Introduction

• The data structure above is LinkedList<Integer>, from the Java library.

• We haven't looked "under the hood" to see how it works; we merely use it above.

• The example shows how to create a list, how to add stuff and how to extract stuff in order.

Lists can contain any kind of object, for example:

import java.util.*;
import java.awt.*;
import java.awt.image.*;

public class ListExample2 
{
    public static void main (String[] argv) 
    {
        // Create an empty list that can hold objects of type Image:
        LinkedList<Image> imageList = new LinkedList<Image> ();

        // Retrieve images from files using ImageTool, and add one
        // by one into the list.
        ImageTool imTool = new ImageTool ();
        Image image1 = imTool.readImageFile ("alum1.jpg");
        imageList.add (image1);
        Image image2 = imTool.readImageFile ("alum2.jpg");
        imageList.add (image2);
        Image image3 = imTool.readImageFile ("alum3.jpg");
        imageList.add (image3);

        viewImages (imageList);

    }


    // The list as method parameter:

    static void viewImages (java.util.List<Image> imageList)
    {
        ImageTool imTool = new ImageTool ();
        for (Image image : imageList) {        // Note the for-loop
            imTool.showImage (image);
        }
    }

}

In-Class Exercise 4: Download the above program, along with ImageTool.java and the three image files (alum1.jpg, alum2.jpg, alum3.jpg), compile and execute.

Can an array be used for a list?
      ⇒ Yes, for example

public class ListExample3 {

    public static void main (String[] argv)
    {
        // Plain old array: must specify size
        int[] intList = new int [10];

        for (int i=0; i < 10; i++) {
            intList[i] = i*i;
        }

        // Print.
        for (int i=0; i < 10; i++) {
            System.out.println ("Element #" + i + " = " + intList[i]);
        }
    }
    
}

• When using an array, we must specify the size in advance.

• For some applications, we may not know the desired size ahead of time.

• Later we will see that arrays can be very efficient.

A queue

What is a queue?

• In a list:
  • We usually add items at the end, but sometimes also in sorted order.
  • Deletion is usually an afterthought, to "fix" the list.
  • Access is usually front to back, or for a particular element in the list.

• In a queue:
  • Items are always added at the end.
  • Items are always deleted from the beginning.
  • The whole purpose is to create "order-of-arrival".

import java.util.*;

public class QueueExample {

    public static void main (String[] argv)
    {
        // We'll use a list, but treat it like a queue.
        // Note: it's a list meant to hold strings.
        LinkedList<String> queue = new LinkedList<String> ();

        // Add some names:
        queue.add ("Alice");
        queue.add ("Bob");
        queue.add ("Chen");
        queue.add ("Dagmar");
        queue.add ("Eva");
        queue.add ("Faisal");
        
        System.out.println ("Queue contents: " + queue);

        // Extract one by one:
        while (! queue.isEmpty() ) {
            String name = queue.remove ();
            System.out.println ("Next comes " + name);
            System.out.println ("Queue contents: " + queue);
        }
    }
    
}

import java.util.*;

public class StackExample {

    public static void main (String[] argv)
    {
        // The Java library has a Stack data structure. Here, we'll
        // make a stack to hold strings.
        Stack<String> toDoList = new Stack<String> ();

        // Add some strings.
        toDoList.push ("Pay bills");
        toDoList.push ("Clean room");
        toDoList.push ("Do homework");
        toDoList.push ("See movie");
        toDoList.push ("Hang out");
        
        // Print.
        System.out.println ("Priorities: ");
        while (! toDoList.isEmpty() ) {
            String nextPriority = toDoList.pop ();
            System.out.println (" " + nextPriority);
        }
    }
    
}

public class TreeExample {

    public static void main (String[] argv)
    {
        // Java's library has tree variant called TreeSet.
        // We'll make an instance to hold String's.
        TreeSet<String> unusualWords = new TreeSet<String> ();

        // Put some stuff in.
        unusualWords.add ("queueing");        // Six vowels in a row.
        unusualWords.add ("psychorhythms");   // Longest with one vowel.
        unusualWords.add ("verisimilitudes"); // Longest with alternating vowel/consonant.
        unusualWords.add ("frillless");       // Three letters in a row.
        unusualWords.add ("bookkeeper");      // Three double-letter's in a row.
        unusualWords.add ("cwm");             // Pronounced "koom" (type of crevasse).
        unusualWords.add ("feedback");        // Contains all letters 'a' to 'f'.

        // Print.
        System.out.println (unusualWords);
    }

}