字符串拼接用“+” 使用的是 StringBuilder

Java 语言本身并不支持运算符重载，“+”和“+=”是专门为 String 类重载过的运算符，也是 Java 中仅有的两个重载过的运算符。

public class Test {
    public Test() {
    }

    public static void main(String[] args) {
        String s1 = "hello1 ";
        String s2 = "hello2 ";
        String s3 = "hello3 ";
        String s4 = "hello4 ";
        (new StringBuilder()).append(s1).append(s2).append(s3).append(s4).toString();
    }
}

执行 javap -c Test.class

 public static void main(java.lang.String[]);
      17: invokespecial #7                  // Method java/lang/StringBuilder."<init>":()V
      20: aload_1
      21: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      24: aload_2
      25: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      28: aload_3
      29: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      32: aload         4
      34: invokevirtual #8                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      37: invokevirtual #9                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      40: astore        5
      42: return

可以看出字符串拼接采用的是StringBuilder