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?
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.