Why doesn't Java offer operator overloading?

Question

There are a lot of posts complaining about operator overloading.

I felt I had to clarify the "operator overloading" concepts, offering an alternative viewpoint on this concept.

Code obfuscating?

This argument is a fallacy.

Obfuscating is possible in all languages...

It is as easy to obfuscate code in C or Java through functions/methods as it is in C++ through operator overloads:

// C++
T operator + (const T & a, const T & b) // add ?
{
   T c ;
   c.value = a.value - b.value ; // subtract !!!
   return c ;
}

// Java
static T add (T a, T b) // add ?
{
   T c = new T() ;
   c.value = a.value - b.value ; // subtract !!!
   return c ;
}

/* C */
T add (T a, T b) /* add ? */
{
   T c ;
   c.value = a.value - b.value ; /* subtract !!! */
   return c ;
}

...Even in Java's standard interfaces

For another example, let's see the Cloneable interface in Java:

You are supposed to clone the object implementing this interface. But you could lie. And create a different object. In fact, this interface is so weak you could return another type of object altogether, just for the fun of it:

class MySincereHandShake implements Cloneable
{
    public Object clone()
    {
       return new MyVengefulKickInYourHead() ;
    }
}

As the Cloneable interface can be abused/obfuscated, should it be banned on the same grounds C++ operator overloading is supposed to be?

We could overload the toString() method of a MyComplexNumber class to have it return the stringified hour of the day. Should the toString() overloading be banned, too? We could sabotage MyComplexNumber.equals to have it return a random value, modify the operands... etc. etc. etc..

In Java, as in C++, or whatever language, the programmer must respect a minimum of semantics when writing code. This means implementing a add function that adds, and Cloneable implementation method that clones, and a ++ operator than increments.

What's obfuscating anyway?

Now that we know that code can be sabotaged even through the pristine Java methods, we can ask ourselves about the real use of operator overloading in C++?

Clear and natural notation: methods vs. operator overloading?

We'll compare below, for different cases, the "same" code in Java and C++, to have an idea of which kind of coding style is clearer.

Natural comparisons:

// C++ comparison for built-ins and user-defined types
bool    isEqual          = A == B ;
bool    isNotEqual       = A != B ;
bool    isLesser         = A <  B ;
bool    isLesserOrEqual  = A <= B ;

// Java comparison for user-defined types
boolean isEqual          = A.equals(B) ;
boolean isNotEqual       = ! A.equals(B) ;
boolean isLesser         = A.comparesTo(B) < 0 ;
boolean isLesserOrEqual  = A.comparesTo(B) <= 0 ;

Please note that A and B could be of any type in C++, as long as the operator overloads are provided. In Java, when A and B are not primitives, the code can become very confusing, even for primitive-like objects (BigInteger, etc.)...

Natural array/container accessors and subscripting:

// C++ container accessors, more natural
value        = myArray[25] ;         // subscript operator
value        = myVector[25] ;        // subscript operator
value        = myString[25] ;        // subscript operator
value        = myMap["25"] ;         // subscript operator
myArray[25]  = value ;               // subscript operator
myVector[25] = value ;               // subscript operator
myString[25] = value ;               // subscript operator
myMap["25"]  = value ;               // subscript operator

// Java container accessors, each one has its special notation
value        = myArray[25] ;         // subscript operator
value        = myVector.get(25) ;    // method get
value        = myString.charAt(25) ; // method charAt
value        = myMap.get("25") ;     // method get
myArray[25]  = value ;               // subscript operator
myVector.set(25, value) ;            // method set
myMap.put("25", value) ;             // method put

In Java, we see that for each container to do the same thing (access its content through an index or identifier), we have a different way to do it, which is confusing.

In C++, each container uses the same way to access its content, thanks to operator overloading.

Natural advanced types manipulation

The examples below use a Matrix object, found using the first links found on Google for "Java Matrix object" and "C++ Matrix object":

// C++ YMatrix matrix implementation on CodeProject
// http://www.codeproject.com/KB/architecture/ymatrix.aspx
// A, B, C, D, E, F are Matrix objects;
E =  A * (B / 2) ;
E += (A - B) * (C + D) ;
F =  E ;                  // deep copy of the matrix

// Java JAMA matrix implementation (seriously...)
// http://math.nist.gov/javanumerics/jama/doc/
// A, B, C, D, E, F are Matrix objects;
E = A.times(B.times(0.5)) ;
E.plusEquals(A.minus(B).times(C.plus(D))) ;
F = E.copy() ;            // deep copy of the matrix

And this is not limited to matrices. The BigInteger and BigDecimal classes of Java suffer from the same confusing verbosity, whereas their equivalents in C++ are as clear as built-in types.

Natural iterators:

// C++ Random Access iterators
++it ;                  // move to the next item
--it ;                  // move to the previous item
it += 5 ;               // move to the next 5th item (random access)
value = *it ;           // gets the value of the current item
*it = 3.1415 ;          // sets the value 3.1415 to the current item
(*it).foo() ;           // call method foo() of the current item

// Java ListIterator<E> "bi-directional" iterators
value = it.next() ;     // move to the next item & return the value
value = it.previous() ; // move to the previous item & return the value
it.set(3.1415) ;        // sets the value 3.1415 to the current item

Natural functors:

// C++ Functors
myFunctorObject("Hello World", 42) ;

// Java Functors ???
myFunctorObject.execute("Hello World", 42) ;

Text concatenation:

// C++ stream handling (with the << operator)
                    stringStream   << "Hello " << 25 << " World" ;
                    fileStream     << "Hello " << 25 << " World" ;
                    outputStream   << "Hello " << 25 << " World" ;
                    networkStream  << "Hello " << 25 << " World" ;
anythingThatOverloadsShiftOperator << "Hello " << 25 << " World" ;

// Java concatenation
myStringBuffer.append("Hello ").append(25).append(" World") ;

Ok, in Java you can use MyString = "Hello " + 25 + " World" ; too... But, wait a second: This is operator overloading, isn't it? Isn't it cheating???

:-D

Generic code?

The same generic code modifying operands should be usable both for built-ins/primitives (which have no interfaces in Java), standard objects (which could not have the right interface), and user-defined objects.

For example, calculating the average value of two values of arbitrary types:

// C++ primitive/advanced types
template<typename T>
T getAverage(const T & p_lhs, const T & p_rhs)
{
   return (p_lhs + p_rhs) / 2 ;
}

int     intValue     = getAverage(25, 42) ;
double  doubleValue  = getAverage(25.25, 42.42) ;
complex complexValue = getAverage(cA, cB) ; // cA, cB are complex
Matrix  matrixValue  = getAverage(mA, mB) ; // mA, mB are Matrix

// Java primitive/advanced types
// It won't really work in Java, even with generics. Sorry.

Discussing operator overloading

Now that we have seen fair comparisons between C++ code using operator overloading, and the same code in Java, we can now discuss "operator overloading" as a concept.

Operator overloading existed since before computers

Even outside of computer science, there is operator overloading: For example, in mathematics, operators like +, -, *, etc. are overloaded.

Indeed, the signification of +, -, *, etc. changes depending on the types of the operands (numerics, vectors, quantum wave functions, matrices, etc.).

Most of us, as part of our science courses, learned multiple significations for operators, depending on the types of the operands. Did we find them confusing, them?

Operator overloading depends on its operands

This is the most important part of operator overloading: Like in mathematics, or in physics, the operation depends on its operands' types.

So, know the type of the operand, and you will know the effect of the operation.

Even C and Java have (hard-coded) operator overloading

In C, the real behavior of an operator will change according to its operands. For example, adding two integers is different than adding two doubles, or even one integer and one double. There is even the whole pointer arithmetic domain (without casting, you can add to a pointer an integer, but you cannot add two pointers...).

In Java, there is no pointer arithmetic, but someone still found string concatenation without the + operator would be ridiculous enough to justify an exception in the "operator overloading is evil" creed.

It's just that you, as a C (for historical reasons) or Java (for personal reasons, see below) coder, you can't provide your own.

In C++, operator overloading is not optional...

In C++, operator overloading for built-in types is not possible (and this is a good thing), but user-defined types can have user-defined operator overloads.

As already said earlier, in C++, and to the contrary to Java, user-types are not considered second-class citizens of the language, when compared to built-in types. So, if built-in types have operators, user types should be able to have them, too.

The truth is that, like the toString(), clone(), equals() methods are for Java (i.e. quasi-standard-like), C++ operator overloading is so much part of C++ that it becomes as natural as the original C operators, or the before mentioned Java methods.

Combined with template programming, operator overloading becomes a well known design pattern. In fact, you cannot go very far in STL without using overloaded operators, and overloading operators for your own class.

...but it should not be abused

Operator overloading should strive to respect the semantics of the operator. Do not subtract in a + operator (as in "do not subtract in a add function", or "return crap in a clone method").

Cast overloading can be very dangerous because they can lead to ambiguities. So they should really be reserved for well defined cases. As for && and ||, do not ever overload them unless you really know what you're doing, as you'll lose the the short circuit evaluation that the native operators && and || enjoy.

So... Ok... Then why it is not possible in Java?

Because James Gosling said so:

I left out operator overloading as a fairly personal choice because I had seen too many people abuse it in C++.

James Gosling. Source: http://www.gotw.ca/publications/c_family_interview.htm

Please compare Gosling's text above with Stroustrup's below:

Many C++ design decisions have their roots in my dislike for forcing people to do things in some particular way [...] Often, I was tempted to outlaw a feature I personally disliked, I refrained from doing so because I did not think I had the right to force my views on others.

Bjarne Stroustrup. Source: The Design and Evolution of C++ (1.3 General Background)

Would operator overloading benefit Java?

Some objects would greatly benefit from operator overloading (concrete or numerical types, like BigDecimal, complex numbers, matrices, containers, iterators, comparators, parsers etc.).

In C++, you can profit from this benefit because of Stroustrup's humility. In Java, you're simply screwed because of Gosling's personal choice.

Could it be added to Java?

The reasons for not adding operator overloading now in Java could be a mix of internal politics, allergy to the feature, distrust of developers (you know, the saboteur ones that seem to haunt Java teams...), compatibility with the previous JVMs, time to write a correct specification, etc..

So don't hold your breath waiting for this feature...

But they do it in C#!!!

Yeah...

While this is far from being the only difference between the two languages, this one never fails to amuse me.

Apparently, the C# folks, with their "every primitive is a struct, and a struct derives from Object", got it right at first try.

And they do it in other languages!!!

Despite all the FUD against used defined operator overloading, the following languages support it: Kotlin, Scala, Dart, Python, F#, C#, D, Algol 68, Smalltalk, Groovy, Perl 6, C++, Ruby, Haskell, MATLAB, Eiffel, Lua, Clojure, Fortran 90, Swift, Ada, Delphi 2005...

So many languages, with so many different (and sometimes opposing) philosophies, and yet they all agree on that point.

Food for thought...

James Gosling likened designing Java to the following:

"There's this principle about moving, when you move from one apartment to another apartment. An interesting experiment is to pack up your apartment and put everything in boxes, then move into the next apartment and not unpack anything until you need it. So you're making your first meal, and you're pulling something out of a box. Then after a month or so you've used that to pretty much figure out what things in your life you actually need, and then you take the rest of the stuff -- forget how much you like it or how cool it is -- and you just throw it away. It's amazing how that simplifies your life, and you can use that principle in all kinds of design issues: not do things just because they're cool or just because they're interesting."

You can read the context of the quote here

Basically operator overloading is great for a class that models some kind of point, currency or complex number. But after that you start running out of examples fast.

Another factor was the abuse of the feature in C++ by developers overloading operators like '&&', '||', the cast operators and of course 'new'. The complexity resulting from combining this with pass by value and exceptions is well covered in the Exceptional C++ book.

Check out Boost.Units: link text

It provides zero-overhead Dimensional analysis through operator overloading. How much clearer can this get?

quantity<force>     F = 2.0*newton;
quantity<length>    dx = 2.0*meter;
quantity<energy>    E = F * dx;
std::cout << "Energy = " << E << endl;

would actually output "Energy = 4 J" which is correct.

The Java designers decided that operator overloading was more trouble than it was worth. Simple as that.

In a language where every object variable is actually a reference, operator overloading gets the additional hazard of being quite illogical - to a C++ programmer at least. Compare the situation with C#'s == operator overloading and Object.Equals and Object.ReferenceEquals (or whatever it's called).

Assuming you wanted to overwrite the previous value of the object referred to by a, then a member function would have to be invoked.

Complex a, b, c;
// ...
a = b.add(c);

In C++, this expression tells the compiler to create three (3) objects on the stack, perform addition, and copy the resultant value from the temporary object into the existing object a.

However, in Java, operator= doesn't perform value copy for reference types, and users can only create new reference types, not value types. So for a user-defined type named Complex, assignment means to copy a reference to an existing value.

Consider instead:

b.set(1, 0); // initialize to real number '1'
a = b; 
b.set(2, 0);
assert( !a.equals(b) ); // this assertion will fail

In C++, this copies the value, so the comparison will result not-equal. In Java, operator= performs reference copy, so a and b are now referring to the same value. As a result, the comparison will produce 'equal', since the object will compare equal to itself.

The difference between copies and references only adds to the confusion of operator overloading. As @Sebastian mentioned, Java and C# both have to deal with value and reference equality separately -- operator+ would likely deal with values and objects, but operator= is already implemented to deal with references.

In C++, you should only be dealing with one kind of comparison at a time, so it can be less confusing. For example, on Complex, operator= and operator== are both working on values -- copying values and comparing values respectively.

Groovy has operator overloading, and runs in the JVM. If you don't mind the performance hit (which gets smaller everyday). It's automatic based on method names. e.g., '+' calls the 'plus(argument)' method.