Question

When I run the following code:

public class LianXi1 
{ 
   public static void main(String args[]) 
   { 
     int a=12; 
     int b=23; 
     System.out.println("case 1"+a); //

     System.out.println("case 2"+b); //

   } 
} 

I get this result:

@ubuntu:~/mycode/test$ java LianXi1 
case 112
case 223

But I don’t understand the result, who can help?

Answer 1

When you add a string and an integer together, it performs “String concatenation”, where it’s converting your integer into a string and sticking it on the end of the other string.

"Case 1" + 12 

… is the same as

"Case 1" + "12"

So your result will be the first string with the characters 12 stuck on after it.

Thus: Case 112

From the Java documentation on Strings:

“The Java language provides special support for the string
concatenation operator ( + ), and for conversion of other objects to
strings. String concatenation is implemented through the StringBuffer
class and its append method. String conversions are implemented
through the method toString, defined by Object and inherited by all
classes in Java. For additional information on string concatenation
and conversion, see Gosling, Joy, and Steele, The Java Language
Specification.”

But be careful! Adding works left to right, so what would the follwing print?

System.out.println(1 + 2 + "test" + 3 + 4);

First, it does 1 + 2, which is equal to 3.

Then it does 3 (the result of the last step) + "test", which causes "3test"

Next, it does "3test" + 3, which results in "3test3".

And finally, "3test3 + 4 is "3test34.

As you can see, its a good idea to put parentheses around things to ensure they come out in the order you want.

(1 + 2) + "test" + (3 + 4) would be “3test7” because the math in the parentheses has precedence.