Question

Below code shows error at line 4

class MyClass{

    public MyClass(int a){ } //Line 2

    public static void main(String a[]){
        MyClass n = new MyClass(); //Line 4
        System.out.print("TRUE");
    }
}

But, once i remove line 2, it runs without any error. Although, I didn’t add default constructor. Why ?

Answer 1

Remember that Compiler provides your class with a default constructor only if you have not given any explicit constructor. As soon as you declare your own constructor, parameterized or 0-arg, the compiler won’t give you the default constructor.

Now in your code, you have declared a parameterized constructor, compiler won’t give a default one. So, you actually don’t have any 0-arg constructor, and hence you cannot use it.

once i remove line 2, it runs without any error. Although, I didn’t
add default constructor. Why ?

Of course if you remove your line 2, then you haven’t declared any explicit constructor, in which case Compiler adds a default 0-arg constructor, and hence your code succeeds. Also note that the default constructor is the one give by compiler. When you declare your 0-arg constructor, it’s not called a default one, but just a 0-arg constructor.

So, whenever you are declaring a parameterized constructor, make sure that you also declare a 0-arg constructor explicitly, if of course you are using it.

public MyClass() {
}