How do you instantiate classes with cyclic references to each other without having to use `NULL` in Java?

Question

Suppose that you have the following classes which are perfectly legal in Java.

class Ping {
    Pong value;

    Ping(Pong value) { this.value = value; }
}

class Pong {
   Ping value;

   Pong(Ping value) { this.value = value; }
}

Is there any way to create an instance of Pong or Ping without giving their constructors a NULL value?

Answer 1

You could use something like this

class Ping {
    Pong value;
    Ping() {this.value = new Pong(this)}
    Ping(Pong value) {this.value = value}
}
class Pong {
    Ping value;
    Pong() {this.value = new Ping(this)}
    Pong(Ping value) {this.value = value}
}

Sadly this seems to be bad practice as described here: Java leaking this in constructor. So a better implementation would be to assign Pong after the creation of Ping.

class Ping {
    Pong value;
    Ping() {}
    Ping(Pong value) {this.value = value}
    public setPong(Pong pong) {
        this.value = pong;
    }
}
class Pong {
    Ping value;
    Pong() {}
    Pong(Ping value) {this.value = value}
    public setPing(Ping ping) {
        this.value = ping;
    }
}
Ping ping = new Ping();
Pong pong = new Pong(ping);
ping.setPong(pong);