Math Class

[Sadashiv Borkar]

Hi All,
While going to JLS I found out that the java.lang.Math class is having a constructor that is a private one thats the reason we r unable to create a object of Math class.
Can anyone please clarify what is the reason for declaring the constructor as a private one when there is no function available with the help of which i can create an object.Why can't the class be an abstract class.Whats the advantage of declaring the constructor as a private against declaring the class as an abstract.
Thanx in Advance
Sada

[Anonymous]

The Math class isn't abstract because it contains a bunch of static methods. An abstract class would define only abstract methods, but they would all have to be overridden in subclasses or classes that implement the abstract class. Math's static methods contain code, and so the class its self can't be abstract.

[Cindy Glass]

The developers of the Math class did not want you to ever create an instance of Math. When you think about it - what is a MATH??? Therefore they made the constructor private. What they do want you to do is to use the methods of the math class without making an instance. That is why all of the methods are static (don't need an instance to use the methods)

[Oliver Grass]

Hey Nick,
sorry, but that's not right...

Originally posted by Nick Riviera:
An abstract class would define only abstract methods, but they would all have to be overridden in subclasses or classes that implement the abstract class. Math's static methods contain code, and so the class its self can't be abstract.

First: the abstract keywords tells you, that you can't instantiate this class. That's all. You can have methods which are NOT abstract, and you won't get a compile error.
Second: Of course you can have static methods in an abstract class. No compiler will prevent you to do that...
See the following code for clarification

public abstract class AbstractTest {
public static void method1() {
System.out.println("method1");
}
public void method2() {
System.out.println("method1");
}
public static String method3() {
System.out.println("method1");
return "method1";
}
public void method4() {
System.out.println("method1");
}

}

Hope that clarifies the concept of abstract classes
cheers
Oliver

[Dave Boden]

OK, but the original question was "Why use private constructors when you could declare the class abstract"


The answer is:

1. Sun (quite rightly) don't want us idiots subclassing Math and changing the functionality. That's why the Math class is declared final.
   
2. You can't declare an abstract class final. Therefore, if Math were implemented as an abstract class, it could be subclassed.
   

So... it's not just a random choice between the two methods. By using the private constructors technique you can stop people both instantiating and subclassing Math. Yippe kay aye!

[Sadashiv Borkar]

Hi Dave,
Thanx for the answer it was really a nice one which cleared my doubts.
bye
take care
sada

[Neil Wheatley]

Almost 12 years since Dave's answer and I found it really useful. Thank you Dave.

[Campbell Ritchie]

Welcome to the Ranch

That is probably a record, a 12‑year‑old thread reopened. There is a Java Language Specification section about uninstantiable classes here.