CODEWITHSUNDEEP
javaoverloadingprimitive-types
Chthonic Project
Chthonic Project

Reputation: 8346

Java primitives and overloading

I have always heard (and thought of) Java as a strongly typed language. But only recently did I notice something that I have been using almost on a daily basis: int and double overloading.

I can write the following, and it is valid Java code:

int i = 1;
double j = 1.5;
double k = i + j;

But, if I have a method, one of whose arguments is a double, I need to specify it:

public static <K, V> V getOrDefault(K k, Map<K, V> fromMap, V defaultvalue) {
    V v = fromMap.get(k);
    return (v == null) ? defaultvalue : v;
}

When I call the above method on a Map<String, Double>, the defaultvalue argument cannot be an int:

getOrDefault(aString, aStringDoubleMap, 0); // won't compile
getOrDefault(aString, aStringDoubleMap, 0d); // compiles and runs just fine

Why does Java overload an int to double (just like it does in addition), and then autobox it to Double? I think the answer lies in how Java does operator overloading (i.e. the overloading happens in the + operator, and not from int to double), but I am not sure.

Here's hoping that SO can help me out on this.

Upvotes: 4

Views: 171

Answers (4)

Sotirios Delimanolis
Sotirios Delimanolis

Reputation: 279930

That's because primitives don't work with generics. They need to be boxed.

For the invocation 

getOrDefault(aString, aStringDoubleMap, 0); // won't compile

to work, Java would have to box the 0 to an Integer, then somehow convert that to a Double. That's not allowed by the language. It's similar to why you can't do 

Double value = 3; // Type mismatch: cannot convert from int to Double

From the JLS, on invocation contexts

If the type of the expression cannot be converted to the type of the parameter by a conversion permitted in a loose invocation context, then a compile-time error occurs.

The type of the expression, 0, an integer literal, is int. Loose invocation contexts are defined as

Loose invocation contexts allow a more permissive set of conversions, because they are only used for a particular invocation if no applicable declaration can be found using strict invocation contexts. Loose invocation contexts allow the use of one of the following:

  • an identity conversion (§5.1.1)
  • a widening primitive conversion (§5.1.2)
  • a widening reference conversion (§5.1.5)
  • a boxing conversion (§5.1.7) optionally followed by widening reference conversion
  • an unboxing conversion (§5.1.8) optionally followed by a widening primitive conversion

int to Double is not supported by any of those.

If you simply had

public static void main(String[] args) throws Exception {
    method(3);
}

public static void method(double d) {
}

it would work.

Upvotes: 4

Renjith
Renjith

Reputation: 3274

Java does not support operator overloading(An exception is String concat (+)) operator.

double k = i + j;

Here what is happening is implicit casting.A data type of lower size is widened to a data type of higher size. This is done implicitly by the JVM.

And for getOrDefault, primitives wont work with generics.And here comes autoboxing. When you call getOrDefault(aString, aStringDoubleMap, 0d);, the 0d will be autoboxed to Double object. But JVM cannot autobox 0 to a Double object in your first case.

Java will not perform a widening primitive conversion (0 to 0d) and a boxing conversion (double to Double) implicitly.

Check this link

An implicit cast from int to double, followed by boxing to Double, is not allowed.

0 can be only autoboxed to Integer. 0d can be autoboxed to Double.

Upvotes: 1

Andrew Aitken
Andrew Aitken

Reputation: 681

You're looking for the exciting section 5.2 of the Java Language specification.

Basically when you add an int and double it performs a widening conversion. But it doesn't know to do this when trying to Autobox an int to a Double. It's explicitly disallowed in fact.

Upvotes: 3

dsh
dsh

Reputation: 12213

The int -> double conversion is a widening conversion. Widening conversions do not lose data, so they are performed automatically.

Upvotes: -1

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