Java - How to only create an object with valid attributes?

Question

I'm doing a basic Java course and I came to a problem: How do I create an object only if I have passed valid parameters to the Constructor?

Should I make an alternate class and call the constructor from there after the validation is realized?

Or should/could I use a static method in the class for the validation?

What is the best practice in this case?

Answer 1

The standard practice is to validate the arguments in the constructor. For example:

class Range {
  private final int low, high;
  Range(int low, int high) {
    if (low > high) throw new IllegalArgumentException("low can't be greater than high");
    this.low = low;
    this.high = high;
  }
}

Side note: to verify that arguments are not null, which is fairly common, you can use:

import static java.util.Objects.requireNonNull;

Constructor(Object o) {
  this.o = requireNonNull(o); //throws a NullPointerException if 'o' is null
}

---

UPDATE

To reply to your specific comment about social security number. One way would be to add a method to the class:

//constructor
public YourClass(String ssn) {
  if (!isValidSSN(ssn)) throw new IllegalArgumentException("not a valid SSN: " + ssn);
  this.ssn = ssn;
}

public static boolean isValidSSN(String ssn) {
  //do some validation logic
}

The calling code could then look like:

String ssn = getSsnFromUser();
while(!YourClass.isValidSSN(ssn)) {
  showErrorMessage("Not a valid ssn: " + ssn);
  ssn = getSsnFromUser();
}
//at this point, the SSN is valid:
YourClass yc = new YourClass(ssn);

With that design, you have achieved two things:

• you validate the user input before using it (which you should always do - users are very good at typos)
• you have made sure that if YourClass is misused an exception is thrown and it will help you detect bugs

You could go further by creating a SSN class that holds the SSN and encapsulates the validation logic. YourClass would then accept a SSN object as an argument which is always a valid SSN by construction.