Initialising immutable fields – in a few modern languages

A_Location.java

// Java - has not changed since Java 1.0 up until Java 11.
// yes, you need to name each field exactly four times and its type two times.
class Location {
  final double lon;
  final double lat;
  
  Location(double lon, double lat) {
    this.lon = lon;
    this.lat = lat;
  }
}

Location.dart

// Dart
// fields are still named twice, but this has advantages when adding more constructors
class Location {
  final double lat;
  final double lon;
  Location(this.lat, this.lon);
}

Location.kt

// Kotlin
// showing of its roots in functional programming with "algebraic data types"
data class GeoLoc(val lat: Double, val lon: Double) { }

// the 'data' keyword is not needed for the initializer and definition syntax to work
// but it is very helpful because it creates sensible implementations of equals() and hashCode()
// which need extra work in other languages

Location.ts

// TypeScript
// also only one mention per field: the feature is called "parameter properties"
class Location {
  constructor(readonly lat: number, readonly lon: number) {
  }
}

X_Auto_Location.java

// using AutoValue, https://github.com/google/auto/blob/master/value/userguide/index.md
// like the Kotlin "data class" this also gives sensible implementations for equals() and hashCode()
import com.google.auto.value.AutoValue;

@AutoValue
abstract class Location {
  static Location create(double lat, double lon) {
    return new AutoValue_Location(lat, lon);
  }

  abstract double lat();
  abstract double lon();
}

X_Lombok_Location.java

// using Lombok, https://projectlombok.org/features/Value
// makes fields private and creates getters and no setters, 
// automatically creates constructor with all arguments, also creates equals and hashCode()
import lombok.Value;

@Value
class Location {
  double lat;
  double lon;
}