Concatenation of Strings and characters

Question

The following statements,

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

Produce the output stringA5.

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The following however,

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

Produce string70.

Where is the difference?

Answer 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.

Answer 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