List Implementations That Use Arrays

Chapter 5

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Chapter ContentsUsing a Fixed-Size Array to Implement the ADT List• An Analogy• The Java Implementation

Using Dynamic Array Expansion to Implement the ADT List• Expanding an Array• A New Implementation of a List

Using a Vector to Implement the ADT List• A Summary of Methods in the Class Vector

The Pros and Cons of Using an Array to Implement the ADT List• The Class ArrayList• The Interface Serializable

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An Analogy

Consider a classroom with 40 desks in fixed position• Desks are wasted if less than 40 students• Not enough desks if more than 40 students

An array is like the classroom• Each desk an array location

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An Analogy

Fig. 5-1 A classroom that contains desks in a fixed position.

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An Analogy

Suppose we have some students in classroom in order alphabeticallyWe add a new student• We desire to maintain the alphabetic order• We must shift some students

We remove a student in the middle of the sequence• Again, we must shift some students

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Adding a Student

Fig. 5-2 Seating a new student between two existing students: at least one other student must move

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The Java Implementation

Private data fields for implementation of AList• Implements interface ListInterface of

Chapter 4

Note the full specification, pgs 103-105

private Object entry[]; // array of list entries

private int length; // current number of entries in list

private static final int MAX_SIZE = 50; // max length of list

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AList Implementation (1):

public class AList<T> implements ListInterface<T>{ private T[] entry; // array of list entries private int length; // current number of entries in list private static final int MAX_SIZE = 50; // max length of list

public AList() { this(MAX_SIZE); } // end default constructor

public AList(int maxSize) { length = 0; entry = (T[]) new Object[maxSize]; } // end constructor

public boolean add(Object newEntry) { /* < Implementation deferred > */ } // end add

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AList Implementation (2):

public boolean add(int newPosition, T newEntry) { /* < Implementation deferred > */ } // end add

public T remove(int givenPosition) { /* < Implementation deferred > */ } // end remove

public void clear() { length = 0; //< But see Question 8. > } // end clear

public boolean replace(int givenPosition, T newEntry) { /* < Implementation deferred > */ } // end replace

public T getEntry(int givenPosition) { /* < Implementation deferred > */ } // end getEntry

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AList Implementation (3):

public boolean contains(T anEntry) { /* < Implementation deferred > */ } // end contains

public int getLength() { return length; } // end getLength

public boolean isEmpty() { return length == 0; } // end isEmpty

public boolean isFull() { return length == entry.length; } // end isFull

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AList Implementation (4):

public void display() { for (int index = 0; index < length; index++) System.out.println(entry[index]); } // end display

/* < This class will define two private methods that will be discussed later. > */} // end AList

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AList add() Methods

First add method adds a new item at the end of the list• Assign new value at end• Increment length of list

Second add method adds item in mid-list• Requires a utility method, makeRoom()• This shifts elements ahead

See my Eclipse Code

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Adding Items in Mid-list

Fig. 5-3 Making room to insert Carla as third entry in an array.

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The remove() Method

Must shift existing entries to avoid gap in the array• Except when removing last entry

Method must also handle error situation• When position specified in the remove is

invalid• When remove() is called and the list is empty• Invalid call returns null value

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Removing a List Entry

Fig. 5-4 Removing Bob by shifting array entries.

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Dynamic Array Expansion

An array has a fixed size• If we need a larger list, we are in trouble

When array becomes full• Move its contents to a larger array (dynamic

expansion)• Copy data from original to new location• Manipulate names so new location keeps

name of original array

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Dynamic Array Expansion

Fig. 5-5 The dynamic expansion of an array copies the array's contents to a larger second array.

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Dynamic Array Expansion

Fig. 5-6 (a) an array; (b) the same array with two references; (c) the two arrays, reference to original array now referencing a new, larger array

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A New Implementation of a ListChange the isFull to always return false• We will expand the array when it becomes full• We keep this function so that the original

interface does not change

The add() methods will double the size of the array when it becomes fullNow declare a private method isArrayFull • Called by the add() methods

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A new add() method (1):

public boolean isFull() { return false;}

private boolean isArrayFull() { return length == list.length;}

public boolean add(T newEntry) { if (isArrayFull()) doubleArray(); // add new entry after last current entry entry[length] = newEntry; length++; return true;} // end add

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A new add() method (2):

NOTE: See System.arraycopy() for built-in array copy method.

/** Task: Doubles the size of the array of list entries. */private void doubleArray(){ T[] oldList = entry; // save reference to array of // list entries int oldSize = oldList.length; // save old max size of array

entry = (T[]) new Object[2*oldSize]; // double size of array

// copy entries from old array to new, bigger array for (int index = 0; index < oldSize; index++) entry[index] = oldList[index];} // end doubleArray

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Using a Vector to Implement the ADT List

Java's Vector class provides capabilities of an array• Able to expand dynamically• Hides the details of the process

Vector• Found in package java.util• Has methods for manipulating entries• Enables implementing the ADT List

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Using a Vector

Fig. 5-7 A client uses the methods given in ListInterface, but the implementation of the list

uses Vector methods to perform its operations

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Using a Vector

Elements of the class

• Class Vector comes from package java.util

• Data field entry is an instance of a Vector

import java.util.Vector;public class VectorList implements ListInterface{

private Vector entry; // entries in list

. . .

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Using a Vector

The add() methods • The first uses the addElement method from

the Vector class• The other uses the insertElementAt

method

The remove() method• Uses the removeElementAt method

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Pros and Cons of Array Use for the ADT List

When using an array or vector …• Retrieving an entry is fast• Adding an entry at the end of the list is fast• Adding or removing an entry that is between

other entries requires shifting elements in the array

• Increasing the size of the array or vector requires copying elements

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Java Class LibraryHas a class similar to class AList defined in this chapter• Class ArrayList• Uses dynamic array expansion

Interface Serializable• Represent an object as a sequence of bytes to

be written to a file• Add the words implements Serializable

to class definitionpublic class AList implements ListInterface, Serializable{ . . .

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SerializableThis is an interface with no methods. Read the ADT specification. Read the last part of Appendix C of our text.