why would this code run without an error?

[sura watthana]

I don't understand why the code below would run without error since object orbiting has not been instantiated in the class Planet?

public interface HeavenlyBody { String describe(); }

class Star implements HeavenlyBody {
String starName;
public String describe() { return "star " + starName; }
}

class Planet {
String name;
Star orbiting;
public String describe() {
return "planet " + name + " orbiting " + orbiting.describe();
}
}

[Ernest Friedman-Hill]

It's not a complete program, as there's no "main" routine. The program could certainly run without error if no one called Planet.describe().