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 you identify the person in the third picture?

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);
    }

}