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by: Taylor Kahl

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# Week 6 Notes CSC 2310

Taylor Kahl
GSU
GPA 4.21

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This covers the lecture from Wednesday Feb 17 (since we had a test on Monday). Includes material from the textbook chapter 10 on ArrayLists
COURSE
Princliples of Computer Programming
PROF.
Kebina Manandhar
TYPE
Class Notes
PAGES
4
WORDS
KARMA
25 ?

## Popular in ComputerScienence

This 4 page Class Notes was uploaded by Taylor Kahl on Saturday February 20, 2016. The Class Notes belongs to CSC 2310 at Georgia State University taught by Kebina Manandhar in Winter 2016. Since its upload, it has received 57 views. For similar materials see Princliples of Computer Programming in ComputerScienence at Georgia State University.

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Date Created: 02/20/16
From lecture on Wednesday, Feb 17: Exercise: Write a method intersect that accepts two sorted array lists of integers as parameters and returns a new list that contains only the elements that are found in both lists. My solution: public static ArrayList<Integer> intersect(ArrayList<Integer> list1, ArrayList<Integer> list2) { ArrayList<Integer> newList = new ArrayList<Integer>(); for (int i=0; i<list1.size(); i++) { if (list2.contains(list1.get(i))) { newList.add(list1.get(i)); } } return newList; } This method goes through every element in list1, then checks if that element is also an element in list2. If it is, the element is added to newList. Objects storing collections:  Remember that objects have fields, values stored within the object  Objects may store collections, such as Arrays and ArrayLists, as fields  Example: Course objects storing grades and students’ names public class Course { private double[] grades; //an Array of doubles private ArrayList<String> studentNames; //an ArrayList of Strings public Course() { //a constructor for new Course objects that initializes its fields grades = new double[4]; studentNames = new ArrayList<String>(); } } Sorting ArrayLists:  The method Collections.sort sorts ArrayLists o integers and doubles are sorted from least to greatest o Strings are sorted in alphabetical order  Collections.sort is part of the java.util package o has to be imported  Example of sorting: import java.util.*; public class ArrayListSorting { public static void main(String[] args) { ArrayList<String> stringList = new ArrayList<String>(); stringList.add("c"); stringList.add("b"); stringList.add("a"); stringList.add("d"); System.out.println("Before sorting: "+stringList); Collections.sort(stringList); System.out.println("After sorting: "+stringList); } } Output: Before sorting: [c, b, a, d] After sorting: [a, b, c, d] The Comparable Interface:  Sorting objects are possible using the Comparable interface o Ex: the String class implements the Comparable interface  But if we tried to use the Collections.sort method on Point objects, the program won’t compile o Point does not implement the Comparable interface  You can implement this method to compare objects of the classes you create  The Comparable interface looks something like this: public interface Comparable<E> { public int compareTo(E other); }  Any class that implements this interface must define the compareTo method o compareTo returns 3 possible integers:  negative to indicate less than  0 to indicate equal  positive to indicate greater than o Integers compare the quantity of values o Strings compare the alphabetical order of values o Examples calling the compareTo method:  if (x.compareTo(y) < 0) {  if (x.compareTo(y) != 0) {  Example: writing the Point class to implement the Comparable interface public class Point implements Comparable<Point> { //btw the <> you have to specify the type of object public int compareTo(Point other) { if (x < other.x) { return -1; } else if (x > other.x) { return 1; } else if (y < other.y) { return -1; } else if (y > other.y) { return 1; } else { return 0; } } } This compareTo method will compare Point objects based on their x and y values. First it will compare which x value is greatest. If they are equal, it will compare which y value is greatest. If they are also equal, the Points are equal ***Now that the Point class implements Comparable, we can use the Collections.sort method to sort an ArrayList<Point>  Other methods that use compareTo:  binarySearch – searches for the index of an Array where a particular value is o Example: finding the index of a String in an Array String[] a = {“a1”, “bob”, “cari”, “dan”, mike”}; int index = Arrays.binarySearch(a, “dan”); //index = 3  Tricks for more efficient compareTo methods:  Subtraction trick – subtracting related values gives you the desired result: a negative, a positive or 0 o Example: a revised compareTo method for Points: public int compareTo(Point other) { if (x != other.x) { return x – other.x; } else { return y – other.y; } } ***Note, this won’t work for double values because the compareTo method has a return type of int

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