Characters and Strings

1
Lecture 3.2

• Characters and Strings

2
Objectives

• Declare and manipulate data of the char data
  type.
• Write string processing programs using String and
  StringBuffer objects.
• Differentiate the String and StringBuffer classes
  and use the correct class in solving a given
  task.
• Distinguish the primitive and reference data
  types and show how the memory allocation between
  the two is different.
• Tell the difference between equality and
  equivalence testings for String objects.
• Show, by using the state-of-memory diagrams, how
  objects are passed to methods and returned from
  methods.

3
Characters

• In Java single characters are represented using
  the data type char. Character constants are
  written as symbols enclosed in single quotes, for
  example, 'a', 'X', and '5'.
• To represent characters in computer, U. S.
  computer manufacturers devised several coding
  schemes.
• One coding scheme widely used today is ASCII
  (American Standard Code for Information
  Interchange).
• To accommodate the character symbols of
  non-English languages, the Unicode Consortium
  established the Unicode Worldwide Character
  Standard, commonly known as Unicode.

4
ASCII Table
5
Character Processing
6
Question-1

• Determine the output of the following statements
• System.out.println ( (char) 65 )
• System.out.println ( (int) C )
• System,out.println ( Y )
• If ( A lt ? )
• System.out.println ( A )
• else
• System.out.println ( ? )

7
Strings

• A string is a sequence of characters that is
  treated as a single value.
• The String data type is used to represent strings
  in Java.
• We have been using String objects all along. For
  example, to display a text with messageBox, we
  write

8
String is an Object

• String is a class in the java.lang package.
• Because String is a class, we need to create an
  instance of String in Java for string processing.
  Like any other objects, we need a declaration and
  object creation for the instances of the String
  class. For example,

But we normally use a shorthand, instead,
treating String objects much like primitive data.
For example,
9
Accessing Individual Elements

• Individual characters in a String accessed with
  the charAt method.

10
Determining the Size

• We determine the number of characters in a String
  with the length method.

Error because no object is created for str3, so
it is a null.
11
Example Counting Vowels
Heres the code to count the number of vowels in
the input string.
Alternatively could have used the toUpperCase
method of class String
12
Example Counting Words
Problem Inner loops could cause index to become
equal to numberOfCharacters, which is an error.
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Example Counting Words - 2
Problem wordCount will be one more than the
actual count if the sentence ends with one or
more spaces.
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Example Counting Java
Continue reading words and count how many times
the word Java occurs in the input, ignoring the
case.
15
Other Useful String Operators

• See the String class documentation for details.

16
Questions-2

• Assume
• String str1 programming
• String str2 language
• Determine the value of each of the following is
  they are valid
• str1.compareTo (str2)
• str2.compareTo (str2)
• str2.substring (1,1)
• str2.substring (0,7)
• str2.charAt (11)
• str1.length ( ) str2.length

17
Answers-2

• Positive number
• 0
• // Empty string
• languag characters from location 0 to 7-16
• Invalid out of pounds error
• 11 8 19

18
Primitive versus Reference Types

• Data types are classified into two groups
  primitive and reference.
• Non-numerical data types char and boolean and all
  of the numerical data types are primitive.
• All of the objects you have learned so far are
  reference data types.

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Effect of Assignment on Primitives
Code
State of Memory
20
Memory Allocation for Reference Data Type
Code
State of Memory
21
Effect of Assignment on References - 1
Code
Both word1 and word2 are allocated memory (to
store references), but the objects themselves are
not yet created, so they both contain null.
State of Memory
Null References
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Effect of Assignment on References - 2
Code
One String object is created and assigned to
word1, so word1 contains the address of this
object.
State of Memory
23
Effect of Assignment on References - 3
Code
Content of word1, which is an address, is
assigned to word2, making word2 refer to the same
object.
State of Memory
24
Equality () vs. equalsCase 1
word1 and word2 point to the same object.
word1 word2
word1.equals( word2 )
25
Equality () vs. equalsCase 2
word1
word2
word1 and word2 point to different objects having
the same string.
word1 word2
word1.equals( word2 )
26
Equality () vs. equalsCase 3
word1
word2
word1 and word2 point to different objects with
different strings.
word1 word2
word1.equals( word2 )
27
StringBuffer

• A String object is immutable, which means that
  once a String object is created, we cannot change
  it.
• We can read individual characters in a string,
  but we cannot add, delete, or modify characters
  of a String object.
• Remember that the methods of the String class,
  such as toUpperCase and substring, do not modify
  the original string they return a new string.
• Java adopts this immutability restriction to
  implement an efficient memory allocation scheme
  for managing String objects.
• Creating a new string from the old one will work
  for most cases, but sometimes manipulating the
  content of a string directly is more convenient.
• Manipulation here means operations such as
  replacing a character, appending a string with
  another string, deleting a portion of a string,
  and so forth.

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Sample StringBuffer Processing - 1

• Replace all vowels in the sentence with X.

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Sample StringBuffer Processing - 2

• Creates a sentence with words having even number
  of letters. Stop when the input word is STOP.

30
Question-3

• Determine the value of str after the following
  statements have are executed
• String str Caffeine
• StringBuffer str1 new StringBuffer
  (str.substring (4,8))
• str1.insert (3, f)
• str De str1
• Find the errors in the following code
• String str Caffeine
• StringBuffer str1 str.substring (1,3)
• str1.append (e)
• System.out.print (str1)
• str1 str1 str

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Answer-3

• Deeinfe
• Cannot assign a String value to a StringBuffer
  variable.

32
Passing Objects as Parameters

• Example What is the output of this code ?
• class Tester
• public void myMethod (StringBuffer strBuf)
• strBuf.setCharAt (0, Y)
• Tester tester
• StringBuffer word new StringBuffer (Java)
• tester new Tester ( )
• tester.myMethod (word)
• outoutBox.printLine ( Word is word )

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Passing Objects to Methods - 1
Code
public void myMethod( StringBuffer strBuf
) strBuf.setCharAt( 0, Y )
//StringBuffer word is //created
here tester.myMethod( word )
A. Local variables do not exist before the
method execution
State of Memory
34
Passing Objects to Methods - 2
Code
public void myMethod( StringBuffer strBuf
) strBuf.setCharAt( 0, Y )
//StringBuffer word is //created
here tester.myMethod( word )
B. The value of the argument, which is an
address, is copied to the parameter.
State of Memory
35
Passing Objects to Methods - 3
Code
public void myMethod( StringBuffer strBuf
) strBuf.setCharAt( 0, Y )
//StringBuffer word is //created
here tester.myMethod( word )
C. The content of the object referenced by
strBuf is changed.
State of Memory
36
Passing Objects to Methods - 4
Code
public void myMethod( StringBuffer strBuf
) strBuf.setCharAt( 0, Y )
//StringBuffer word is //created
here tester.myMethod( word )
D. The parameter is erased. The argument still
points to the same (now modified) object.
State of Memory
37
Question-4

• Determine the output of the following code
• String str Internet
• Tester tester new Tester ( )
• System.out.println (str is tester.toUpper
  (str))

38
Returning an Object from Methods

• Passing an object as an argument to a method
  means passing the address of the object to the
  method.
• The previous four slides illustrate the effect of
  passing an object to a method.
• The same rule applies when we return an object
  from a method. It means the address of the
  object is passed back from the method.