If StringBuider is preferred over String, then what are the cons of using StringBuilder always and everywhere instead of String?

Question

Java

The first question is in the title.
Second, why doesn't the Java compiler automatically optimize concatenation operations and so on using StringBuilder? Or can it be turned on somehow?

Answer 1

Programming is an engineering discipline. It cannot contain magic recipes that work always and everywhere. The art of an engineer in finding compromises that give the most beneficial combination of qualities in specific situations. StringBuildergives an advantage only when there are many operations to change the row. For example, when growing a string in a loop. In all other cases, it is StringBuilderslower and requires more memory. Also, it is not thread-safe, but strings are.
In terms of optimization, the compiler replaces string concatenations with the StringBuilder.append() method , but only when it is certain that this will not change the execution logic. It's easy to see. Let's take this code:

public class Example {
    public static String concat(String s1, String s2) {
        return s1 + s2;
    }

    public static void main(String[] args) {
        System.out.println(concat("Hello ", "World"));
    }
}

Let's compile
AND look into the resulting bytecode

public static java.lang.String concat(java.lang.String, java.lang.String);
    Code:
       0: new           #2                  // class java/lang/StringBuilder
       3: dup
       4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
       7: aload_0
       8: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      11: aload_1
      12: invokevirtual #4                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      15: invokevirtual #5                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      18: areturn