Avoid creating unnecessary objects

It is often appropriate to reuse a single object instead of creating a new functionally equivalent object each time it is needed. Reuse can be both fast and more stylish. An object can always be reused if it is immutable.

String s = new String(“string”); // DON’T DO THIS

The statement create a new String instance each time it is executed, and none of those object creation is necessary. The argument to the String constructor (“string”) is itself a String instance.

class Person {
  private final Date birthDate;
  ...
  //DON'T DO THIS
  public boolean isBabyBoomer() {
    // Unnecessary allocation of expensive object
    Calendar gmtCalendar = Clanedar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCalendar.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
    Date boomStart = gmtCalendar.getTime();
    gmtCalendar.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
    Date boomEnd = gmtCalendar.getTime();
    return birthDate.compareTo(boomStart) >= 0 && birthDate.compareTo(boomEnd) < 0;
  }

The isBabyBoomer method unnecessarily create a new Calendar, TimeZone and two Date instances each time is is invoked. The version that follows avoids this inefficiency:

class Person {
  private static final Date BOOM_START;
  private static final Date BOOM_END;

  private final Date birthDate;
  
  static {
    Calendar gmtCalendar = Clanedar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCalendar.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
    BOOM_START = gmtCalendar.getTime();
    gmtCalendar.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
    BOOM_END = gmtCalendar.getTime();
  }

  public boolean isBabyBoomer() {
    return birthDate.compareTo(BOOM_START) >= 0 && birthDate.compareTo(BOOM_END) < 0;
  }
}

The improved version of the Person class creates Calendar, TimeZone and Date instance only once, when it is initialized, instead of creating them every time isBabyBoomer is invoked.