// S#1. stream works with StringBuilder 
    System.out.println( 
            Stream.of("a", "b", "c", "d", "ef", "ghi", "j")
                    .collect(StringBuilder::new,
                            StringBuilder::append,
                            StringBuilder::append)
                    .toString()
    );

    // S#2. stream works fine with Int 
    // my custom class that conforms to arg types of collect method
    // method signature: collect(Supplier, accumulator, combiner)
    Int anInt = new Int();
    System.out.println(
            Stream.of("a", "b", "c", "d", "ef", "ghi", "j")
                    .collect(() -> anInt, Int::accumulator, Int::combiner)
                    .total
    );
    class Int {
      int total;
      static void accumulator(Int result, String str) { result.total += str.length(); }
      static void combiner(Int a, Int b) { a.total += b.total; }
    }

    // S#3. compiler error with Int2 
    // custom class I created which has method accumulator(String) similar to StringBuidler append(String)
    // StringBuilder::append (used as above in S#1)
    Int2 int2 = new Int2();
    System.out.println(
            Stream.of("a", "b", "c", "d", "ef", "ghi", "j")
                    .collect(() -> int2, int2::accumulator, int2::combiner)
    );
    class Int2 {
      void accumulator(String s) {}
      void combiner(Int2 b) {}
    }

但是我想知道stringbuilder版本的代码是如何编译的，而s#3（int2版本）却没有。stringbuilder扩展自charsequence、abstractstringbuilder类。是不是因为我的自定义类（int2）中缺少的这种层次关系才起作用
非常感谢你的帮助。