Concatenation of Strings and characters

The following statements,

String string = "string";   

string = string +((char)65) + 5;
System.out.println(string);

Produce the output stringA5.

The following however,

String string = "string";

string += ((char)65) + 5;
System.out.println(string);

Produce string70.

Where is the difference?

Solution 1:

You see this behavior as a result of the combination of operator precedence and string conversion.

JLS 15.18.1 states:

If only one operand expression is of type String, then string conversion (§5.1.11) is performed on the other operand to produce a string at run time.

Therefore the right hand operands in your first expression are implicitly converted to string: string = string + ((char)65) + 5;

For the second expression however string += ((char)65) + 5; the += compound assignment operator has to be considered along with +. Since += is weaker than +, the + operator is evaluated first. There we have a char and an int which results in a binary numeric promotion to int. Only then += is evaluated, but at this time the result of the expression involving the + operator has already been evaluated.

Solution 2:

Case 1

string = string +((char)65) + 5;

everything is treated as String but in second case

Sequence of operation performed:

• string +((char)65 = stringA
• stringA + 5 = stringA5

Case 2

 string += ((char)65) + 5;

first right hand side is calculated means first operation will be like ((char)65) + 5, So result of ((char)65) + 5 is 70 and after that += operation.

Sequence of operation performed:

• (char)65 + 5 = 70
• string + 70 = string70

Lets see 1 more example

String string = "string";
string += ((char)65) + 5 + "A";
System.out.println(string); 

Output string70A

Reason Same first right hand side is calculated and sequesce of opertion performed is

• (char)65 + 5 = 70
• 70 + "A" = 70A
• string + 70A = string70A