That is a constructor. It's what's known as a "default constructor", i.e. it intializes all the values to some pre-determined initial value, in this case -1 and "unknown".
As a second note, if you are going to be providing other constructors that create an initial value (say, a constructor which also sets the city name), I would recommend calling that constructor from the the default constructor. This reduces the amount of code you have to check for problems.
public City() { // call the other constructor that actually assigns values this("unknown", -1); } public City(String name, int population) { this.name = name; this.population = population; }