Arrays Continued

1
Arrays Continued

• Objectives
• Learn how to manipulate strings
• Use String methods appropriately
• Understand objects in arrays
• Use polymorphism in arrays

2
The Importance of String

• Strings are everywhere in programming
• In user interface
• A significant part of user input is through
  strings
• Output also often involves strings
• Program interfaces are also becoming more
  text-based
• XML

3
In The Textbook

• Strings
• p6365 first introduction (Lesson 3.2)
• p315319 more String operations
• Arrays of objects
• p334336 Lesson 10.5

4
Strings in Java

• Each string is an object of String
• The String class has variables and methods
• Each string composes an ordered list of
  characters (note character is denoted by )
• Each String object has methods that manipulate
  the ordered list of characters

5
Escape Characters

• Some special characters cannot appear in a string
  for various reasons
• As Strings are quoted by , to include , one
  needs to use \
• New line character and tab are represented by \n
  and \t
• \ itself becomes \\

6
We Already Know Some Things About String

• Declaration and instantiation
• Initialization
• Concatenation
• The operator
• Precedence of concatenation
• concatenation has the same precedence as addition
• Examples n34, n34, 34n

7
Methods of String (int and char Returns)

• The length() method
• Returns the number of characters, including
  spaces (Note in array length is an attribute)
• char charAt(int anIndex) returns the character
  at anIndex (starting at 0)
• e.g. String sgreat s.charAt(2) returns e
• int indexOf(char a) returns the index of first
  occurrence of a, -1 if a is not part of string
• e.g. s.indexOf(a) returns 3
• int indexOf(char a, int index) and int
  indexOf(String sub)
• s.indexOf(e,3) returns 1

8
String Comparison Methods (int and boolean
Returns)

• int compareTo(String a)
• lexicographically compare with a
• returns 0 if equal, a negative number if I am
  less than a and positive number if a is less than
  myself
• boolean equals(String a)
• Returns true if a is equal to me in content,
  false otherwise
• boolean equalsIgnorCase(String a)

9
More String Methods (String Returns)

• String substring(int beginIndex) returns
  substring from beginIndex till the end of the
  string
• e.g. s.substring(2) returns String eat
• String substring(int start, int end) return
  substring between indices start and end

10
More String Methods (String returns)

• String toLowerCase()
• String toUpperCase()
• String trim() returns string without leading and
  trailing spaces
• Note if the return is of String type, the
  original string is not changed

11
Exercises

• 1. What does the following code do?

String original "Hi there!" String word
""   // Visit all the characters in the
string for (int i 0 i lt original.length()
i)   // Or stop when a space is found if
(original.charAt(i) ' ') break   //
Add the non-space character to the word word
original.charAt(i)
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Exercises

• 2. What does the following code do?

String strThe rain in Spain falls mainly on
the plain int begin0 while (beginltstr.length()
) int endstr.indexOf( ,begin) if
(end-1) endstr.length() String
wordstr.substring(begin,end) System.out.println
(word) beginend1
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Want to Know More String Methods?

• Refer to the documentation!

14
Working with Arrays of Objects

• Polymorphism, Casting, and instanceof
• It is quite common to declare and instantiate an
  array of some interface type.
• For instance, below is code that reserves 10
  cells for shapes
• Shape shapes new Shape10

15
Storing Different Objects in One Array

• We can now store in this array instances of any
  class that implements Shape, such as Rect,
  Circle, or Wheel, as follows
• As long as we send Shape messages to the elements
  of this array, we can ignore the fact that they
  are of different concrete classes (thats what
  polymorphism is all about).

shapes0 new Rect(20, 20, 40, 40) // Cell 0
refers to a Rect shapes1 new Circle(100, 100,
20) // Cell 1 refers to a Circle shapes2
new Wheel(200, 200, 20, 6) // Cell 2 refers to
a Wheel
16
Sending One Common Message

• Let us now draw all the shapes in the array

Pen pen new StandardPen() for (int i 0 i lt
3 i) shapesi.draw(pen)
17
Sending a Message Specific to One Class

• Let us assume that we know the position of the
  wheel object in the array (in our example, it's
  at position 2).
• Then, to set its spokes to 5, we perform the
  following steps
• Access the array element with the subscript.
• Cast the element, which is masquerading as a
  Shape, to a Wheel.
• Send the setSpokes(5) message to the result.
• Here is the code
• ((Wheel) shapes2).setSpokes(5)

18
Casting

• Note the use of parentheses to override the
  precedence of the method selector, which would
  otherwise be run before the cast operation.
• Failure to cast in this code causes a
  compile-time error.
• Suppose we don't know the position of a wheel in
  the array of shapes, but we wish to set the
  spokes of each wheel to 5.
• We must first determine that a shape is a wheel
  before casting, and Java's instanceof operator
  comes to our rescue.

19
Using instanceof

• Here is a loop that solves the problem
• We only send setSpokes() to a Wheel

for (int i 0 i lt shapes.length i) if
(shapesi instanceof Wheel) ((Wheel)
shapesi).setSpokes(5)
20
Summary of Using Objects of Different Types in
Array

• When the element type of an array is a reference
  type or interface, objects of those types or any
  subtype (subclass or implementing class) can be
  directly inserted into the array.
• After accessing an object in an array, care must
  be taken to send it the appropriate messages or
  to cast it down to a type that can receive the
  appropriate messages
• The beauty of polymorphism lies in the ability of
  the client to send one message to the array and
  the actual binding of methods to object happens
  at run-time

21
Exercises

• Exercise 10.5 on Page 336