ternary operator with strings

[Mary McCartney]

Consider these 2 methods, both of which convert null strings to empty strings:

private static final String N2E1(String str){
if (str == null){
str = "";
}
return str;
}

private static final String N2E2(String str){
return str == null ? "" : str;
}

Assuming it will be called a lot, is there a performance difference, or will the compiler just optimize N2E1 to do what N2E2 does anyway?

Thanks for your help!

[Ernest Friedman-Hill]

They would likely compile to identical bytecode.

[Ken Blair]

Almost.

String methodA(String str) {
return str != null ? str : "";
}
 
String methodB(String str) {
if (str != null) {
return str;
} else {
return "";
}
}

compiles to

java.lang.String methodA(java.lang.String);
  Code:
   0:   aload_1
   1:   ifnull  8
   4:   aload_1
   5:   goto    10
   8:   ldc     #2; //String
   10:  areturn
 
java.lang.String methodB(java.lang.String);
  Code:
   0:   aload_1
   1:   ifnull  6
   4:   aload_1
   5:   areturn
   6:   ldc     #2; //String
   8:   areturn

[Ernest Friedman-Hill]

Originally posted by Ken Blair:
Almost.

Here the ternary operator compiles to slightly worse code!

But it's likely that HotSpot would optimize the double branch to the methodB version with the multiple returns.

[Ilja Preuss]

I'd also assume that a goto is a very cheap operation on most platforms.

[Nishant Verma]

Ernest Friedman-Hill, Ken Blair, Ilja Preuss

You guys rock really

[S Thanigaivel]

Hi Ken, can you explain in detail the below calculations and also how you derived it?


compiles to

java.lang.String methodA(java.lang.String);
  Code:
   0:   aload_1
   1:   ifnull  8
   4:   aload_1
   5:   goto    10
   8:   ldc     #2; //String
   10:  areturn
 
java.lang.String methodB(java.lang.String);
  Code:
   0:   aload_1
   1:   ifnull  6
   4:   aload_1
   5:   areturn
   6:   ldc     #2; //String
   8:   areturn

[/QB]

[Ernest Friedman-Hill]

The disassembly comes from the "javap" tool that comes in the JDK. "javap -c MyClassName" will show a display like the one given here.

[S Thanigaivel]

Originally posted by Ernest Friedman-Hill:
The disassembly comes from the "javap" tool that comes in the JDK. "javap -c MyClassName" will show a display like the one given here.

Thats interesting! Thank you Ernest

[Mary McCartney]

I was not aware of that javap tool. Thanks everyone for the info!