Declarative approach via getters

d.java

class User {
  private String name;
  private int age;
  
  // ... constructor
  
  public String name() {
    return this.name;
  }
  
  public 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 User('John', 26)
).value()