> Really? Does 3+"" really convert the 3 to a string in Java?
Yes. It is one of my favorite Java features...
If a binary + has one argument of type java.lang.String, then the
other object is coerced to string as well (unless the other operand is
also a string), using the following procedure:
If it is a primitive type, an instance of the appopriate helper is
created, in this case java.lang.Integer, so we only have
java.lang.Object specializations now (*).
Then, .toString() is invoked for the object, and the strings are
concatenated.
This allows to write things like
System.out.println("Name:"+name" Age:"+age);
so they did not have to copy the printf(3C) family of function, nor to
introduce operator overloading for iostream-style output.
Teaching this stuff to students, it turns out that there are two major
sources of confusion:
a) System.out.println("abc"+3+3) prints "abc33", whereas
System.out.println(3+3+"abc") prints "6abc", so the binary +
is not commutative.
b) Both System.out.println("result "+myobj) and
System.out.println(myobj) work, but for different reasons. In the
first case, it works because of the magic binary '+'. In the second
case, it works because println is overloaded for all types, i.e.
println(int);
println(double);
...
println(Object); // invokes println(o.toString());
println(String); // does the real work
Regards,
Martin
(*) Implementations don't have to create those temporaries; the usual
as-if rule applies.
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