String

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String Exception
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Lesson plan

• Class Object
• String
• Exercise for midterm

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Class Object

• A class definition provides a description of a
  typical object within that class.
• An individual object is an instance of a class.
• A class has its behavior (methods) and attributes
  (fields).

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Class Object

• Access a dynamic method or field by
• ltobject namegt.ltmethod namegt
• ltobject namegt.ltfield namegt
• Example
• df.format(loanAmount)
• . Access a static method by
• ltclass namegt.ltmethod namegt
• Example
• JOptionPane. showInputDialog(null,"Loan Amount
  (dollars.cents)")

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String

• What is it?
• A String is a sequence of characters that is
  treated as a single value
• String class handles all operations related to
  String
• String class is defined in java.lang.

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Create a String object

• Syntax
• String ltvariable namegt
• ltvariable namegtnew String(constant)
• OR
• String ltvariable namegt
• ltvariable namegt constant

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Create a String object

• Example
• String strVar
• strVar new String(CS 172 Course)
• OR
• String strVar
• strVar CS 172 Course

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Internal Representation of a String
Individual characters in a string object are
indexed from 0


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Compute Length of a string

• Method length()
• Returns the length of a string
• Example
• String strVar
• strVar new String(CS 172 Course)
• int len strVar.length()

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Substring

• Method
• Extract a substring from a given string by
  specifying the beginning and ending positions
• Example
• String strVar, strSubStr
• strVar new String(CS 172 Course)
• strSubStr strVar.substring(0,6)

strSubStr CS 172
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Index position of a substring within another
string

• Method
• Find an index position of a substring within
  another string.
• Example
• String strVar1 CS 172 Course
• String strVar2 Course
• int index
• index strVar1.indexOf(strVar2)

index 7
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Index position of a substring within another
string

• Example
• String strVar1 CS 172 Course
• String strVar2 C
• int index
• index strVar1.indexOf(strVar2)

index 0
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String concatenation

• Method
• Create a new string from two strings by
  concatenating the two strings.
• Example
• String strVar1 CS 172
• String strVar2 Course
• String sumStr
• sumStr strVar1strVar2

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Practice exercise

• Determine values of some variables given codes

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String comparison

• Two methods
• equals
• equalsIgnoreCase
• string1.equals(string2)
• Or
• string1.equalsIgnoreCase(string2)

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String comparison

• Methods
• equals
• equalsIgnoreCase
• compareTo
• String string1 CS 172
• String string2 172
• Boolean isEqual
• isEqual string1.equals(string2)

isEqual false
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String comparison

• equalsIgnoreCase
• Example
• String string1 COMPSCI
• String string2 compsci
• Boolean isEqual
• isEqual string1.equals(string2)

isEqual true
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String comparison

• compareTo
• Example
• String string1 Adam
• String string2 Brian
• int compareResult
• compareResult string1.compareTo(string2)

compareResult lt 0
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String comparison

• - string1.compareTo(string2)
• Compares two strings lexicographically
• will return 0 if two strings are equal
• will return negative value if string1 is less
  than string 2
• will return positive value if string1 is
  greater than string 2
• The comparison is based on the Unicode value of
  each character in the strings

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String comparison

• The comparison is based on the Unicode value of
  each character in the strings
• let k be the smallest index valid for both
  strings
• compareTo returns the difference of the two
  character values at position k in the two string
  -- that is, the value
• character at the position k of string 1
  character at the position k of string 2

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Character at position k of a string

• Example
• String sampleCS 172 Course
• char aChar
• aChar sample.charAt(3)

aChar1
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Character at position k of a string

• Example
• String sampleCS 172 Course
• char aChar
• aChar sample.charAt(3)

aChar1
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Implicit type of conversion

• Implicit type conversion is common
• Example
• int num 172
• String s CS "num

sCS 172
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Practice (answers)Monday (3)
int sum1 0 for (int i0 ilt 5 i) for (int
j0 jlt 5 j) sum1i System.out.println
(" Sum1 is " sum1)

i1 j0 -gt sum1 0i011 j1 -gt sum1
01 i 012 j5 -gt sum1 016 6
i0 j0 -gt sum1 0i 00 0 j1
-gt sum1 0i 00 0 .. j5 -gt sum1
0i 00 0
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Practice (answers)Monday (3)

i2 j0 -gt sum1 6i 621 j1
-gt sum1 622 622 .. j5 -gt sum1
6i 626 18
i3 j0 -gt sum1 183 1831 j1
-gt sum1 1833 1832 .. j5 -gt sum1
1836 36
i4 j0 -gt sum1 3641 j1 -gt
sum1 3642 .. j5 -gt sum1 3646
60
i5 j0 -gt sum1 6051 j1 -gt
sum1 6052 .. j5 -gt sum1 6056
90
SUM1 90
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Practice (answers)Wednesday (3)

int sum 0 int j0 while (jlt11)
j int i5 while (igtj) sum sum
(ij) i-- System.out.println( Sum
sum)
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Practice (answers)Wednesday (3)

j0 j 1 i5 sum sum (15) 6 i4 sum
sum (14) 65 11 i3 sum sum (13)
114 15 i2 sum sum(12) 153 18
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Practice (answers)Wednesday (3)

j 2 i5 sum sum (25) 187
25 i4 sum sum (24) 25 6 31 i3 sum
sum (23) 315 36
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Practice (answers)Wednesday (3)

j 3 i5 sum sum (35) 368
44 i4 sum sum (34) 44 7 51
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Practice (answers)Wednesday (3)

j 4 i5 sum sum (45) 519 60
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Practice

Write a loop that prints out a string in
reverse. input CS 172 output 271 SC
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Practice

public class StringReverse public static void
main(String args) String strSource "CS
172" String strDes"" int len
strSource.length()-1 for (int ilen igt0
i--) strDes strDes strSource.charAt(i)
System.out.println(" Reserve string is "
strDes)
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Practice

Multiple choices Determine the output of the
codes
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StringBuffer class

• A String object is immutable. Once it is created,
  we cant add/delete/modify characters of a String
  object
• If we need to modify the content of a string
  directly, we must use StringBuffer class

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Create a StringBuffer object

• StringBuffer word new StringBuffer(Java)
• word.setCharAt(0,D)
• word.setCharAt(1,i)

word DIVA
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Create a StringBuffer object

• StringBuffer word new StringBuffer(Java)
• word.setCharAt(0,D)
• word.setCharAt(1,i)

word DIVA
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Delete a substring from a StringBuffer object

• StringBuffer word new StringBuffer(CS172
  Course)
• word.delete(0,1)

word 172 Course
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Append a string

• StringBuffer word new StringBuffer(CS172)
• word.append(Course)

word 172 Course
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Insert a string

• StringBuffer word new StringBuffer(CS
  Course)
• word.insert(3,172)

word CS 172 Course
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Practice

• Write a method to sort three strings
  alphabetically

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Practice

• Identify objects, class
• What will be the output from the following code?