What are Java Records and what do they automatically generate?

Records (JEP 395, finalised in Java 16) are a concise syntax for declaring immutable data-carrier classes. A single record declaration replaces a full class with private final fields, a canonical constructor, accessors, equals(), hashCode(), and toString().

// Declaration — the header defines the record's components
public record Person(String name, int age) {}

// Equivalent traditional class (simplified):
// private final String name;
// private final int age;
// public Person(String name, int age) { this.name = name; this.age = age; }
// public String name() { return name; }   // accessors, NOT getName()
// public int age()     { return age; }
// public boolean equals(Object o) { ... }
// public int hashCode() { ... }
// public String toString() { return "Person[name=" + name + ", age=" + age + "]"; }

Person p = new Person("Alice", 30);
System.out.println(p.name()); // Alice    (accessor, not getName)
System.out.println(p);        // Person[name=Alice, age=30]

// Compact canonical constructor — validate without repeating assignments
public record Range(int lo, int hi) {
    Range { // compact form — no parameter list
        if (lo > hi) throw new IllegalArgumentException(lo + " > " + hi);
        // assignments lo = lo; hi = hi; happen automatically at end
    }
}

// Records CAN have:
// - static fields and methods
// - instance methods (non-component)
// - custom constructors that delegate to canonical
// - implement interfaces

// Records CANNOT:
// - extend any class (implicitly extend java.lang.Record)
// - declare instance fields beyond components
// - be abstract

Records work seamlessly with Java 21 features: they are the natural carrier type for record patterns, they compose well with sealed interfaces for algebraic data types, and their auto-generated equals()/hashCode() make them safe as map keys and set elements.

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