"Private Constructors" - What's that for?

Question

I read a sentence from a book, saying that "Suppose that you want to create a class but you don't want it to be instantiated as an object. Just declear the class's constructors as private."
It's out of my imagination. What's the real user case for this weird "Private Consturctor"? Who knows?
Thanks.
CL

Answer 1

public class TestFact {
  private TestFact() {}
  public static TestFact makeTestFact(String s) {
    if ("Dog".equals(s)) {
      return new Dog();
    if ("Cat".equals(s)) {
      return new Cat();
    return null;
  protected String whatever() {
    return "foo";
  private static class Dog extends TestFact {
    public String toString() {
      return whatever() + " dog";
  private static class Cat extends TestFact { }
}I must be really bored at work today...