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Chapter 10
How to work with
arrays
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 1
Objectives
Applied
 Given a list of values or objects, write code that creates a onedimensional array that stores those values or objects.
 Use for loops and enhanced for loops to work with the values or
objects in an array.
 Use the methods of the Arrays class to fill an array, compare two
arrays, sort an array, or search an array for a value.
 Implement the Comparable interface in any class you create.
 Create a reference to an array and copy elements from one array
to another.
 Given a table of values or objects, write code that creates a twodimensional array that stores those values or objects. The array
can be either rectangular or jagged.
 Use for loops and enhanced for loops to work with the values or
objects in a two-dimensional array.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 2
Objectives (continued)
 Given the Java code for an application that uses any of the
language elements presented in this chapter, explain what each
statement in the application does.
Knowledge
 In general, explain what an array is and how you work with it.
 Describe the operation of the enhanced for loop and explain why
it’s especially useful with arrays.
 Explain when you need to implement the Comparable interface in
a class you create.
 Explain what happens when you assign a new array to an existing
array variable.
Describe the difference between a rectangular array and a jagged
array, and explain the difference in how you create them.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 3
The syntax for declaring and instantiating an array
Two ways to declare an array
type[] arrayName;
type arrayName[];
How to instantiate an array
arrayName = new type[length];
How to declare and instantiate an array in one statement
type[] arrayName = new type[length];
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 4
Examples of array declarations
Code that declares an array of doubles
double[] prices;
Code that instantiates an array of doubles
prices = new double[4];
Code that declares and instantiates an array of
doubles in one statement
double[] prices = new double[4];
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 5
Other examples of array declarations
An array of String objects
String[] titles = new String[3];
An array of Product objects
Product[] products = new Product[5];
Code that uses a constant to specify the array length
final int TITLE_COUNT = 100;
// array size set at
// compile time
String[] titles = new String[TITLE_COUNT];
Code that uses a variable to specify the array length
int titleCount = 100;
// array size not set
// until run time
String[] titles = new String[titleCount];
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 6
How to create an array
 An array can store more than one primitive type or object.
 An element is one of the items in an array.
 To create an array, you must declare a variable of the correct type
and instantiate an array object that the variable refers to.
 To declare an array variable, you code a set of empty brackets after
the type or the variable name.
 To instantiate an array, you use the new keyword and specify the
length, or size, of the array in brackets following the array type.
 You can specify the array length by coding a literal value or by
using a constant or variable of type int.
 When you instantiate an array of primitive types, numeric types are
set to zeros and boolean types to false.
 When you create an array of objects, they are set to nulls.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 7
The syntax for referring to an element of an array
arrayName[index]
The syntax for creating an array and assigning
values in one statement
type[] arrayName = {value1, value2, value3, ...};
How to assign values to the elements of an array
 To refer to the elements, you use an index that ranges from zero
(the first element in the array) to one less than the total number of
elements.
 If you specify an index that’s less than zero or greater than the
upper bound of the array, an ArrayIndexOutOfBoundsException
will be thrown.
 You can instantiate an array and provide initial values in a single
statement by listing the values in braces. The number of values
determines the size of the array.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 8
Code that accesses each element to assign…
…values to an array of double types
double[] prices = new double[4];
prices[0] = 14.95;
prices[1] = 12.95;
prices[2] = 11.95;
prices[3] = 9.95;
//prices[4] = 8.95; // this would throw an
// ArrayIndexOutOfBoundsException
…values to an array of String types
String[]
names[0]
names[1]
names[2]
names = new String[3];
= "Ted Lewis";
= "Sue Jones";
= "Ray Thomas";
…objects to an array of Product objects
Product[] products = new Product[2];
products[0] = new Product("java");
products[1] = new Product("jsps");
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 9
Examples that create arrays and assign values in
one statement
double[] prices = {14.95, 12.95, 11.95, 9.95};
String[] names = {"Ted Lewis", "Sue Jones", "Ray Thomas"};
Product[] products = {new Product("java"),
new Product("jsps")};
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 10
The syntax for getting the length of an array
arrayName.length
Code that puts the numbers 0 through 9 in an
array
int[] values = new int[10];
for (int i = 0; i < values.length; i++)
{
values[i] = i;
}
How to use for loops with arrays
 You can use the length field of an array to determine how many
elements are defined for the array.
 For loops are often used to process each element in an array.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 11
Code that prints an array of prices to the console
double[] prices = {14.95, 12.95, 11.95, 9.95};
for (int i = 0; i < prices.length; i++)
{
System.out.println(prices[i]);
}
The console output
14.95
12.95
11.95
9.95
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 12
Code that computes the average of the array of
prices
double sum = 0.0;
for (int i = 0; i < prices.length; i++)
{
sum += prices[i];
}
double average = sum/prices.length;
Another way to compute the average in a for loop
double sum = 0.0;
for (int i = 0; i < prices.length; sum += prices[i++]);
average = sum/prices.length;
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 13
The syntax of the enhanced for loop
for (type variableName : arrayName)
{
statements
}
How to use enhanced for loops with arrays
 Java 5.0 provides a new form of the for loop called an enhanced for
loop.
 The enhanced for loop is sometimes called a foreach loop because
it lets you process each element of an array.
 Within the parentheses of an enhanced for loop, you declare a
variable with the same type as the array followed by a colon and
the name of the array.
 With each iteration of the loop, the variable that’s declared by the
for loop is assigned the value of the next element in the array.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 14
Code that prints an array of prices to the console
double[] prices = {14.95, 12.95, 11.95, 9.95};
for (double price : prices)
{
System.out.println(price);
}
The console output
14.95
12.95
11.95
9.95
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 15
Code that computes the average
of the array of prices
double sum = 0.0;
for (double price : prices)
{
sum += price;
}
double average = sum/prices.length;
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 16
The Arrays class
java.util.Arrays
Static methods of the Arrays class
Method
fill(arrayName,
value)
fill(arrayName, index1,
index2, value)
equals(arrayName1,
arrayName2)
Murach’s Beg. Java 2, JDK 5, C10
Description
Fills all elements of the array with the
specified value.
Fills elements of the array with the
specified value from the index1 element
to, but not including, the index2
element.
Returns a boolean true value if both
arrays are of the same type and all of the
elements are equal to each other.
© 2005, Mike Murach & Associates, Inc.
Slide 17
Static methods of the Arrays class (continued)
Method
Sort(arrayName)
sort(arrayName,
index1, index2)
binarySearch(
arrayName, value)
Murach’s Beg. Java 2, JDK 5, C10
Description
Sorts the elements into ascending order.
Sorts the elements into ascending order
from the index1 element to, but not
including, the index2 element.
Returns an int value for the index of the
specified value in the specified array;
returns a negative number if the
specified value is not found. Array must
first be sorted by the sort method.
© 2005, Mike Murach & Associates, Inc.
Slide 18
The methods of the Arrays class
 All of these methods accept arrays of primitive data types and
arrays of objects for the arrayName argument.
 The methods all accept primitive types and objects for the value
argument.
 All of the index arguments must be int types.
 If an index argument is less than zero or greater than one less than
the length of the array, the method will throw an
ArrayIndexOutOfBoundsException.
 If you use the sort method on an array of objects created from a
user-defined class, the class must implement the Comparable
interface.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 19
Code that uses the fill method
int[] quantities = new int[5];
Arrays.fill(quantities, 1); // all elements are set to 1
Code that uses the fill method to fill 3 elements in
an array
int[] quantities = new int[5];
Arrays.fill(quantities, 1, 4, 100);
// elements 1, 2, and 3 are set to 100
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 20
Code that uses the equals method
String[] titles1 = {"War and Peace",
"Gone With the Wind"};
String[] titles2 = {"War and Peace",
"Gone With the Wind"};
if (titles1 == titles2)
System.out.println("titles1 == titles2 is true");
else
System.out.println("titles1 == titles2 is false");
if (Arrays.equals(titles1, titles2))
System.out.println("Arrays.equals(titles1, titles2)
is true");
else
System.out.println("Arrays.equals(titles1, titles2)
is false");
The console output
titles1 == titles2 is false
Arrays.equals(titles1, titles2) is true
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 21
Code that uses the sort method
int[] numbers = {2,6,4,1,8,5,9,3,7,0};
Arrays.sort(numbers);
for (int num : numbers)
{
System.out.print(num + " ");
}
The console output
0 1 2 3 4 5 6 7 8 9
Code that uses the sort and binarySearch
methods
String[] productCodes = {"mcbl", "jsps", "java"};
Arrays.sort(productCodes);
int index = Arrays.binarySearch(productCodes, "mcbl");
// sets index to 2
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 22
The Comparable interface defined in the Java API
public interface Comparable
{
int compareTo(Object obj);
}
How to implement the Comparable interface
 Define the compareTo method. Then, the sort method of the Arrays
class uses the compareTo method to sort an array of objects created
from that class.
 The compareTo method must return -1 if the current object is less
than the passed object, 0 if the objects are equal, and 1 if the
current object is greater than the passed object.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 23
An Item class that implements the Comparable
interface
public class Item implements Comparable
{
private int number;
private String description;
public Item(int number, String description)
{
this.number = number;
this.description = description;
}
public int getNumber()
{
return number;
}
public String getDescription()
{
return description;
}
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 24
An Item class that implements the Comparable
interface (continued)
public int compareTo(Object o)
{
Item i = (Item) o;
if (this.getNumber() < i.getNumber())
return -1;
if (this.getNumber() > i.getNumber())
return 1;
return 0;
}
}
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 25
Code that sorts an array of Item objects
Item[] items = new Item[3];
items[0] = new Item(102, "Duct Tape");
items[1] = new Item(103, "Bailing Wire");
items[2] = new Item(101, "Chewing Gum");
Arrays.sort(items);
for (Item i : items)
System.out.println(i.getNumber() + ": " +
i.getDescription());
The console output
101: Chewing Gum
102: Duct Tape
103: Bailing Wire
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 26
How to create a reference to an array
Example 1: Code that creates a reference to an array
double[] grades = {92.3, 88.0, 95.2, 90.5};
double[] percentages = grades;
percentages[1] = 70.2;
// changes grades[1] too
System.out.println("grades[1]=" + grades[1]);
// prints 70.2
Example 2: Code that reuses an array variable
double[] grades = new grades[5];
grades = new grades[20]
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 27
How to copy elements of one array to another
The syntax of the arraycopy method of the System class
System.arraycopy(fromArray, intFromIndex, toArray,
intToIndex, intLength);
Example 1: Code that copies the values of an array
double[] grades = {92.3, 88.0, 95.2, 90.5};
double[] percentages = new double[grades.length];
System.arraycopy(grades, 0, percentages, 0,
grades.length);
percentages[1] = 70.2;
// doesn't change grades[1]
System.out.println("grades[1]=" + grades[1]);
// prints 88.0
Example 2: Code that copies part of one array to another
double[] grades = {92.3, 88.0, 95.2, 90.5};
Arrays.sort(grades);
double[] lowestGrades = new double[2];
System.arraycopy(grades, 0, lowestGrades, 0, 2);
double[] highestGrades = new double[2];
System.arraycopy(grades, 2, highestGrades, 0, 2);
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 28
How to refer to and copy arrays
 To create a reference to an existing array, code an assignment
statement that assigns an array variable to the existing array. Then,
two variables will point to the same array in memory.
 To copy the values of one array into another, use the arraycopy
method of the System class.
 When you copy an array, the target array must be the same type as
the sending array and it must be large enough to receive all of the
elements that are copied to it.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 29
How to work with rectangular arrays
 Two-dimensional arrays use two indexes and allow data to be
stored in a table that consists of rows and columns.
 This can also be thought of as an array of arrays where each row is
a separate array of columns.
 A rectangular array is a two-dimensional array whose rows all
have the same number of columns.
How to create a rectangular array
The syntax for creating a rectangular array
type[][] arrayName = new type[rowCount][columnCount];
A statement that creates a 3x2 array
int[][] numbers = new int[3][2];
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 30
How to assign values to a rectangular array
The syntax for referring to an element of a rectangular
array
arrayName[rowIndex][columnIndex]
The indexes for a 3x2 array
[0][0]
[1][0]
[2][0]
[0][1]
[1][1]
[2][1]
Code that assigns values to the array
numbers[0][0]
numbers[0][1]
numbers[1][0]
numbers[1][1]
numbers[2][0]
numbers[2][1]
=
=
=
=
=
=
1;
2;
3;
4;
5;
6;
Code that creates a 3x2 array and initializes it in one
statement
int[][] numbers = { {1,2} {3,4} {5,6} };
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 31
Code that processes a rectangular array with
nested for loops
int[][] numbers = { {1,2}, {3,4}, {5,6} };
for (int i = 0; i < numbers.length; i++)
{
for (int j = 0; j < numbers[i].length; j++)
System.out.print(numbers[i][j] + " ");
System.out.print("\n");
}
The console output
1
3
5
2
4
6
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 32
The syntax for creating a jagged array
type[][] arrayName = new type[rowCount][];
How to work with jagged arrays
 A jagged array is a two-dimensional array whose rows have
different numbers of columns.
 When you create a jagged array, you specify the number of rows in
the array, but you leave the size of each column array unspecified
and set it later.
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 33
Code that creates a jagged array of integers
int[][] numbers = new int[3][];
numbers[0] = new int[10];
numbers[1] = new int[15];
numbers[2] = new int[20];
Code that creates and initializes a jagged array of
strings
String[][] titles =
{{"War and Peace", "Wuthering Heights", "1984"},
{"Casablanca", "Wizard of Oz", "Star Wars", "Birdy"},
{"Blue Suede Shoes", "Yellow Submarine"}};
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 34
Code that creates and initializes a jagged array of
integers
int number = 0;
int[][] pyramid = new int[4][];
for (int i = 0; i < pyramid.length; i++)
{
pyramid[i] = new int[i+1];
for (int j = 0; j < pyramid[i].length; j++)
pyramid[i][j] = number++;
}
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 35
Code that prints the contents of the jagged array
of integers
for (int i = 0; i < pyramid.length; i++)
{
for (int j = 0; j < pyramid[i].length; j++)
System.out.print(pyramid[i][j] + " ");
System.out.print("\n");
}
The console output
0
1 2
3 4 5
6 7 8 9
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 36
Code that uses foreach loops to print
a jagged array
for (int[] row : pyramid)
{
for (int col : row)
System.out.print(col + " ");
System.out.print("\n");
}
Murach’s Beg. Java 2, JDK 5, C10
© 2005, Mike Murach & Associates, Inc.
Slide 37