10
Arrays Chapter Lists
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Array Basics
An array is an indexed collection of data values. If your program needs to deal with 100 integers, 500 Account objects, 365 real numbers, etc., you will use an array. In Java, an array is a collection of data values of the same type.
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Arrays of Primitive Data Types
Array Declaration
<data type> [ ] <variable> <data type> <variable>[ ] //variation 1 //variation 2
Array Creation
<variable> = new <data type> [ <size> ]
Example
Variation 1
rainfall
Variation 2
double rainfall [ ]; = new double[12];
double[ ] rainfall; rainfall = new double[12];
An array is like an object!
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Accessing Individual Elements
Individual elements in an array accessed with the indexed expression.
double[] rainfall = new double[12];
rainfall
0
1
2
3
4
5
6
7
8
9
10
11
The index of the first position in an array is 0.
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rainfall[2]
This indexed expression refers to the element at position #2
Array Processing Sample1
double[] rainfall = new double[12]; double annualAverage, sum = 0.0; for (int i = 0; i < rainfall.length; i++) { rainfall[i] = Double.parseDouble( JOptionPane.showinputDialog(null, "Rainfall for month " + (i+1) ) ); sum += rainfall[i]; } annualAverage = sum / rainfall.length;
The public constant length returns the capacity of an array.
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Array Processing Sample 2
double[] rainfall = new double[12]; String[] monthName = new String[12]; monthName[0] = "January"; monthName[1] = "February"; ... double annualAverage, sum = 0.0;
The same pattern for the remaining ten months.
for (int i = 0; i < rainfall.length; i++) { rainfall[i] = Double.parseDouble( JOptionPane.showinputDialog(null, "Rainfall for " + monthName[i] )); sum += rainfall[i]; } annualAverage = sum / rainfall.length;
The actual month name instead of a number.
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Array Processing Sample 3
Compute the average rainfall for each quarter.
//assume rainfall is declared and initialized properly double[] quarterAverage = new double[4]; for (int i = 0; i < 4; i++) { sum = 0; for (int j = 0; j < 3; j++) { //compute the sum of sum += rainfall[3*i + j]; } quarterAverage[i] = sum / 3.0; } //Quarter (i+1) average //one quarter
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Array Initialization
Like other data types, it is possible to declare and initialize an array at the same time.
int[] number = { 2, 4, 6, 8 }; double[] samplingData = { 2.443, 8.99, 12.3, 45.009, 18.2, 9.00, 3.123, 22.084, 18.08 }; String[] monthName = { "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" };
number.length samplingData.length monthName.length
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4 9 12
Variable-size Declaration
In Java, we are not limited to fixed-size array declaration. The following code prompts the user for the size of an array and declares an array of designated size:
int size; int[] number; size= Integer.parseInt(JOptionPane.showInputDialog(null, "Size of an array:")); number = new int[size];
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Arrays of Objects
In Java, in addition to arrays of primitive data types, we can declare arrays of objects An array of primitive data is a powerful tool, but an array of objects is even more powerful. The use of an array of objects allows us to model the application more cleanly and logically.
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The Person Class
We will use Person objects to illustrate the use of an array of objects.
Person latte; latte = new Person( ); latte.setName("Ms. Latte"); latte.setAge(20); latte.setGender('F'); System.out.println( "Name: " + latte.getName() System.out.println( "Age : " + latte.getAge() ); ); The Person class supports the set methods and get methods.c
System.out.println( "Sex : " + latte.getGender() );
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Creating an Object Array - 1
Code
A
Person[ ]
person; Only the name person is declared, no array is allocated yet.
person = new Person[20]; person[0] = new Person( );
person
State of Memory
After A is executed
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Creating an Object Array - 2
Code
Person[ ] person; Now the array for storing 20 Person objects is created, but the Person objects themselves are not yet created.
B
person = new Person[20]; person[0] = new Person( );
person 0 State of Memory 1 2 3 4 16 17 18 19
After B is executed
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Creating an Object Array - 3
Code
Person[ ] person; One Person object is created and the reference to this object is placed in position 0.
person = new Person[20];
C
person
person[0] = new Person( );
0 State of Memory
1
2
3
4
16 17 18 19
Person
After C is executed
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Person Array Processing Sample 1
Create Person objects and set up the person array.
String int char name, inpStr; age; gender;
for (int i = 0; i < person.length; i++) { name age = inputBox.getString("Enter name:"); = inputBox.getInteger("Enter age:"); //read in data values
inpStr = inputBox.getString("Enter gender:"); gender = inpStr.charAt(0); person[i] = new ); Person( person[i].setName person[i].setAge }
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//create a new Person and assign values ); );
( name ( age
person[i].setGender( gender );
Person Array Processing Sample 2
Find the youngest and oldest persons.
int int minIdx = 0; maxIdx = 0; //index to the youngest person //index to the oldest person
for (int i = 1; i < person.length; i++) { if ( person[i].getAge() < person[minIdx].getAge() ) { minIdx = i; //found a younger person
} else if (person[i].getAge() > person[maxIdx].getAge() ) { maxIdx } } //person[minIdx] is the youngest and person[maxIdx] is the oldest = i; //found an older person
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Object Deletion Approach 1
int delIdx = 1;
A
person[delIdx] = null;
Delete Person B by setting the reference in position 1 to null.
person 0 1 2 3
person 0 1 2 3
A
B
C
D
A
C
D
Before A is executed
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After A is executed
Object Deletion Approach 2
int delIdx = 1, last = 3;
A
person[delIndex] = person[last]; person[last] = null;
Delete Person B by setting the reference in position 1 to the last person.
person 0 1 2 3
person 0 1 2 3
A
B
C
D
A
D
C
Before A is executed
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After A is executed
Person Array Processing Sample 3
Searching for a particular person. Approach 2 Deletion is used.
int i = 0; while ( person[i] != null && !person[i].getName().equals("Latte") ) { i++; } if ( person[i] == null ) { //not found - unsuccessful search System.out.println("Ms. Latte was not in the array"); } else { //found - successful search System.out.println("Found Ms. Latte at position " + i); }
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Passing Arrays to Methods - 1
Code A
minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { ...
}
At
A before searchMinimum
A. Local variable
number does not exist before the method execution
arrayOne
State of Memory
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Passing Arrays to Methods - 2
Code
minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { ...
B
}
The address is copied at B
arrayOne number
B. The value of the
argument, which is an address, is copied to the parameter.
State of Memory
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Passing Arrays to Methods - 3
Code
minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { ...
C
}
While at C
arrayOne
inside the method
number
C. The array is
accessed via number inside the method.
State of Memory
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Passing Arrays to Methods - 4
Code
minOne = searchMinimum(arrayOne); public int searchMinimum(float[] number)) { ...
D
}
At D after searchMinimum
arrayOne number
D. The parameter is
erased. The argument still points to the same object.
State of Memory
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Collections The java.util standard package contains different types of classes for maintaining a collection of objects. These classes are collectively referred to as the Java Collection Framework (JCF). JCF includes classes that maintain collections of objects as sets, lists, or maps.
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Java Interface A Java interface defines only the behavior of objects
It includes only public methods with no method bodies. It does not include any data members except public constants No instances of a Java interface can be created
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JCF Lists JCF includes the List interface that supports methods to maintain a collection of objects as a linear list L = (l0, l1, l2, . . . , lN) We can add to, remove from, and retrieve objects in a given list. A list does not have a set limit to the number of objects we can add to it.
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List Methods
Here are five of the 25 list methods:
boolean add void Object clear get ( Object o ) ( ( int idx ) ) ) )
Adds an object o to the list Clears this list, i.e., make the list empty Returns the element at position idx
boolean remove ( int idx
Removes the element at position idx
int
size
(
Returns the number of elements in the list
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Using Lists To use a list in a program, we must create an instance of a class that implements the List interface. Two classes that implement the List interface:
ArrayList LinkedList
The ArrayList class uses an array to manage data. The LinkedList class uses a technique called linked-node representation.
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Sample List Usage
Here's an example of manipulating a list:
import java.util.*; List<Person> Person person; friends;
friends = new ArrayList<Person>( ); person = new Person("jane", 10, 'F'); friends.add( person ); person = new Person("jack", friends.add( person ); Person p = friends.get( 1 );
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6, 'M');
Using an Iterator
The iterator method of the ArrayList class returns an object that you can use to loop through all the elements of the list
Iterator iter = friends.iterator(); while (iter.hasNext() System.out.println( iter.next();
The next() method returns each object in the list in turn
the return type of next is Object so to use the Person methods, you have to cast the object to a person Person p = (Person)iter.next();
Use hasNext() to determine when all the elements of the list have been processed
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