String concatenation: concat() vs "+" operator

In Java, you can use the concat() method of the java.lang.String class or the + operator to concatenate (join) two or more strings.

The concat() method returns a new string that is the result of concatenating the specified string to the end of this string. Here is an example of how you can use the concat() method to concatenate two strings:

---

public class StringConcatenationExample {
  public static void main(String[] args) {
    String str1 = "Hello";
    String str2 = "World";
    String str3 = str1.concat(str2);
    System.out.println(str3);  // Outputs "HelloWorld"
  }
}

The + operator is also used to concatenate strings in Java. When used with two strings, it returns a new string that is the result of concatenating the two strings. Here is an example of how you can use the + operator to concatenate two strings:

---

public class StringConcatenationExample {
  public static void main(String[] args) {
    String str1 = "Hello";
    String str2 = "World";
    String str3 = str1 + str2;
    System.out.println(str3);  // Outputs "HelloWorld"
  }
}

Both the concat() method and the + operator are used to concatenate strings in Java. The main difference between these two approaches is that the concat() method is a method of the java.lang.String class, while the + operator is a built-in operator in Java.

The + operator is generally preferred for readability and flexibility. While concat() can be slightly faster for simple two-string joins because it avoids StringBuilder overhead, the + operator is optimized by the Java compiler. Modern Java (9+) uses StringConcatFactory to generate efficient bytecode, making it the standard choice for most concatenation tasks.

For example, when using the + operator to concatenate two strings, the Java compiler may generate code that uses the StringBuilder class to concatenate the strings, like this:

---

public class StringConcatenationExample {
  public static void main(String[] args) {
    String str1 = "Hello";
    String str2 = "World";
    String str3 = new StringBuilder().append(str1).append(str2).toString();
    System.out.println(str3);  // Outputs "HelloWorld"
  }
}

In summary, use the + operator for general string concatenation due to its readability and compiler optimizations. Use concat() when you specifically need a method call and are certain the argument is not null, as it avoids the overhead of creating a StringBuilder instance.