Java String, the string pool and StringBuilder

A Java String is immutable: once created it never changes. Methods like toUpperCase return a new String.

public class Strings {
  public static void main(String[] args) {
    String a = "uni"; String b = "uni"; String c = new String("uni");
    System.out.println((a == b) + " " + (a == c) + " " + a.equals(c));
    String s = "hello";
    s.toUpperCase();
    System.out.println(s);
    StringBuilder sb = new StringBuilder();
    for (int i = 1; i <= 5; i++) sb.append(i).append(i < 5 ? "," : "");
    System.out.println(sb.reverse());
  }
}

true false true
hello
5,4,3,2,1

What the output shows

• a == b is true because identical string literals share one object from the string pool.
• a == c is false because new String always creates a separate object. == compares references; equals compares the characters. Always use equals (or equalsIgnoreCase) for String content.
• s is still "hello" because the result of toUpperCase was thrown away. Write s = s.toUpperCase().

When to use StringBuilder

Joining Strings with + inside a loop creates a new String each time, copying everything built so far, which is O(n²) for n pieces. StringBuilder appends into one growing buffer, which is O(n) overall. Use it whenever you build a string in a loop. It also offers reverse, insert and deleteCharAt, which are handy in coding rounds. StringBuffer is the older, synchronised version; prefer StringBuilder unless several threads share it.