Java Comparable and Comparator
1 like65 views
The document discusses the differences between the Comparable and Comparator interfaces in Java. Comparable allows objects to be sorted based on natural ordering by implementing the compareTo() method. Comparator allows objects to be compared and sorted based on arbitrary criteria by implementing the compare() method. Guidelines are provided for when to use each interface, such as using Comparable for a class's natural ordering and Comparator when multiple orderings are possible or the class cannot be modified. Examples of implementing each are also included.
Java Comparable and Comparator
- 1. Java Comparable vs Comparator Sujit Kumar Zenolocity LLC © 2013-2023
- 2. java.lang.Comparable Interface • Allows an object of a class to be compared to objects of the same class using a 1 or more attributes of the same class. • compareTo method is the ONLY member of the Comparable interface. Allows objects of a class to be sorted by natural ordering. • Implementing Comparable allows: • calling Collections.sort & Collections.binarySearch • calling Arrays.sort and Arrays.binarySearch • using objects as keys in a TreeMap • using objects as elements in a TreeSet
- 3. compareTo method • Return value is int, 0 => equal, -ve => less than and +ve => greater than • anticommutation : x.compareTo(y) = - (y.compareTo(x)) • exception symmetry : x.compareTo(y) throws exactly the same exceptions as y.compareTo(x) • transitivity : if x.compareTo(y) > 0 and y.compareTo(z) > 0, then x.compareTo(z) > 0 (and same for less than) if x.compareTo(y) == 0, then x.compareTo(z) has the same sign as y.compareTo(z)
- 4. Example Implementation of compareTo public int compareTo(Employee emp) { int result = this.id.compareTo(emp.getId()); if (result == 0) { result = this.age > emp.age ? 1 : this.age < emp.age ? -1 : 0; } return result; }
- 5. Performance Considerations • One can greatly increase the performance of compareTo by comparing first on attributes which are most likely to differ. • Preferable to always sort items in an RDBMS first using ORDER BY instead of in memory in java using Comparable.
- 6. java.util.Comparator Interface • public int compare (Object o1, Object o2); • Logical difference between Comparator and Comparable is : Comparator in Java compares any two objects, while Comparable interface compares the "this" reference with the object specified. • Return value semantics is same as the compareTo method of Comparable interface.
- 7. Guidelines on when to use Comparable and Comparator • For natural (intuitive) ordering use Comparable. Use Comparator if you want to have an ordering different from the natural order. If there is more than one intuitive comparison possible, use a Comparator. • Use comparable if class is in your control. Use Comparator if class is NOT in your control and you cannot make the class author implement Comparable. • Comparable should be used when you compare instances of same class (homogeneous). Comparator can be used to compare instances of same or different classes (homogeneous and heterogeneous).
- 8. Guidelines (continued…) • Comparator has a distinct advantage of being self descriptive. Examples: • If you are writing Comparator to compare two Employees based upon the salary then name that comparator as SalaryComparator. • If you are writing Comparator to compare two Employees based upon the age then name that comparator as AgeComparator.
- 9. Examples of Comparator public static Comparator<Employee> EmployeeAgeComparator = new Comparator<Employee>() { public int compare(Employee emp1, Employee emp2) { return emp1.age.compareTo(emp2); } }; public static Comparator<Employee> EmployeeNameComparator = new Comparator<Employee>() { public int compare(Employee emp1, Employee emp2) { return emp1.name.compareTo(emp2) } };
- 10. How to Remember? • Am I (this) Comparable to (compareTo) my friend? • Can I compare (Comparator) apples to oranges?