ArrayList Tutorial

SET08112 ArrayList Interface Tutorial

In this tutorial you will use the ArrayList and the HashMap or TreeMap to solve some common problems. You will use methods such as:

• java.util.Collections.sort
• add
• get

Problem 1 - Sort a List

The file from http://progzoo.net/students.txt contains a list of students:

Whitaker, Forest 
Winslet, Kate 
De Havilland, Olivia 
Donat, Robert 
Douglas, Michael 
Dressler, Marie 
...

Create a programme SortStudents.java that reads in this file and outputs the students in Alphabetical order.

• You can use an ArrayList to hold the students.
• ArrayList<String> names = new ArrayList<String>();
• You can use java.util.Collections.sort to sort the ArrayList

Hint

Add this line after you have filled the ArrayList but before you print it.

java.util.Collections.sort(names);

.

import java.util.ArrayList; import java.io.BufferedReader; import java.io.FileReader; public class SortStudents{ public static void main(String[]argv) throws Exception{ ArrayList<String> names = new ArrayList<String>(); String line; BufferedReader fh = new BufferedReader(new FileReader("students.txt")); while (null!=(line=fh.readLine())) names.add(line); for (String s:names) System.out.println(s); } }

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Problem 2 - List Routes

The file http://progzoo.net/year1.txt contains a list of all students in year one. Each line contains the name of the student and the name of the route (programme) they follow separated by a tab.

Brando, Marlon 	BSC (HONS) DIGITAL MEDIA F/T
Bridges, Jeff 	BSC (HONS) INTERACTIVE MEDIA DESIGN F/T
Brody, Adrien 	BENG (HONS) COMPUTER SECURITY AND FORENSICS F/T

Write a program to read in this file and output a list of routes with no duplicates. You can use a program very similar to the one above – but this time:

• Use line.split("\t") to get the route
• Use name.contains(st[1]) to add the route only if it is not already in the ArrayList
• You do not need to sort before output

.

import java.util.ArrayList; import java.io.BufferedReader; import java.io.FileReader; public class SortStudents{ public static void main(String[]argv) throws Exception{ ArrayList<String> names = new ArrayList<String>(); String line; BufferedReader fh = new BufferedReader(new FileReader("year1.txt")); while (null!=(line=fh.readLine())){ String [] st = line.split("\t"); names.add(st[1]); } for (String s:names) System.out.println(s); } }

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Problem 3 - Who's missing?

The file present.txt contains those students from students.txt who attended a particular tutorial. We want a list of all those students who were not present.

Brody, Adrien
Irons, Jeremy
Spacek, Sissy

How to do this…

• Create an array list called present
• Open the file present.txt and add every line to the ArrayList present
• Open the file student.txt
• For each line in the file – if that student is not in present print the name.

if (!present.contains(line)) System.out.println(line);

.

import java.util.ArrayList; import java.io.BufferedReader; import java.io.FileReader; public class SortStudents{ public static void main(String[]argv) throws Exception{ ArrayList<String> names = new ArrayList<String>(); String line; BufferedReader fh = new BufferedReader(new FileReader("present.txt")); while (null!=(line=fh.readLine())){ names.add(line); } } }

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Problem 4 - Count Routes

Use the file year1.txt to print a list of all the routes in alphabetical order. We also want a count of how many students are in each route.

BENG (HONS) COMPUTER NETWORKS AND DISTRIBUTED SYSTEMS F/T	10
BENG (HONS) COMPUTER SECURITY AND FORENSICS F/T	28
BENG (HONS) COMPUTING F/T	22
BENG (HONS) EMBEDDED COMPUTER SYSTEMS F/T	1
...

How to do this:

• See the section “Use Map to aggregate” from the lecture
• Use the HashMap structure to map String to Integer, the String will be the route name, the Integer will be the number of students
• HashMap supports methods put and get
• You can use two passes to process get the answer.
  • On the first pass run over the list and put the value 0 against every route you encounter.
  • On the second pass use put to add 1 to each route you encounter.
• It is better to use a single pass:
  • For each route if the key exists then +1 of the value, if the key does not exist then put 1.

.

import java.util.HashMap; import java.io.BufferedReader; import java.io.FileReader; public class CountRoutes{ public static void main(String[]argv) throws Exception{ HashMap<String,Integer> routes = new HashMap<String,Integer>(); String line; BufferedReader fh = new BufferedReader(new FileReader("year1.txt")); while (null!=(line=fh.readLine())){ String [] s = line.split("\t"); routes.put(s[1],42); } for (String r:routes.keySet()) System.out.printf("%s\t%d\n", r,routes.get(r)); } }

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Problem 5 – Use Map to count regions

Read in the file bbc.txt and output a list of region and the number of countries in each. This problem is just like the previous problem. If you use a TreeMap in place of a hash map then the output will be in order.

.

import java.util.HashMap; import java.io.BufferedReader; import java.io.FileReader; public class CountRegions{ public static void main(String[]argv) throws Exception{ HashMap<String,Integer> region = new HashMap<String,Integer>(); String line; BufferedReader fh = new BufferedReader(new FileReader("bbc.txt")); while (null!=(line=fh.readLine())){ String [] s = line.split("\t"); region.put(s[1],42); } for (String r:region.keySet()){ System.out.printf("%-19s%3d\n", r,region.get(r)); } } }

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Problem 6 – Use Map to find associates

Read in the file bbc.txt and output a list of countries by region. One way to do this is to build up a list of countries for each region. To do that you could use a structure such as:

TreeMap<String,ArrayList<String>> region
   = new TreeMap<String,ArrayList<String>>();

.

import java.util.TreeMap; import java.util.ArrayList; import java.io.BufferedReader; import java.io.FileReader; public class FindRegions{ public static void main(String[]argv) throws Exception{ TreeMap<String,ArrayList<String>> region = new TreeMap<String,ArrayList<String>>(); String line; BufferedReader fh = new BufferedReader(new FileReader("bbc.txt")); while (null!=(line=fh.readLine())){ String [] s = line.split("\t"); } } }

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