiated with the listed parameters:

Employee e = new Employee("fbo11", "Frederik", "Bonte", "f.bonte@saxion.nl", 52);

e.id();
e.firstName();
e.lastName();

Things are allowed

Records are bound to certain rules, so there are limits to what you can and cannot do.

Add your own version of the constructor

Yes, you may add checks that limit the provided arguments for instance: You don’t even need to write out all the constructor arguments.

record Rectangle(double length, double width) {
    public Rectangle {
        if (length <= 0 || width <= 0) {
            throw new java.lang.IllegalArgumentException(
                String.format("Invalid dimensions: %f, %f", length, width));
        }
    }
}

(From docs.oracle.com)

You may also overload the constructor, so that you need to provide fewer arguments:

record Rectangle(double length, double width) {
    public Rectangle(double size) {
		this(size, size);
    }
}

You may add additional methods

Yes, this is allowed, but if you are going to write a lot of these, why not simply use a class?

record Rectangle(double length, double width) {
    public double surface() {
		return length()*width();
    }
}

You may add static methods and variables

In the example the static variable is also final but that is not required!

If you need your record to provide a main function, then that is allowed.

public record SomeData(int x, int y, Color color) {
	// Please note: final is optional here.
	public static final Color DEFAULT_COLOR = Color.RED;
	
	public SomeData(int x, int y) {
		this(x,y,DEFAULT_COLOR);
	}

	public static void main(String[] args) {
		// This is legal :)
	}
}

Things that are NOT^* allowed

You may not extend records

Just like the stored data, the record class is final. That means cannot create a new class that extends from a record.

Conclusion

Records save time on designing classes. If you only need to store data, a record will do just fine. However, the moreadditions you make to your record, the less sense it makes to have chosen a record.