みんな大好きJavaの文字列結合に関して調べてみた。とりあえずコンパイラは結構仕事をしているみた。StringBuilder直より命令数少ないし、Java11だとさらなる最適化がされておる...

Base.java

    public static void checkJavacOptimizationPlus() {
        String hello = "hello(v)";
        String world = "world(v)";

        final String HELLO = "hello(c)";
        final String WOLRD = "world(c)";

        String mixedStr2 = "hello(l)" + "world(l)" + HELLO + WOLRD + hello + world;

    }

    public static void checkJavacOptimizationSB() {
        String hello = "hello(v)";
        String world = "world(v)";

        final String HELLO = "hello(c)";
        final String WOLRD = "world(c)";

        StringBuilder sb = new StringBuilder();
        sb.append("hello(l)");
        sb.append("world(l)");
        sb.append(HELLO);
        sb.append(WOLRD);
        sb.append(hello);
        sb.append(world);
        String sbStr = sb.toString();
    }

checkJavacOptimizationPlus with Java11

  public static void checkJavacOptimizationPlus();
    Code:
       0: ldc           #22                 // String hello(v)
       2: astore_0
       3: ldc           #23                 // String world(v)
       5: astore_1
       6: ldc           #24                 // String hello(c)
       8: astore_2
       9: ldc           #25                 // String world(c)
      11: astore_3
      12: aload_0
      13: aload_1
      14: invokedynamic #29,  0             // InvokeDynamic #6:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
      19: astore        4
      21: return

checkJavacOptimizationPlus.dump

  public static void checkJavacOptimizationPlus();
    Code:
       0: ldc           #23                 // String hello(v)
       2: astore_0
       3: ldc           #24                 // String world(v)
       5: astore_1
       6: ldc           #25                 // String hello(c)
       8: astore_2
       9: ldc           #26                 // String world(c)
      11: astore_3
      12: new           #6                  // class java/lang/StringBuilder
      15: dup
      16: invokespecial #7                  // Method java/lang/StringBuilder."<init>":()V
      19: ldc           #29                 // String hello(l)world(l)hello(c)world(c)
      21: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: aload_0
      25: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      28: aload_1
      29: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      32: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      35: astore        4
      37: return

checkJavacOptimizationSB.dump

  public static void checkJavacOptimizationSB();
    Code:
       0: ldc           #23                 // String hello(v)
       2: astore_0
       3: ldc           #24                 // String world(v)
       5: astore_1
       6: ldc           #25                 // String hello(c)
       8: astore_2
       9: ldc           #26                 // String world(c)
      11: astore_3
      12: new           #6                  // class java/lang/StringBuilder
      15: dup
      16: invokespecial #7                  // Method java/lang/StringBuilder."<init>":()V
      19: astore        4
      21: aload         4
      23: ldc           #30                 // String hello(l)
      25: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      28: pop
      29: aload         4
      31: ldc           #31                 // String world(l)
      33: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      36: pop
      37: aload         4
      39: ldc           #25                 // String hello(c)
      41: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      44: pop
      45: aload         4
      47: ldc           #26                 // String world(c)
      49: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      52: pop
      53: aload         4
      55: aload_0
      56: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      59: pop
      60: aload         4
      62: aload_1
      63: invokevirtual #9                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      66: pop
      67: aload         4
      69: invokevirtual #11                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      72: astore        5
      74: return

decompiled.java

    public static void checkJavacOptimizationPlus() {
        String hello = "hello(v)";
        String world = "world(v)";
        String HELLO = "hello(c)";
        String WOLRD = "world(c)";
        String mixedStr2 = "hello(l)world(l)hello(c)world(c)" + hello + world;
    }

    public static void checkJavacOptimizationSB() {
        String hello = "hello(v)";
        String world = "world(v)";
        String HELLO = "hello(c)";
        String WOLRD = "world(c)";
        StringBuilder sb = new StringBuilder();
        sb.append("hello(l)");
        sb.append("world(l)");
        sb.append("hello(c)");
        sb.append("world(c)");
        sb.append(hello);
        sb.append(world);
        String sbStr = sb.toString();
    }