1. Programming with Arrays 1
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. Partially Filled Arrays
The exact size needed for an array may be unknown till run time.
Solution:
Declare a larger array: someArray[MAX]
Keep track of # valid elements in count
someArray[0] ~ someArray[count – 1]
Where count <= someArray.length
CSS161: Fundamentals of Computing

3. An Array Implementation of a List
List is a data structure to maintain a dynamic size of items in a sequential order.
10.0
20.0
30.0
40.0
50.0
60.0
70.0
List.add( ) X
10.0
20.0
30.0
40.0
50.0
60.0
70.0
List.deleteLast( ) X
10.0
20.0
30.0
40.0
50.0
60.0
List.delete( 3 )
10.0
20.0
30.0
50.0
60.0
List.getElement( 3 ) returns 50.0
CSS161: Fundamentals of Computing

4. List Implementation (1 of 5)
/**
List.java: demonstrates a use of a partially filled array of doubles.
The class enforces the following invariant: All elements are at the beginning
of the array in locations 0, 1, 2, and so forth up to a highest index with no
gaps. */
public class List {
private int maxNumberElements; //Same as a.length
private double[] a;
private int numberUsed; //Number of indices currently in use
Sets the maximum number of allowable elements to 10.
List( )
maxNumberElements = 10;
a = new double[maxNumberElements];
numberUsed = 0; }
CSS161: Fundamentals of Computing

5. List Implementation (2 of 5)
/**
Precondition arraySize > 0. */
List(int arraySize) {
if (arraySize <= 0)
System.out.println("Error Array size zero or negative.");
System.exit(0); }
maxNumberElements = arraySize;
a = new double[maxNumberElements];
numberUsed = 0;
List(List original)
if (original == null)
System.out.println("Fatal Error: aborting program.");
maxNumberElements =
original.maxNumberElements;
numberUsed = original.numberUsed;
for (int i = 0; i < numberUsed; i++)
a[i] = original.a[i];
CSS161: Fundamentals of Computing

6. List Implementation (3 of 5)
/**
Adds newElement to the first unused array position. */
public void add(double newElement) {
if (numberUsed >= a.length)
System.out.println("Error: Adding to a full array.");
System.exit(0); }
else
a[numberUsed] = newElement;
numberUsed++;
public double getElement(int index)
if (index < 0 || index >= numberUsed)
System.out.println("Error:Illegal or unused index.");
return a[index];
CSS161: Fundamentals of Computing

7. List Implementation (4 of 5)
/**
index must be an index in use or the first unused index. */
public void resetElement(int index, double newValue) {
if (index < 0 || index >= maxNumberElements)
System.out.println("Error:Illegal index.");
System.exit(0); }
else if (index > numberUsed)
System.out.println(
"Error: Changing an index that is too large.");
else
a[index] = newValue;
public void deleteLast( )
if (empty( ))
System.out.println("Error:Deleting from an empty array.");
numberUsed--;
CSS161: Fundamentals of Computing

8. List Implementation (5 of 5)
/**
Deletes the element in position index. Moves down all elements with
indices higher than the deleted element. */
public void delete(int index) {
if (index < 0 || index >= numberUsed)
System.out.println("Error:Illegal or unused index.");
System.exit(0); }
for (int i = index; i < numberUsed; i++)
a[i] = a[i + 1]; // what if numberUsed reached a.length?
numberUsed--;
public boolean empty( )
return (numberUsed == 0);
public boolean full( )
return (numberUsed == maxNumberElements);
public int getMaxCapacity( ) {
return maxNumberElements; }
public int getNumberOfElements( )
return numberUsed;
CSS161: Fundamentals of Computing

9. CSS161: Fundamentals of Computing
List Driver
public class ListDriver {
public static void main( String[] args ) {
List list = new List( 20 );
for ( int i = 0; i < 20; i++ )
list.add( i );
list.deleteLast( ); // what if this statement is omitted?
list.delete( 10 );
for ( int i = 0; i < list.getNumberOfElements( ); i++ )
System.out.println( "list(" + i + ") = " + list.getElement( i ) ); }
css161]$ java ListDriver
list(0) = 0.0
list(1) = 1.0
list(2) = 2.0
list(3) = 3.0
list(4) = 4.0
list(5) = 5.0
list(6) = 6.0
list(7) = 7.0
list(8) = 8.0
list(9) = 9.0
list(10) = 11.0
list(11) = 12.0
list(12) = 13.0
list(13) = 14.0
list(14) = 15.0
list(15) = 16.0
list(16) = 17.0
list(17) = 18.0
css161]$
CSS161: Fundamentals of Computing

10. CSS161: Fundamentals of Computing
Question
What if we delete the following statement from ListDriver.java program?
list.deleteLast( );
Keep track of the program execution.
CSS161: Fundamentals of Computing

11. CSS161: Fundamentals of Computing
“for-each” Loop
ordinary for loop
for-each loop
double[] a = new double[10];
for ( int i = 0; i < a.length; i++ )
a[i] = 0.0;
System.out.println( a[i] );
a[i] = 2 * i;
double[] a = new double[10];
for ( double element : a )
element = 0.0;
System.out.println( element );
element = 2 * i; // ??? Was i defined ???
CSS161: Fundamentals of Computing

12. Vararg Specification Method with a Variable Number of Parameters
Syntax
Type… Array_Name
Examples
int… arg
double… a
String… unwanted
public static int max( int… arg ) {
// arg can be used as an array of ints }
max( score1, score2 );
max( score1, score2, score3 );
score1
score2
score1
score2
score3
CSS161: Fundamentals of Computing

13. CSS161: Fundamentals of Computing
Display 6.7 (1 of 2)
CSS161: Fundamentals of Computing

14. CSS161: Fundamentals of Computing
Display 6.7 (2 of 2)
import java.util.Scanner;
public class VariableParameterDemo {
public static void main(String[] args)
System.out.println("Enter scores for Tom, Dick, and Harriet:");
Scanner keyboard = new Scanner(System.in);
int tomsScore = keyboard.nextInt( );
int dicksScore = keyboard.nextInt( );
int harrietsScore = keyboard.nextInt( );
int highestScore = UtilityClass.max(tomsScore, dicksScore, harrietsScore);
System.out.println("Highest score = " + highestScore); }
Enter scores for Tom, Dick, and Harriet:
Highest score = 100
CSS161: Fundamentals of Computing

15. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on textbook p371’s exercises
CSS161: Fundamentals of Computing

16. Privacy Leaks with Array Instance Variables
Just as when an accessor returns a reference to any private object
public double[] getArray() {
return anArray;//BAD! }
The example above will result in a privacy leak
CSS161: Fundamentals of Computing

17. Privacy-Leak Preventive Example 1
The previous accessor method would simply return a reference to the array anArray itself
Instead, an accessor method should return a reference to a deep copy of the private array object
Below, both a and count are instance variables of the class containing the getArray method
public double[] getArray() {
double[] temp = new double[count];
for (int i = 0; i < count; i++)
temp[i] = a[i];
return temp }
CSS161: Fundamentals of Computing

18. Privacy-Leak Preventive Example 2
If a private instance variable is an array that has a class as its base type, then copies must be made of each class object in the array when the array is copied:
public ClassType[] getArray() {
ClassType[] temp = new ClassType[count];
for (int i = 0; i < count; i++)
temp[i] = new ClassType(someArray[i]);
return temp; }
CSS161: Fundamentals of Computing

19. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on textbook p375’s exercises 18 ~ 19.
CSS161: Fundamentals of Computing