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Chapter 6: Arrays and Strings
 Introducing Arrays
 Declaring Arrays
 Creating Arrays
 Initializing Arrays
 Array
of Objects
 Copying Arrays
 Multidimensional Arrays
 String , StringBuffer, and
StringTokenizer
 Command-Line Parameters
Introducing Arrays
• Array is a data structure that
represents a collection of the
same types of data.
• Java treats these arrays as
objects.
• An Array of 10 Elements
of type double
double[] myList = new double[10]
myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
Declaring Arrays
 datatype[]
arrayname;
Example:
int[] myList;
 datatype
arrayname[];
Example:
int myList[];
Creating Arrays
arrayName = new datatype[arraySize];
Example:
myList = new double[10];
Declaring and Creating
in One Step
 datatype[]
arrayname = new
datatype[arraySize];
double[] myList = new double[10];
 datatype
arrayname[] = new
datatype[arraySize];
double myList[] = new double[10];
Initializing Arrays
 Using
a loop:
for (int i = 0; i < myList.length; i++)
myList[i] = (double)i;
 Declaring,
creating, initializing in one
step:
double[] myList = {1.9, 2.9, 3.4, 3.5};
Example 6.2
Using Arrays in Sorting
 Objective:
Use the selectionSort() method
to write a program that will sort a list of
double floating-point numbers.
TestSelectionSort
Run
Example 6.3
Testing Linear Search
 Objective:
Implement and test the linear
search method by creating an array of 10
elements of int type randomly and then
displaying this array. Prompt the user to
enter a key for testing the linear search.
TestLinearSearch
Run
Array of Objects
 Declaring
and creating:
Circle[] circleArray = new Circle[10];
 Initializing:
for (int i=0; i<circleArray.length; i++)
{
circleArray[i] = new Circle();
}
Copying Arrays
Using a loop:
int[] sourceArray = {2, 3, 1, 5, 10};
int[] targetArray = new int[sourceArray.length];
for (int i = 0; i < sourceArrays.length; i++)
targetArray[i] = sourceArray[i];
The arraycopy Utility
arraycopy(sourceArray, src_pos, targetArray,
tar_pos, length);
Example:
System.arraycopy(sourceArray, 0, targetArray, 0,
sourceArray.length);
Multidimensional Arrays
int[][] matrix = new int[10][10];
or
int matrix[][] = new int[10][10];
for (int i=0; i<matrix.length; i++)
for (int j=0; j<matrix[i].length; j++)
{
matrix[i][j] = (int)(Math.random()*1000);
}
Example 6.7
Testing Multidimensional Arrays
 Objective:
Use two-dimensional arrays to
create two matrices, and then add the two
matrices.
TestMatrixAddition
Run
The String Class
 Declaring
a String:
– String message = "Welcome to Java!"
– String s = new String();
 String
Comparisons
 String Concatenation
 Substrings
 String Length
 Retrieving Individual Characters
in a String
String Comparisons
 equals
String s1 = "Welcome";
String s2 = "welcome";
if (s1.equals(s2))
{ ... }
Substrings
String is an immutable class; its values
cannot be changed individually.
String s1 = "Welcome to Java";
String s2 = s1.substring(0,10) + "HTML";
Finding String Length
Finding string length using the length()
method:
message = "Welcome";
message.length() (returns 7)
Retrieving Individual
Characters in a String
 Do
not use message[0]
 Use
message.charAt(index)
 Index
starts from 0
The StringBuffer Class
The StringBuffer class is an alternative to the
String class. In general, a string buffer can be
used wherever a string is used.
StringBuffer is more flexible than String. You
can add, insert, or append new contents
into a string buffer. However, the value of
a string is fixed once the string is created.
StringBuffer Constructors

public StringBuffer()
No characters, initial capacity 16 characters.

public StringBuffer(int length)
No characters, initial capacity specified by the
length argument.

public StringBuffer(String str)
Represents the same sequence of characters
as the string argument. Initial capacity 16
plus the length of the string argument.
Appending New Contents
into a String Buffer
StringBuffer strBuf = new StringBuffer();
strBuf.append("Welcome");
strBuf.append(' ');
strBuf.append("to");
strBuf.append(' ');
strBuf.append("Java");
The StringTokenizer Class
Constructors

StringTokenizer(String s, String delim,
boolean returnTokens)

StringTokenizer(String s, String delim)

StringTokenizer(String s)
The StringTokenizer Class
Methods

boolean hasMoreTokens()

String nextToken()

String nextToken(String delim)
Example 6.9
Testing StringTokenizer
 Objective:
Using a string tokenizer, retrieve
words from a string and display them on the
console.
TestStringTokenizer
Run
Command-Line Parameters
class TestMain
{
public static void main(String[] args)
{ ... }
}
java TestMain arg0, arg1, arg2, ..., argn
Processing
Command-Line Parameters
In the main method, get the arguments from
args[0], args[1], ..., args[n], which corresponds
to arg0, arg1, ..., argn in the command line.
Example 6.10
Using Command-Line
Parameters
 Objective:
Write a program that will
perform binary operations on integers.
The program receives three parameters:
an operatorTestCommandParameters
and two integers.
Run
Java TestCommandParameters + 2 3
Run
Java TestCommandParameters - 2 3
Run
Java TestCommandParameters / 2 3