I have a class :
class Employee{...} then :
Employee aEmployee = new Employee(...); int i = 10; String str = aEmployee + i; it generates a ERROR when compiled, Why ?
EDIT: I didn't override the toString() method in the Employee class, but if I try this:
Employee aEmployee = new Employee(...); String h = "hello"; String str = aEmployee + h; this time will be fine, both compiling and running.
So: why is it OK after changing the int variable i to a String variable h?
aEmployee is not a String it's an Object.
Without knowing how your Employee object is set up, it's difficut to provide you with a suitable solution, but ...
String str = aEmployee.toString() + i; Will stop the compiler from complaining, but may not give you the result you're expecting.
You'd be better trying to perform this action with a known property of theEmployee
String str = aEmployee.aMethodThatReturnsString() + i; 
Employee is not a String, and cannot be concatenated using the + operator.
In fact, in Java, there is no such thing as operator overloading (besides a few that are baked into the language, String is a prime example of that).
May be
String str = aEmployee.EName + i; Where EName is a property of Employee Class and of type string
Employee aEmployee = new Employee(...);
aEmployee is an Object Reference Variable of Type Employee pointing to Employee object on the heap.
String str = aEmployee + i;
In the above statement you are trying to concatenate a String and an Object Reference Variable.
Try doing this.....
String str = aEmployee.toString() + i;
If you have not overridden the toString() method in Employee class, then you will get something like this, getClass().getName()@hashCode + i
Because aEmployee is a object of the Employee class so that object not belongs to the String class. if you need to concatenate with string you have to use .toString() method.
String str = aEmployee.toString() + i; 
Employee aEmployee = new Employee(...); aEmployee is an object reference not an String .so you can not concatenate it .
to combine you need to convert object to toString() then can concatenate.
The reason that the first example is failing to compile is because of the nature of the + operator. In Java, + is an overloaded operator whose meaning depends on the static types of the left and right operand sub-expressions.
If both operands are both primitive numeric types (or their wrapper types), the operator is a numeric addition.
If either or both operands are Strings, the operator is a String concatenation.
If neither of the above is true, the operator is not defined - a compilation error.
In your first example, Employee is neither a String or a number, and i is not a String either, so the + operator is not meaningful.
In the second example, you have changed the second operand to a String, so the operator is now recognized as a String concatenation. In fact, the Java treats this as equivalent to:
String str = aEmployee.toString() + h; As a corollary, you could have made the first example compile by changing it to:
String str = aEmployee.toString() + i; 
Employeeclass.