How do I convert from int to String?

Question

I'm working on a project where all conversions from int to String are done like this:

int i = 5; String strI = "" + i; 

I'm not familiar with Java.

Is this usual practice or is something wrong, as I suppose?

Answer 1

Normal ways would be Integer.toString(i) or String.valueOf(i).

The concatenation will work, but it is unconventional and could be a bad smell as it suggests the author doesn't know about the two methods above (what else might they not know?).

Java has special support for the + operator when used with strings (see the documentation) which translates the code you posted into:

StringBuilder sb = new StringBuilder(); sb.append(""); sb.append(i); String strI = sb.toString(); 

at compile-time. It's slightly less efficient (sb.append() ends up calling Integer.getChars(), which is what Integer.toString() would've done anyway), but it works.

To answer Grodriguez's comment: ** No, the compiler doesn't optimise out the empty string in this case - look:

simon@lucifer:~$ cat TestClass.java public class TestClass {   public static void main(String[] args) {     int i = 5;     String strI = "" + i;   } } simon@lucifer:~$ javac TestClass.java && javap -c TestClass Compiled from "TestClass.java" public class TestClass extends java.lang.Object{ public TestClass();   Code:    0:    aload_0    1:    invokespecial    #1; //Method java/lang/Object."<init>":()V    4:    return  public static void main(java.lang.String[]);   Code:    0:    iconst_5    1:    istore_1 

Initialise the StringBuilder:

   2:    new    #2; //class java/lang/StringBuilder    5:    dup    6:    invokespecial    #3; //Method java/lang/StringBuilder."<init>":()V 

Append the empty string:

   9:    ldc    #4; //String    11:    invokevirtual    #5; //Method java/lang/StringBuilder.append: (Ljava/lang/String;)Ljava/lang/StringBuilder; 

Append the integer:

   14:    iload_1    15:    invokevirtual    #6; //Method java/lang/StringBuilder.append: (I)Ljava/lang/StringBuilder; 

Extract the final string:

   18:    invokevirtual    #7; //Method java/lang/StringBuilder.toString: ()Ljava/lang/String;    21:    astore_2    22:    return } 

There's a proposal and ongoing work to change this behaviour, targetted for JDK 9.