Topics

Review

• Storing Values Read from a File
• ArrayLists
• Creating an ArrayList Object
• Loading Values into an ArrayList
• Looping Through an ArrayList
• Classes for Primitive Values
• Creating an ArrayList from a File
• Being a Good Citizen

New Material

• Other ArrayList Methods
• The add(value) Method
• The add(index, value) Method
• The clear() Method
• The get(index) Method
• The remove(index) Method
• The set(index, value) Method
• The size() Method
• ArrayList Search Methods
• The contains(value) Method
• indexOf(value) Method
• lastIndexOf(value) Method
• Writing a Method to Read in Files
• Reading a File of Numbers
• Stopping Execution of a Loop
• The break Statement
• The continue Statement
• The switch Statement
• Do Not Leave Out break in a switch Statement

Homework 8

Homework 8 is due this Sunday at 11:59 PM.

You will find the assignment here.

If you have a problem or a question, make a post on the Class Discussion Area.

Review

Storing Values Read from a File

• To make your programs efficient you want to read in a file once
• And store the information in RAM
• The easiest way to do this is to read in values from the file into an array
• But this poses a major problem
• You need to know the size of an array to declare it
• And you cannot know the size you need until you have read the file

ArrayLists

• An ArrayList object is dynamic so you do not have to specify a size when creating them
• The data contained in an ArrayList object is actually stored in a an array
• When first created an ArrayList object contains an array of a certain size
• But if the object needs more space it simply creates a bigger array
• Then is copies all the data in the original array into this new array
• This is all done automatically

Creating an ArrayList Object

• This is also true of an ArrayList object can only hold data of one data type
• The data types used when creating an ArrayList object must be classes
• To create an ArrayList of strings you would write

  ArrayList<String> list = new ArrayList<>();

• The ArrayList class is contained in the java.util package
• So you must have the following import statement at the top of your code

  import java.util.ArrayList;

Loading Values into an ArrayList

• When you first create an ArrayList object it has no values
• The simplest way to load a value into an ArrayList object is to use the add method
• This method adds a single value to the end of the list
• Here is an example

  import java.util.ArrayList;
  
  public class ArrayListString {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          for (int i = 0; i < args.length; i++){
              list.add(args[i]);
          }
          System.out.println(list);
      }
  }
  
  $ java ArrayListString foo bar blecth
  [foo, bar, blecth]

Looping Through an ArrayList

• Just as with arrays, you can using indexes to loop through the values in an ArrayList object
• Once again, the indexes start with 0
• And end with one less than the length of the ArrayList
• The size method returns the length of its ArrayList

  import java.util.ArrayList;
  
  public class ArrayListString2 {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          list.add("four");
          list.add("five");
          System.out.println("The length of the list is " + );
      }
  }
  
  $ java ArrayListString2
  The length of the list is 5

• To get the value at any index position, we use the get method
• With size and get we can easily write a for loop to move through the values

  import java.util.ArrayList;
  
  public class ArrayListString3 {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          list.add("four");
          list.add("five");
          for (int i = 0; i < list.size(); i++){
              System.out.println(list.get(i));
          }
      }
  }
  
  $ java ArrayListString3
  one
  two
  three
  four
  five

Classes for Primitive Values

• Java has wrapper classes
• These classes contain, or "wrap", a single primitive value
• Such as int or double
• So we write

  ArrayList<Integer> list = new ArrayList<>();

Creating an ArrayList from a File

• Reading a file into an ArrayList object of numbers is straight forward
• Create an ArrayList object
• Create Scanner object on a file
• Loop through the file with using hasNextInt method on the Scanner object
• Here is an example

  import java.util.*;
  import java.io.*;
  
  public class ArrayListIntegerFile {
      public static void main (String[] args)
          throws FileNotFoundException {
          ArrayList<Integer> numbers = new ArrayList<>();
          File file = new File(args[0]);
          Scanner input = new Scanner(file);
          while (input.hasNextInt()){
              numbers.add(input.nextInt());
          }
          System.out.println(numbers.size());
      }
  }
  
  $ java ArrayListIntegerFile numbers.txt 
  30

Being a Good Citizen

• The first of the general ethical principles of the ACM is

  Contribute to society and to human well-being, acknowledging 
  that all people are stakeholders in computing

• I think this breaks down into two responsibilities
  • Make the world a better place
  • Make sure what you do benefits everyone
• Every day technology makes our lives easier
• Weather reports help us plan our day
• Remote connections allow us to work from anywhere on the planet
• We can get detailed directions to anywhere we want to go
• We are lucky to live when we do and enjoy these conveniences
• We enjoy these benefits because the people who came before us made it happen
• We owe them a debt
• But the debt is not to them, but to those who come after us
• We repay our debt by working on improvements that will benefit future generations
• But we must be sure that the benefits go to all levels of society
• Not just to the affluent and well-connected
• Certain groups do not enjoy the same benefits of technology as the majority
• IT work is well paid
• And those who work in it tend to associate with other well paid people
• This can make the important needs of some people invisible
• We must not let our good fortune blind us to the needs of others
• This is particularly important now when immigrants are vilified
• When people of color are considered less than human by some
• If you are not part of the solution, you are part of the problem

New Material

Other ArrayList Methods

• ArrayLists have a number of useful methods
• Below is a table of methods and what they do

  Method	Description
  add(value)	Adds a value to the end of the list
  add(index, value)	Adds a value at a given position
  clear()	Removes all entries from the list
  get(index)	Returns the value at a given position
  remove(index)	Remove the element at a given position
  set(index, value)	Changes the value at a give position
  size()	Returns the number of elements in the list

The add(value) Method

• add(value) creates an element at the end of the list
• This operation is something you cannot do with an array

  import java.util.ArrayList;
  
  public class ListAdd {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          System.out.println(list.size());
          list.add("one");
          System.out.println(list.size());
          list.add("two");
          System.out.println(list.size());
          
          System.out.println(list.size());
          System.out.println(list);
      }
  }
  
  $ java ListAdd 
  0
  1
  2
  3
  [one, two, three]

The add(index, value) Method

• add(index, value) adds a value at a given index
• It moves the element already at that position, and all following, to the right

  import java.util.ArrayList;
  
  public class ListAddIndex {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          System.out.println(list);
          list.add(1, "one and a half");
          System.out.println(list);
      }
  }
  
  $ java ListAddIndex
  [one, two, three]
  [one, one and a half, two, three]

The clear() Method

• The clear() method removes all entries from a list
• This is also something that cannot be done with an array

  import java.util.ArrayList;
  
  public class ListClear {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          System.out.println(list);
          list.clear();
          System.out.println(list);
      }
  }
  
  $ java ListClear
  [one, two, three]
  []

The get(index) Method

• The get(index) method returns the value of the element at a given index
• It performs the same function as using square brackets [ ], with an array

  import java.util.ArrayList;
  
  public class ListGet {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          System.out.println(list);
          System.out.println("The value at index 1: " + list.get(1));
      }
  }
  
  $ java ListGet
  [one, two, three]
  The value at index 1: two

The remove(index) Method

• The remove(index) method delete an entry at a given index position
• Every element after the one removed is shifted one positon to the left
• Once again, this is an operation that cannot be performed on an array

  import java.util.ArrayList;
  
  public class ListRemove {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          System.out.println(list);
          list.remove(1);
          System.out.println(list);
      }
  }
  
  $ java ListRemove
  [one, two, three]
  [one, three]

The set(index, value) Method

• The set(index, value) method changes the value at a given position

  import java.util.ArrayList;
  
  public class ListSet {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          System.out.println(list);
          list.set(1, "2");
          System.out.println(list);
      }
  }
  
  $ java ListSet
  [one, two, three]
  [one, 2, three]

The size() Method

• The size() method returns the number of elements in a list
• It is equivalent the the length attribute of an array

  import java.util.ArrayList;
  
  public class ListSize {
      public static void main (String[] args){
          ArrayList<String> list = new ArrayList<>();
          list.add("one");
          System.out.println("Elements: " + list.size());
          list.add("two");
          System.out.println("Elements: " + list.size());
          list.add("three");
          System.out.println("Elements: " + list.size());
      }
  }
  
  $ java ListSize 
  Elements: 1
  Elements: 2
  Elements: 3

ArrayList Search Methods

• ArrayList provides methods that can be used to search the list

  Method	Description
  contains(value)	Returns true if the value can be found in the list
  indexOf(value)	Returns the index of the first occurence of a value in the list or -1 if the value cannot be found
  lastIndexOf(value)	Returns the index of the first occurence of a value in the list or -1 if the value cannot be found

The contains(value) Method

• contains(value) is a boolean method that returns true if the list contains the given value

  import java.util.ArrayList;
  
  public class ListContains {
      public static void main (String[] args){
          ArrayListt<String> list = new ArrayList<>();
          list.add("one");
          list.add("two");
          list.add("three");
          System.out.println("\"two\" is in list: " + list.contains("two"));
          System.out.println("\"four\" is in list: " + list.contains("four"));
      }
  }
  
  $ java ListContains
  "two" is in list: true
  "four" is in list: false

indexOf(value) Method

• indexOf(value) returns the index of the first occurence of the value in the list
• If the value does not occur in the list it returns -1

  import java.util.ArrayList;
  
  public class ListIndexOf {
      public static void main (String[] args){
          ArrayList<String> teams = new ArrayList<>();
          teams.add("Red Sox");
          teams.add("Pats");
          teams.add("Celtics");
          teams.add("Bruins");
          teams.add("Red Sox");
          System.out.println(teams);
          System.out.println("Index of \"Red Sox\": " + teams.indexOf("Red Sox"));
          System.out.println("Index of \"Yankees\": " + teams.indexOf("Yankees"));
      }
  }
  
  java ListIndexOf
  [Red Sox, Pats, Celtics, Bruins, Red Sox]
  Index of "Red Sox": 0
  Index of "Yankees": -1

lastIndexOf(value) Method

• lastIndexOf(value) returns the index of the last occurence of the value in the list
• If the value does not occur in the list it returns -1

  import java.util.ArrayList;
  
  public class ListLastIndexOf {
      public static void main (String[] args){
          ArrayList<String> teams = new ArrayList<>();
          teams.add("Red Sox");
          teams.add("Pats");
          teams.add("Celtics");
          teams.add("Bruins");
          teams.add("Red Sox");
          System.out.println(teams);
          System.out.println("Last index of \"Red Sox\": " + teams.lastIndexOf("Red Sox"));
          System.out.println("Last index of \"Yankees\": " + teams.lastIndexOf("Yankees"));
      }
  }
  
  $ java ListLastIndexOf
  [Red Sox, Pats, Celtics, Bruins, Red Sox]
  Last index of "Red Sox": 4
  Last index of "Yankees": -1

Writing a Method to Read in Files

• Creating an ArrayList from the contents of a file is something we might want to do often
• Any job you might often want to do is something that should be turned into a method
• Here is an program that contains a method to create an ArrayList from a file

  import java.io.*;
  import java.util.*;
  
  public class FileToList {
      public static void main (String[] args) 
          throws  FileNotFoundException {
          ArrayList<String> list = create_list_from_file(args[0]);
          System.out.println(list);
      }
  
      public static ArrayList>String> create_list_from_file(String filename)
          throws  FileNotFoundException {
          File file = new File(filename);
          ArrayList>String> list = new ArrayList<>();
          Scanner input = new Scanner(file);
          while (input.hasNext()){
              list.add(input.next());
          }
          return list;
      }
  }
  
  $ java FileToList.java fruit.txt 
  [grapes, pears, oranges, cranberries, apples, melons, blueberries]

Reading a File of Numbers

• The create_list_from_file method works will give us a list of strings
• But what if we wanted a list of integers?
• We have to change the return type of the method
• And the type of ArrayList created
• We also need to replace hashNext and next with hasNextInt and nextInt
• Here is the code with the changes highlighted

  import java.io.*;
  import java.util.*;
  
  public class FileToList2 {
      public static void main (String[] args) 
          throws  FileNotFoundException {
          ArrayList<Integer> list = create_list_from_file(args[0]);
          System.out.println(list);
      }
  
      public static ArrayList<Integer> create_list_from_file(String filename)
          throws  FileNotFoundException {
          File file = new File(filename);
          ArrayListArrayList<Integer>  list = new ArrayList<>();
          Scanner input = new Scanner(file);
          while (input.hasNextInt()){
              list.add(input.());
          }
          return list;
      }
  }
  
  $ java FileToList2.java numbers
  [46, 19, 35, 43, 35, 33, 42, 6, 25, 6, 19, 31, 1, 23, 14, 18, 24, 24, 48, 13, 36, 12, 15, 40, 14, 27, 28, 44, 30, 50]
  

Stopping Execution of a Loop

• The loop header determines how a loop
• In a for loop it tells the number of times the loop will run
• In while and do/while loops it gives the condition that will make the loop stop
• But sometimes there are special situations where you want another way to stop running the loop code
• Java has two statements that can do this
  • continue
  • break

The break Statement

• Sometimes you will come across a situation where you simply want a loop to stop running
• To jump completely out of he loop you would use a break statement
• Here is an example

  public class Break {
      public static void main (String[] args){
          for (int i = 1; i <= 10; i++){
              if (i == 5){
                  break;
              } else {
                  System.out.println(i);
              }
          }
  
      }
  }
  
  $ java Break.
  1
  2
  3
  4

The continue Statement

• Maybe you don't want to jump completely out of the loop
• You just want to stop running the loop code for one iteration of the loop
• In that case you would use a continue statement
• Here is an example

  public class Continue {
      public static void main (String[] args){
          for (int i = 1; i <= 10; i++){
              if (i % 2 == 0){
                  continue;
              } else {
                  System.out.println(i);
              }
          }
      }
  }
  
  $ java Continue
  1
  3
  5
  7
  9

The switch Statement

• What if we wanted to write a script that would give the course title for and IT class number?
• Limiting ourselves to the 100 series courses, we might write

  public class ItCourseTitle {
      public static void main (String[] args){
          String course_number = args[0];
          String course_title;
          if (course_number.equals("110")){
              course_title = "Information Technology Problem Solving";
          } else if (course_number.equals("111")){
              course_title = "Managerial Statistics";
          } else if (course_number.equals("114")){
              course_title = "Introduction To Java";
          } else if (course_number.equals("116")){
              course_title = "Introduction to Scripting";
          }  else if (course_number.equals("117")){
              course_title = "Intermediate Scripting";
          } else {
              course_title = "Unknown";
          }
          System.out.println("IT " + course_number + " - " + course_title);
      }
  }
  
  $ java ItCourseTitle 114
  IT 114 - Introduction To Java
  
  $ java ItCourseTitle.java 119
  IT 119 - Unknown

• There is a lot of repeated code in this example
• But Java gives us another way
• Whenever the path taken through the code is different depending on the value of a particular expression we can use a switch statement
• It has the following format

  
  switch(EXPRESSION) {
    case VALUE_1:
      STATEMENT;
      STATEMENT;
      ...
      break;
    case y:
      STATEMENT;
      STATEMENT;
      break;
    ...
    default:
      STATEMENT;
      STATEMENT;
  }

• In other words, each value runs a different set of statements
• We can rewrite the program to give course titles using a switch statement

  public class ItCourseTitle2 {
      public static void main (String[] args){
          String course_number = args[0];
          String course_title;
          switch (course_number){
              case "110":
                  course_title = "Information Technology Problem Solving";
                  break;
              case "111":
                  course_title = "Managerial Statistics";
                  break;
              case "114":
                  course_title = "Introduction To Java";
                  break;
              case "116":
                  course_title = "Introduction to Scripting";
                  break;
              case "117":
                  course_title = "Intermediate Scripting";
                  break;
              default:
                  course_title = "Unknown";
          
          }
          System.out.println("IT " + course_number + " - " + course_title);
      }
  }
  
  $ java ItCourseTitle2 114
  IT 114 - Introduction To Java

Do Not Leave Out break in a switch Statement

• If you leave out break in the code for each case you will get very strange results
• The code for the matching case will run
• But so with the code for every case below the one that matches

  public class ItCourseTitle3 {
      public static void main (String[] args){
          String course_number = args[0];
          String course_title;
          switch (course_number){
              case "110":
                  course_title = "Information Technology Problem Solving";
                  System.out.println("IT " + course_number + " - " + course_title);
              case "111":
                  course_title = "Managerial Statistics";
                  System.out.println("IT " + course_number + " - " + course_title);
              case "114":
                  course_title = "Introduction To Java";
                  System.out.println("IT " + course_number + " - " + course_title);
              case "116":
                  course_title = "Introduction to Scripting";
                  System.out.println("IT " + course_number + " - " + course_title);
              case "117":
                  course_title = "Intermediate Scripting";
                  System.out.println("IT " + course_number + " - " + course_title);
              default:
                  course_title = "Unknown";
          }
      }
  }
  
  $ java ItCourseTitle3 114
  IT 114 - Introduction To Java
  IT 114 - Introduction to Scripting
  IT 114 - Intermediate Scripting

Class Quiz

Attendance

Class Exercise