What is assertion really good for?

Question

Yes, I read Programming With Assertions but still can't find a situation where I can use assertions. When I want to check I value, I use an if-statement or a special exception because I want to react for such a specific case in a specific manner. If an assertion fails, the JVM just throws an error. An error is something very dangerous but if an assertion is not true, the application should handle this and continue with controlled statements (e.g. error message but continue with next object).
What real life situations do you use assertions for?

Answer 1

What real life situations do you use assertions for?That's a tricky one. If by "real life" you mean "production environment" then the answer is "none".
An assertion should never ever happen in production code. It's a check for a condition that you think is impossible to meet. A quick example:
public class MyClass {
  private Foo foo;
  public MyClass(final Foo foo) {
    if (foo == null) {
      throw new IllegalArgumentException(foo);
    this.foo = foo;
  public void frobnicate() {
    assert foo != null;
    foo.frobnicate();
}That code looks harmless enough and (if I didn't make any incredibly stupid mistake and noone is fiddling with evil reflection tricks) the assert should never fail.
Now if someone added a default constructor to that class (for example to make it serializable) and forgot to make sure that each use correctly initialized foo then your assert would fail during development).
(All that is IMHO of course).