Immutability

Immutable objects are thread-safe. When an object is immutable, you cannot make changes to the object. Many threads can read the object at the same time and when a thread needs to change a value, it creates a modified copy. For example, Java strings are immutable. Consider the following code snippet: 

import java.util.HashSet;
import java.util.Set;

public class StringsAreImmutable {
  public static void main(String[] args) {
    String s = "Hi friends!";
    s.toUpperCase();
    System.out.println(s);

    String s1 = s.toUpperCase();
    System.out.println(s1);

    String s2 = s1;
    String s3 = s1;
    String s4 = s1;

    Set set = new HashSet<String>();
    set.add(s);
    set.add(s1);
    set.add(s2);
    set.add(s3);

    System.out.println(set);
  }
}

Running the code gives the following output:

Hi friends!
 HI FRIENDS!
 [Hi friends!, HI FRIENDS!]

When...