Is String concatenation on assignment efficient?

Question

I know that using the "+" concatenation operator for building strings is very inefficient, and that is why it is recommended to use the StringBuilder class, but I was wondering if this kind of pattern is inefficient too?

String some = a + "\t" + b + "\t" + c + "\t" + d + "\t" + e;

I guess here the compiler will optimize the assignment fine, or not?

Answer 1

This particular example will be inlined by the compiler:

String a = "a";
String b = "bb";
String c = "ccc";
String some = a + "\t" + b + "\t" + c;

Java 9+ will inline this using invokedynamic with makeConcatWithConstants making it efficient. As per javap -v output:

Code:
  stack=3, locals=5, args_size=1
     0: ldc           #2                  // String a
     2: astore_1
     3: ldc           #3                  // String bb
     5: astore_2
     6: ldc           #4                  // String ccc
     8: astore_3
     9: aload_1
    10: aload_2
    11: aload_3
    12: invokedynamic #5,  0              // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
    17: astore        4
    19: return

However if the a b and c are compile time constants compiler will further optimize the code:

final String a = "a";
final String b = "bb";
final String c = "ccc";
String some = a + "\t" + b + "\t" + c;

and some will be loaded with a constant value:

Code:
  stack=1, locals=5, args_size=1
     0: ldc           #2                  // String a
     2: astore_1
     3: ldc           #3                  // String bb
     5: astore_2
     6: ldc           #4                  // String ccc
     8: astore_3
     9: ldc           #5                  // String a\tbb\tccc
    11: astore        4
    13: return

In other circumstances e.g. for loop the compiler might not be able to produce optimized code so StringBuilder might be faster.