compact-canonical-constructor.yaml

---
id: 95
slug: "compact-canonical-constructor"
title: "Compact canonical constructor"
category: "language"
difficulty: "intermediate"
jdkVersion: "16"
oldLabel: "Java 16"
modernLabel: "Java 16+"
oldApproach: "Explicit constructor validation"
modernApproach: "Compact constructor"
oldCode: |-
  public record Person(String name,
                       List<String> pets) {
      // Full canonical constructor
      public Person(String name,
                    List<String> pets) {
          Objects.requireNonNull(name);
          this.name = name;
          this.pets = List.copyOf(pets);
      }
  }
modernCode: |-
  public record Person(String name,
                       List<String> pets) {
      // Compact constructor
      public Person {
          Objects.requireNonNull(name);
          pets = List.copyOf(pets);
      }
  }
summary: "Validate and normalize record fields without repeating parameter lists."
explanation: "Records can declare a compact canonical constructor that omits the parameter\
  \ list and field assignments. The compiler automatically assigns parameters to fields\
  \ after your validation logic runs. This is ideal for precondition checks, defensive\
  \ copies, and normalization."
whyModernWins:
- icon: "✂️"
  title: "Less repetition"
  desc: "No need to repeat parameter list or assign each field manually."
- icon: "🛡️"
  title: "Validation"
  desc: "Perfect for null checks, range validation, and defensive copies."
- icon: "📖"
  title: "Clearer intent"
  desc: "Compact syntax emphasizes validation, not boilerplate."
support:
  state: "available"
  description: "Widely available since JDK 16 (March 2021)"
prev: "language/static-members-in-inner-classes"
next: "language/call-c-from-java"
related:
- "language/records-for-data-classes"
- "language/flexible-constructor-bodies"
- "errors/record-based-errors"
docs:
- title: "Record (Java 21)"
  href: "https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/lang/Record.html"
- title: "JEP 395: Records"
  href: "https://openjdk.org/jeps/395"