How do you declare x and y so that x+=y gives a compilation error and x=x+y not?

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 https://stackoverflow.com/questions/7456462/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.

EDIT:

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.

It is not possible.

X x = ...;
Y y = ...;

x += y;         //1
//equivalent to
x = (X) (x+y);  //2

x = x+y;        //3

Suppose the type of x+y is Z. #2 requires a casting conversion from Z to X; #3 requires an assignment conversion from Z to X. "casting conversions are more inclusive than assignment conversions"(1). Therefore, as long as #3 is legal, #2 is legal, and #1 is legal.

On the reverse side, it is possible that #1 is legal, but #3 is illegal, for example

    byte x = 0;
    int y  = 1;
    x+=y;     // ok, x=(byte)(x+y), cast int to byte is allowed.
    x = x+y;  // error, assign int to byte

This information is not useful whatsoever; it is a flaw of Java making such surprising differences.

(1) http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.5

int  i = 5;
String s = "a";
System.out.println(i+=s);  //Error
System.out.println(i+s);   // No error

Basically, works for Any object or any non-string primitive and String combination.

I wonder which company it was? :)

This thing will not always give you compilation error

If you are doing smoething like this :

class A{
public static void main(String args[]){
	String x = "10";
	String y = "s";
	x += y;
	System.out.println(x);
}
}

It will work fine

even if you do

class A{
public static void main(String args[]){
	int x = 10;
	float y = 11.5F;
	x += y;
	System.out.println(x);
}
}

it will work properly.

But if you take x and y two different type of variables like :

class X{
 }
class A{
public static void main(String args[]){
	X x = new X();
	float y = 11.5F;
	x += y;
	System.out.println(x);
}
}

In such cases it will fail to compile.

*Even you can concatinate any int, float etc with String.