Comparing Objects 3/14/16

Transcription

1 Comparing Objects 3/14/16

2 Agenda HW Notes Practice Tests Returned

3 Searching Data One of the most important and useful things that computers do is search through huge amounts of data. Search Engines Patterns in Environmental Data Patterns in Medical Data E-Commerce advertisements Security s/w, Facial Recognition

4 Three ways of comparing data public boolean equals(object other) public int compareto(t other) public int compare(t obj1, T obj2) Every class that may be searched, sorted, put on ArrayLists or arrays should have these 3 methods defined.

5 equals method public boolean equals(object other) is a method of class Object. All classes inherit this method. It compares the addresses of the objects. We often override this method so we are checking if the content of the objects are equal as opposed to the addresses. (Like String class)

6 Override equals method To override Object s equals method, the header of your equals method must be exactly the same as the header of equals in Object.

7 obj1.equals(obj2) (cont d) public class Pet { Or: if (other!= null) private String name;... public boolean equals (Object other) { if (other instanceof Petl) return name.equals(((pet)other).name); else return false; instanceof } is a boolean } operator in Java 14-7

8 Why override Object s equals method? You could define an equals method that took in your Class type as a parameter: public boolean equals(pet p). However, this would not work with ArrayLists and other more general uses. equals is called polymorphically from library methods, such as ArrayList s contains or indexof that is why we have to properly override Object s equals. The equals method is properly defined in String, Integer, Double, etc.

9 To Do Create a Customer class that uses fields of first name, last name, address, customerid, totalamtspent, and an ArrayList<String> representing items bought. Use a static variable to ensure each Customer object is assigned a unique Customer id. Include an equals method for this class as well as a constructor and tostring.

10 To Do Create several Customer objects. Write a copy constructor in your Customer class so you can make copies of your Customer object. Test that the equals method works as expected. (true when contents are the same, false otherwise) Verify that the == operation compares addresses. (true when addresses are the same, false otherwise even if contents are same) Create an ArrayList of these Customer objects. Use the contains and indexof method to verify your equals method is working correctly. Try using the equals method to compare a non-customer object. See what happens.

11 obj1.compareto(obj2) compareto is an abstract method defined in the java.util.comparable<t> interface: public int compareto (T other); T is the type parameter Returns a positive integer if obj1 is greater than obj2, a negative integer if obj1 is less than obj2, zero if they are equal. Remember to think of compareto as a subtraction problem: obj1 obj2, will return positive if obj1 is greater than obj2, etc

12 obj1.compareto(obj2) (cont d) public class Pet implements Comparable<Pet> { private String name;... public int compareto(pet other) { return name.compareto(other.name); } } public boolean equals(object other) { return other instanceof Pet && compareto((pet)other) == 0; } equals is not required by Comparable, but it is a good idea to provide it and make it agree with compareto 14-12

13 obj1.compareto(obj2) (cont d) compareto is called polymorphically from library methods, such as Arrays.sort(Object[] arr), Collections.sort(ArrayList<T> list) Arrays.binarySearch(Object[ ] arr) Objects of classes that implement Comparable are called comparable Strings, Integers, Doubles are comparable

14 obj1.compareto(obj2) (cont d) «interface» Comparable<String> «interface» Comparable<Integer> «interface» Comparable<Double> String Integer Double compareto is based on lexicographical order compareto is based on numerical values 14-14

15 To Do Add a compareto method to your Customer class that agrees with your equals method. Return 0, if the names are the same and the id is the same. Think about how to handle the other conditions. Test your code. Sort your arraylist of Customers using Collections.sort(), Create an array of Customer objects and sort them using Arrays.sort()

16 What if you want more than one way to compare? Create a Comparator object that specifies a way to compare two objects of your class. A comparator is an object of a class that implements the java.util.comparator<t> and has a method: public int compare(t obj1, T obj2)

17 compare(obj1, obj2) compare is an abstract method defined in the java.util.comparator<t> interface: public int compare (T obj1, T obj2); Returns a positive integer if obj1 is greater than obj2, a negative integer if obj1 is less than obj2, zero if they are equal. Sort of like obj1 - obj2 T is the type parameter 14-17

18 compare (obj1, obj2) (cont d) public class PetComparatorByName implements Comparator<Pet> {... public int compare (Pet pet1, Pet pet2) { return pet1.getname(). compareto(pet2.getname()); } } 14-18

19 compare(obj1, obj2) (cont d) A programmer can define different comparators to be used in different situations. compare is called from library methods, such as Arrays.sort(T [ ] arr, Comparator<T> c) (or from your own methods) that take a comparator object as a parameter

20 To Do Create a CustomerId Comparator that compares customers based on their customer Id. This would allow you to sort based on how long a customer has been with you. Create a Name Comparator that compares customers based on name (alphabetically) Create a totalamt Comparator that compares customers based on amount of money they spent in store.