Example of a Java class that is NOT thread-safe

NotThreadSafe.java

public class NotThreadSafe {
  
  private static final NotThreadSafe INSTANCE = new NotThreadSafe();

  private boolean isEven = false;
  
  private NotThreadSafe() {
    super();
  }
  
  public static NotThreadSafe getInstance() {
    return INSTANCE; 
  }
  
  public String getMessage(final int num) {
    isEven = num % 2 == 0;
    System.out.println("isEven=" + isEven);
    if(isEven) {
      System.out.println("Returning foo");
      return "foo";
    } else {
      System.out.println("Returning bar");
      return "bar";
    }
  }
}