[Java] Java was said to be okay to concatenate strings with +, so I checked

What I tried

Make a file like this

PlainString.java

import java.time.LocalDateTime;

class PlainString {
    String getString() {
        //e! String+I will connect with! ??
        return "abc" + LocalDateTime.now();
    }
}

Compile

javac PlainString.java

For JDK8

result

I tried decompiling.

javap -c PlainString
Compiled from "PlainString.java"
class PlainString {
  PlainString();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  java.lang.String getString();
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       7: ldc           #4                  // String abc
       9: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      12: invokestatic  #6                  // Method java/time/LocalDateTime.now:()Ljava/time/LocalDateTime;
      15: invokevirtual #7                  // Method java/lang/StringBuilder.append:(Ljava/lang/Object;)Ljava/lang/StringBuilder;
      18: invokevirtual #8                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      21: areturn
}

It was firmly replaced by StringBuilder.

Conclusion

~~ Imadoki's Java seems to be okay to concatenate strings using + without worrying about it. ~~ ** Added on 2020/2/22 ** We received a comment from @ cfm-art. Thank you very much. If you just want to concatenate, + is fine. If you use + in a loop, StringBuilder will be created each time, so it's useless.

environment

• Ubuntu 18.04.4 LTS
• OpenJDK 8u242-b08-0ubuntu3~18.04

[Updated on February 22, 2020] For JDK13

I was told that StringBuilder is not used after JDK11, so I tried it.

result

I did javac and then javap.

javap -c PlainString                                                                                                      Compiled from "PlainString.java"
class PlainString {
  PlainString();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  java.lang.String getString();
    Code:
       0: invokestatic  #7                  // Method java/time/LocalDateTime.now:()Ljava/time/LocalDateTime;
       3: invokedynamic #13,  0             // InvokeDynamic #0:makeConcatWithConstants:(Ljava/time/LocalDateTime;)Ljava/lang/String;
       8: areturn
}

The StringBuilder disappeared and was replaced by something called makeConcatWithConstants.

Conclusion

It seems okay to use + anywhere for string concatenation.

environment

openjdk version "13.0.2" 2020-01-14 OpenJDK Runtime Environment AdoptOpenJDK (build 13.0.2+8) OpenJDK 64-Bit Server VM AdoptOpenJDK (build 13.0.2+8, mixed mode, sharing)