CSC 313 Java Wrapper Classes, Arrays, Strings, StringBuffer

1
CSC 313 JavaWrapper Classes, Arrays, Strings,
StringBuffer
2
Wrapper classes

• Java provides wrapper classes such as Integer
  and Boolean for primitive types such as int and
  boolean, respectively.
• Wrapper classes have various uses, including data
  conversion.

3
Wrapper Classes for Primitive Types
4
Wrapper Class Methods

• All of these wrapper classes directly or
  indirectly inherit from Object.
• Therefore standard methods are available such
  as
• toString
• Clone
• equals

5
Constants Utility Methods

• The Java platform includes constants and utility
  methods related to the primitive types in wrapper
  classes. For example
• Integer.MAX_VALUE
• Integer.MIN_VALUE
• Character.isDigit
• Character.isSpace

6
Wrapper classes

• The code segment
• String num 123
• int n Integer.parseInt( num )
• illustrates data conversion with the Integer
  wrapper class.
• Wrapper classes also can be used to store
  primitive types in object-only constructs such
  as Vectors and Hashtables.

7
Arrays in Java

• Primitive types such as int and class types such
  as String can be aggregated in arrays.
• Arrays are fixed size. Once storage for an array
  has been allocated, the array cannot grow or
  shrink in size.

8
Arrays in Java

• The code segment int nums new int 100
• and.. int nums nums new int 100
• Both examples declare an array of int named nums
  and allocates storage for 100 ints.
• The arrays cells are initialized to 0, the
  default value for an int

9
Arrays in Java
10
Initializing Arrays

• If you create arrays with objects that already
  exist, you do not have to use the new keyword.
• You can initialize an array with your own values
  when you declare it, thereby defining how many
  elements it will have. // An anonymous
  array of 7 elementsint primes  2, 3, 5, 7, 11
  , 13, 17   
•  An anonymous array is an array object that has
  no name.

11
Initializing Arrays

• If you specify initializing values for an array,
  you must include values for all the elements. If
  you only want to set some of the array elements
  to values explicitly, you should use an
  assignment statement for each element. For
  example
• int primes  new int100
• primes0  2
• primes1  3
• You can also initialize an array with an existing
  array. For example, you could declare the
  following array variables,
• long even  2L, 4L, 6L, 8L, 10L
• long value  even

12
Copying Arrays

• You've created two array variables, but you only
  have one array.
• Both arrays refer to the same set of elements
  and you can access the elements of the array
  through either variable name-for example, even2
  refers to the same variable as value2.

13
Copying Arrays

• Because arrays are reference types, you cannot
  copy the contents of one array into another with
  the assignment operator.
• Use the utility method arraycopy, provided by the
  System class in the package java.lang. Syntax
• void arraycopy (Object source, int SourcePos,
  Object destination, int
  destinationPos, int number)
• System.arraycopy (even2,0,value2,0, 5)
• The arraycopy method can be used to copy a whole
  array or just a part of an array.
• The method fails if the source and destination
  arrays, do not already exist or do not have
  enough space to perform the copy.

See testarrays.java
14
Array Indexes in Java

• Array elements are accessed through an index
  expression. The code segment
• int nums new int 100
• int i 0
• while ( i lt nums.length )
• nums i i 1
• Illustrates loading array nums with numbers 1 to
  100. Note first array position is subscript 0
  as in C.

15
Length Data Member in Arrays

• Every array has a length data member that
  specifies the number of elements in the array.
  As illustrated by the following code segment
• int nums new int 100
• int i 0
• while ( i lt nums.length )
• nums i i 1 i i 1

16
Reusing Array Variables

• An array variable is actually an object which
  can reference different arrays at different
  points in your program. Suppose you've declared
  and defined the variable primes as before
• int primes
• primes  new int10      // Allocate an array of
   10 integer elements
• This produces an array of ten elements of
  type int. A bit later in your program you may
  want the array variable primes to refer to a
  larger array, with 50 elements say. You would
  simply write
• primes  new int50      // Allocate an array of
   50 integer elements 
• When this statement is executed, the previous
  array of 10 elements is discarded, along with the
  data values you may have stored in it, as
  illustrated here.

17
Reusing Array Variables

18
Array Utilities in Java

• Java also provides utility classes for filling,
  searching, sorting, and performing other standard
  operations on arrays. If nums refers to an array
  of integers, the statement
• Arrays.sort( nums )
• sorts the array in ascending order.

19
Arrays of Arrays

• float temperature  new float10365

20
Strings

• Java provides two standard classes, String and
  StringBuffer, for string processing.
• Strings are immutable but
• StringBuffers are not.
• A double quoted string such as hi is a
  reference to a String object.

21
String Objects

• A String variable is simply an object of the
  class String. You declare a String variable and
  initialize it with a declaration such as
• String myString  "My inaugural string"
• This declares the variable myString as type
  String, and initializes it with the value "My
  inaugural string". You can store another string
  in a String variable, once you've declared it, by
  using an assignment. For example
• myString  "Strings can be knotty"

22
String Objects are Immutable

23
Operations on Strings

• Joining Strings use the operatormyString  "Th
  e quick brown fox"  " jumps over the lazy dog"
• Use System's arraycopy method to efficiently
  copy data from one array into another.

24
The toString () method

• Every class has a toString()instance method that
  returns an appropriate string representation. For
  example, the statement
• System.out.println( new Date() )
• prints a Date as a human-readable string.
• toString() is an example of a polymorphic method.

25
Standard utility classes

• The java.util package has an assortment of
  utility classes such as Date for representing
  dates, Random for generating random numbers of
  various types, Vector for dynamically sized
  aggregates of class types, StringTokenizer for
  tokenizing strings, and so on.

26
End of Array Lecture!