Define default constructor for record

Question

I have a record and want to add default constructor to it.

public record Record(int recordId) {
   public Record {

   }
}

But it created constructor with int param.

public final class Record extends java.lang.Record {
    private final int recordId;
    public Record(int);
    //other method
}

How can we add a default constructor to a record?

what you defined is a compact constructor which gets its declaration derived from the record header. There is a 1-1 match in name, type and variable arity between a record component and the corresponding formal parameter in the compact constructor, same applies for the canonical constructor. A record can only have a canonical constructor. If declared the compact constructor is the canonical constructor — Vicente Romero, Apr 30 '20 at 18:29

score 62 · Accepted Answer · edited Apr 14 '20 at 09:36

62

To split hairs, you cannot ever define a default constructor, because a default constructor is generated by the compiler when there are no constructors defined, thus any defined constructor is by definition not a default one.

If you want a record to have a no-arg constructor, records do allow adding extra constructors or factory methods, as long as the "canonical constructor" that takes all of the record fields as arguments is called.

public record Record(int recordId) {
   public Record() {
      this(0); 
   }
}

edited Apr 14 '20 at 09:36

Lii

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answered Apr 11 '20 at 06:03

ThisIsNoZaku

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A constructor taking no argument is called `default`. The compiler creates a default constructor by default. you can propose another implementation of the default constructor. Then, the compiler does not use the by default one anymore. – Marine Galantin Nov 29 '22 at 19:38

score 30 · Answer 2 · edited Jun 20 '20 at 09:12

Explicit constructor

In your case, you can explicitly specify a no-argument constructor with the delegation to the canonical constructor with a default value if you want to and this can be done as -

public Record(){
    this(Integer.MIN_VALUE);
}

In short, any non-canonical constructor should delegate to one, and that should hold true for the data-carrying nature of these representations.

Compact Constructor

On the other hand, note that the representation you had used in your code.

public Record {}

is termed as a "compact constructor" which represents a constructor accepting all arguments and that can also be used for validating the data provided as attributes of the record. A compact constructor is an alternate way of declaring the canonical constructor.

Define default constructor for record

2 Answers2

Explicit constructor

Compact Constructor

Linked