I ran into this question in an interview and couldn't come up with a solution. I know the vice versa can be done as shown in What does the "+=" operator do in Java?
So the question was like below.
..... x = .....; ..... y = .....; x += y; //compile error x = x + y; //works properly
Try this code
Object x = 1; String y = ""; x += y; //compile error x = x + y; //works properly
not entirely sure why this works, but the compiler says
The operator += is undefined for the argument type(s) Object, String
and I assume that for the second line,
toString is called on the Object.
It makes sense as the
+= operator is meaningless on a general Object. In my example I cast an int to an Object, but it only depends on
x being of type Object:
Object x = new Object();
It only works if
x is Object though, so I actually think it is more that String is a direct subclass of Object. This will fail for
x + y:
Foo x = new Foo();
for other types that I have tried.