Declarative approach via interface with getters

declarative-interface.java

interface User {
  String name();
  int age();
}

class SimpleUser implements User {
  private String name;
  private int age;
  //...constructor
  String name () {
    return this.name;
  }
  int age() {
    return this.age;
  }
}


class JSONUser {
  private User;
  // constructor...
  public void value() {
    return String.format("{ \"name\":\"%s\", \"age\":\"%d\" }", this.user.name(), this.user.age())
  }
}

new JSONUser(
  new SimpleUser('John', 26)
).value()