Constructor Types of Java Record Classes
With Record class type, various terminologies are introduced to Java developers. Compact constructor is one of them, and well known all-args constructor or all fields constructor is called a canonical constructor. Aside from these two constructor types, there is another type called a custom constructor for record classes. In the following sections, I’ll describe them with examples.
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Canonical Constructor
Almost every developer knows the canonical constructor type. It is the all-args constructor. In my previous record class type blog posts, I have used the canonical constructor to show examples. You can check them too. Their links are at the end of this post.
By writing a canonical constructor, you can manipulate the args and set the fields as you wish. In the below example, we intended to trim the name argument and convert the email argument to lowercase.
public record UserRecord(String name, int age, String email) {
public UserRecord(String name, int age, String email) {
this.name = name.trim();
this.age = age;
this.email = email.toLowerCase(Locale.ROOT);
}
}Canonical constructors give us a chance to make validations and data manipulations for our record classes. However, it has its drawbacks. You need to repeat some parts of the code, even though you don’t change anything. The age field is an example of that. There is no change or any manipulation or normalization, but we should assign the age argument to the age field. But compact constructors save us from dealing with disadvantages of this kind.
Compact Constructor
A compact constructor can be seen as a no-args constructor. We cannot assign argument values to fields in this constructor type. However, we can handle validations or normalization. Code in the example below does the same operation that we had in the canonical constructor example with fewer lines of code.
public record UserRecord(String name, int age, String email) {
public UserRecord {
name = name.trim();
email = email.toLowerCase(Locale.ROOT);
}
}But as I mentioned it is not possible to assign argument values to the field, hence data manipulation is handled and the result is assigned back to the argument. If you attempt to assign an argument to a field you will end up with a “cannot assign a value to final variable ‘field_name’” error message.
Custom Constructor
We can create custom constructors for our specific requirements. Let's continue with the previous example. We have a UserRecord record class. In our business case, we don’t need to know the email address of the user. So we can set the email field with a null or empty value. In that case, our code for instance creation will look like this:
var user = new UserRecord(" a ", 18, null);But we can take care to avoid null parameter passing with a custom constructor.
public record UserRecord(String name, int age, String email) {
public UserRecord(String name, int age) {
this(name, age, null);
}
}With this new custom constructor, We can create a record instance without null parameter passing.
var user = new UserRecord(" a ", 18);The most critical part of the custom constructor is that you cannot make assignments. Instead, the custom constructor should call the canonical constructor of the record class with the manipulated arguments or with their custom values such as null for our example.
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