Is it good practice to make the constructor throw an exception?

Question

Is it a good practice to make the constructor throw an exception? For example I have a class Person and I have age as its only attribute. Now I provide the class as

class Person{
  int age;
  Person(int age) throws Exception{
   if (age<0)
       throw new Exception("invalid age");
   this.age = age;
  }

  public void setAge(int age) throws Exception{
  if (age<0)
       throw new Exception("invalid age");
   this.age = age;
  }
}

Answer 1

Throwing exceptions in a constructor is not bad practice. In fact, it is the only reasonable way for a constructor to indicate that there is a problem; e.g. that the parameters are invalid.

I also think that throwing checked exceptions can be OK¹, assuming that the checked exception is 1) declared, 2) specific to the problem you are reporting, and 3) it is reasonable to expect the caller to deal with a checked exception for this².

However explicitly declaring or throwing java.lang.Exception is almost always bad practice.

You should pick an exception class that matches the exceptional condition that has occurred. If you throw Exception it is difficult for the caller to separate this exception from any number of other possible declared and undeclared exceptions. This makes error recovery difficult, and if the caller chooses to propagate the Exception, the problem just spreads.

---

^{1 - Some people may disagree, but IMO there is no substantive difference between this case and the case of throwing exceptions in methods. The standard checked vs unchecked advice applies equally to both cases.}

^{2 - For example, the existing FileInputStream constructors will throw FileNotFoundException if you try to open a file that does not exist. Assuming that it is reasonable for FileNotFoundException to be a checked exception3, then the constructor is the most appropriate place for that exception to be thrown. If we threw the FileNotFoundException the first time that (say) a read or write call was made, that is liable to make application logic more complicated.}

^{3 - Given that this is one of the motivating examples for checked exceptions, if you don't accept this you are basically saying that all exceptions should be unchecked. That is not practical ... if you are going to use Java.}

---

Someone suggested using assert for checking arguments. The problem with this is that checking of assert assertions can be turned on and off via a JVM command-line setting. Using assertions to check internal invariants is OK, but using them to implement argument checking that is specified in your javadoc is not a good idea ... because it means your method will only strictly implement the specification when assertion checking is enabled.

The second problem with assert is that if an assertion fails, then AssertionError will be thrown, and received wisdom is that it is a bad idea to attempt to catch Error and any of its subtypes.