What function is more efficient?

Question

I'm new to Java and i wanted to know if there is a difference between these 2 functions:

public static String function1(int x) {
    String res = "";
    if(x > 10)
        res = "a";
    else
        res = "b";

    return res;
}

and:

public static String function2(int x) {
    if(x > 10)
        return "a";

    return "b";
}

and I'm not speaking on the length of the code, only efficiency.

Answer 1

The second version is in theory more efficient, decompiling to:

public static java.lang.String function1(int);
Code:
   0: ldc           #2                  // String
   2: astore_1
   3: iload_0
   4: bipush        10
   6: if_icmple     12
   9: ldc           #3                  // String a
  11: areturn
  12: ldc           #4                  // String b
  14: areturn

whereas the version with the assignment decompiles to:

public static java.lang.String function1(int);
Code:
   0: ldc           #2                  // String
   2: astore_1
   3: iload_0
   4: bipush        10
   6: if_icmple     15
   9: ldc           #3                  // String a
  11: astore_1
  12: goto          18
  15: ldc           #4                  // String b
  17: astore_1
  18: aload_1
  19: areturn

where it can be seen that the additional variable is created and returned.

However in practise the difference in actual runtime performance should be negligible . The JIT compiler would (hopefully) optimise away the useless variable, and in any case unless the code was in a hot code path according to your profiler then this would certainly count as premature optimisation.

Answer 2

Both versions end up creating a string either "a" or "b" and return it out. But version 2 is better in term of efficiency, which doesn't create an redundant empty string "" in memory.