Throw exception for default constructor java

Question

How do I prevent the default constructor from being used in Java?

In my assessment it says:

"We don't want the user to use the default constructor since the user has to specify the HashCode, and maximum load factor"

I thought this would do the trick, but apparently not (dictionary is a class that is used to throw exceptions):

public boolean HashDictionary() throws DictionaryException {}

DictionaryException Class:

public class DictionaryException extends Throwable {

}

Test to make sure it throws an exception when using default contructor(Supplied by lecturer):

try
{
    HashDictionary h = new HashDictionary();
    System.out.println("***Test 1 failed");

}
catch (DictionaryException e) {
        System.out.println("   Test 1 succeeded");
}

I just want to know how I could do this, as I'm not familiar with a method of doing it. Thanks.

Answer 1

You can declare the default one as private if you do not want it to be called.

To answer your comment, you can throw an exception-

public HashDictionary() throws DictionaryException {
    throw new DictionaryException("Default constructor is not allowed.");
}

Answer 2

You can

a) Omit the default constructor

b) Make the default constructor private

c) Throw an exception when the default constructor is used

public HashDictionary() throws DictionaryException {
    throw new DictionaryException("Default constructor should not be used!");
}

Answer 3

Don't do it like that. Just make the default constructor private:

private HashDictionary() {}

EDIT: What was the boolean doing in the Constructor definition? I just copied it...

Answer 4

If you don't want the user to use the default constructor you can either omit the constructor if other non-default constructors are declared, or you can make the default constructor private .

The other issue you may be having is that the declaration you put in your question is not a constructor, it is a method. Constructors do not have return types.

Change this

public boolean HashDictionary() throws DictionaryException {}

to this

public HashDictionary() throws DictionaryException {}

better yet just change it to

private HashDictionary() {}

and then no one can access it outside of the class itself.

Answer 5

I ended up adding this:

 public DictionaryException(String error_string)
 {
     System.out.println(error_string);
 }

as the default constructor for DictionaryException

And then:

public HashDictionary() throws DictionaryException {
    throw new DictionaryException("Default constructor should not be used!");
}

This for the default constructor of the HashDictionary, the issue was that the type was a boolean, so I removed that and it seemed to work.

Answer 6

1. To Forbid the Default Constructor To Be Called from Outside the Class, use:

   private HashDictionary(){}

2. To Forbid the Default Constructor To Be Called from Even Inside the Class, use:

   Declare a parameterized constructor and don't write the default non-parameterized constructor inside the class. public HashDictionary(String string){ }

3. If you want some exception to be thrown while calling out the constructor, then:

   public HashDictionary() throws Exception {
   throw new Exception("throwing exception from constructor");
   }

Regards, Birendra.

Answer 7

If you don't want to use a default constructor but you want to instantiate a class you need to implement a custom constructor. You have to tell the user what is needed to create a valid instance. And that's it! Default constructor won't be available anymore. If a constructor logic is too complicated, feel free to extract private methos, including a private constructor if you feal it's ok in your case. If you have many ways of creating instance you can use static factory-like methods and private ctor. Making a private ctor only to disable the default one makes no sense (at least I don't see one).

If you don't want to instantiate a class (I mean you dont want to use the default ctor and you have no reason to create a custom ctor) this means you want to use only a static fields/methods, so make whole class static.

Oh, I assume that we're talking about non abstract classes.