a parameterised constructor that is not returning an object

[Manohar Reddy Gopireddy]

Recently I have faced a question in one of my interviews. Here is that.

Suppose say i am having a class TestConstructor, and this class is having a parameterised constructor which takes an int as a parameter.

I am using this class somewhere else in my program as follows

int a = 10;
TestConstructor tc = new TestConstructor(a);

here if a is negative then the constructor should not return any object. that means tc is not referring any object.

can you please suggest me how to resolve this problem?

Thanks in advance

[Rob Spoor]

Constructors either throw an exception or lead to a successfully created object. There are no other choices when using public constructors.

You can start using factory methods instead. Make the constructor(s) private (or protected if you need to allow sub classing):

public class TestConstructor
{
    private TestConstructor(int value)
    {
        ...
    }

    public static TestConstructor create(int value)
    {
        return (value < 0 ? null : new TestConstructor(value);
    }
}

int a = 10;   
TestConstructor tc = TestConstructor.create(a); // a valid object
a = -1;
tc = TestConstructor.create(a); // null

Still, throwing exceptions (IllegalArgumentException) from the constructor is not uncommon if anyone provides wrong input to it.

[Manohar Reddy Gopireddy]

Thank you very much Rob

[Campbell Ritchie]

I would prefer to throw an Exception

public class TestConstructor
{
   private int i;

   /**
    * . . .
    *@throws IllegalArgumentException if <tt>i</tt> is negative
    */
   public TestConstructor(int i)
   {
      if (i < 0)
      {
         throw new IllegalArgumentException("Negative values not permitted");
      }
      this.i = i;
   }
 . . .
}

Note that there is no need for an else after the if. You should describe the Exception in the documentation comments.