Strings

1
Strings

• A string is a sequence of characters.
• Java provides two classes to support strings
• String class the instances of String class are
  constants, ie. the content of a String object
  cannot be changed after its creation AKA
  immutable
• StringBuffer class the instances of
  StringBuffer class are mutable, ie. the
  content of a StringBuffer object can be changed
  after its creation.
• The String class has some unique privileges not
  shared by ordinary classes.
• A string can be created using string literals.
  (i.e. without using new operator)
• Operator can be applicable to strings.
• The characters in a string indexed by 0 to n-1,
  where n is the length of the string.

2
String Class

• Creation of a String object.
• String s new String(abc)
• String s abc
• Add operator
• String s1 abc
• String s2 de
• String s3 s1 s2
• String class has a lot of methods. These methods
  are useful to manipulate strings.

3
String Literals

• String literals (e.g. This is a string) or
  string-valued constant expressions (e.g. This is
  a constant expression) are created at
  runtime and stored in a pool.
• Redundant literals re-use existing String
  objects in the pool.
• Example
• String s1 hello
• String s2 hello
• System.out.println(s1 s2) // prints true
• However, strings created with the new keyword are
  not kept in the pool.
• Example
• String s1 hello
• String s2 new String(hello)
• System.out.println(s1 s2) // prints false

4
length and charAt

• int length() this instance method returns the
  length of the string.
• String sabc
• s.length() ? returns 3
• char charAt(int index) this instance method
  returns the character at the given index.
• String sabcde
• s.length() ? 5
• s.charAt(0) ? a
• s.charAt(1) ? b
• s.charAt(4) ? e
• s.charAt(5) ? error

5
indexOf

• int indexOf(char c)
• int indexOf(char c, int index)
• Returns the index of the first occurrence of the
  character c in the current object, no less than
  index (default 0). Returns 1 if there is no such
  occurrence.
• String sababc
• s.indexOf(b) ? 1
• s.indexOf(b,2) ? 3
• int indexOf(String s2)
• int indexOf(String s2, int index)
• Returns the index of the first occurrence of the
  string s2 in the current object, no less than
  index (default 0). Returns 1 if there is no such
  occurrence.
• String sdaabcabc
• String s2ab
• s.indexOf(s2) ? 2
• s.indexOf(s2,3) ? 5

6
substring

• String substring(int startindex)
• String substring(int startindex, int lastindex)
• Returns the substring of the current object
  starting from startindex and ending with
  lastindex-1 (or the last index of the string if
  lastindex is not given)
• String sabcdef
• s.substring(1) ? bcdef
• s.substring(3) ? def
• s.substring(1,4) ? bcd 
• s.substring(3,5) ? de 

7
equals and equalsIgnoreCase

• boolean equals(String s2)
• boolean equalsIgnoreCase(String s2)
• Returns true if the current object is equal to
  s2 otherwise false. equalsIgnorecase disregards
  the case of the characters.
• String sabc
• s.equals(abc) ? true
• s.equals(abcd) ? false
• s.equals(aBc) ? false
• s.equalsIgnoreCase(aBc) ? true

8
StringBuffer Class

• StringBuffer constructors
• StringBuffer()
• StringBuffer(int size)
• Returns an instance of the StringBuffer class
  that is empty but has an initial capacity of
  size characters (default 16 characters).
• StringBuffer(String arg)
• Creates an instance of the StringBuffer class
  from the string arg.
• length and charAt methods are also defined for
  StringBuffer class.

9
append and insert

• StringBuffer append(String s)
• Returns the current object with the String
  parameter s appended to the end.
• StringBuffer insert(int index, char c)
• StringBuffer insert(int index, String s)
• Inserts the character c (or String s) into the
  current StringBuffer object at index index. The
  characters (after index) are shifted to right.
• StringBuffer sb new StringBuffer(abcd)
• sb.insert(0,AB) ? sb is ABabcd
• sb.insert(1,CD) ? sb is ACDBabcd
• sb.insert(8,EFG) ? sb is ACDBabcdEFG
• sb.append(HI) ? sb is ACDBabcdEFGHI

10
Palindrome Example

• import java.io.
• public class Palindrome
• static boolean isPalindrome(String s)
• int i 0
• int j s.length() - 1
• boolean flag true
• while ((iltj) flag)
• if (s.charAt(i) ! s.charAt(j))
• flag false
• else i j--
• return flag

11
Converting to a String

• Objects can return a string representation of
  themselves by calling the toString method.
• For primitives, we have two ways of converting to
  a String
• String.valueOf() (static method of the String
  class)
• Example String.valueOf(5000) produces a String
  object equivalent to 5000
• Or, we can take advantage of the behavior of the
  operator. Remember than the operator means
  concatenation when one of the two arguments is a
  String
• Example Hello World ? Hello World
• Example 5 hello ? 5hello
• Example 5 ? 5 // can be used instead of
  String.valueOf

12
Arrays

• An array is an object just like any other object
  in Java!
• It just has some special syntax (bracket
  notation).

13
Arrays

• An array is a group of contiguous locations that
  all have the same name and the same type.
• x
• x0 x1 x2 x3 x4
  x5
• Accessing array elements
• arraynameindex where index is an integer
  expression
• x02
• i2
• xi1xi

5 -2 8 3 -2 7
14
Declaring and Allocating Arrays

• type arrayname new typearraysize
• int x
• x new int8
• int y new int10
• type arrayname values
• int y 1,3,5,8 allocates an integer array
  with size 4 (index range0-3)
• with initial values.
• Other Notation type arrayname
• int x We prefer the first notation because
  it is consistent with
• other declarations.
• Array Length arrayname.length
• x.length ? 8

15
Arrays of Primitives (1)

• Assume the following declaration
• char initials
• After executing declaration we have

initials
(null)
16
Arrays of Primitives (2)

• Now we execute the line below
• initials new char4
• The situation now looks like

initials
initials char
Note Each slot in the array is initialized with
the default value of the type of the slots.
0
\u0000
1
\u0000
2
\u0000
3
\u0000
17
Arrays of Primitives (3)

• Assume the following line is executed
• initials1 B
• Now the array looks like

initials
initials char
0
\u0000
1
B
2
\u0000
3
\u0000
18
Arrays of Objects (1)

• Assume the following declaration
• Animal animals
• After executing declaration we have

animals
(null)
19
Arrays of Objects (2)

• Assume we have the following declaration and
  assignment
• Animal animals new Animal4
• The situation then looks like this

animals
There are no objects bound to the array slots yet!
animals Animal
0
null
1
null
2
null
3
null
20
Arrays of Objects (3)

• Now assume we execute the following line
• animals2 new Animal()
• The array looks like

animals
animals Animal
0
null
1
null
2
Animal
3
null
21
Arrays of Objects - Pitfalls

• Assume the following code
• Animal hisPets new Animal2
• Animal herPets new Animal3
• Animal fido new Animal(fido)
• hisPets1 fido
• herPets0 fido
• The situation looks like

herPets
hisPets
herPets Animal
hisPets Animal
0
0
null
1
null
Fido Animal
1
2
null
22
Array Element Initialization

• Many times youll want fill an array with objects
  (instead of leaving the slots null). To
  accomplish this, you can use a for loop. On each
  iteration, instantiate an object and then bind
  that object to the current array slot.
• for (int i 0 i lt animals.length i)
• animalsi new Animal()

23
Array Element Usage

• Array elements are variables that are bound to
  objects. Because of this, we can use array
  elements anywhere an individual variable can be
  used.
• Example
• animal2.speak()

24
Array Element Usage

• Assume the following class definition
• public class CarWash
• public void wash(Car c)
• And the following usage
• CarWash cw new CarWash()
• Car myCars new Car3
• myCars0 new Car(Porshe)
• myCars1 new Car(BMW)
• myCars2 new Car(Benz)
• cw.wash(myCars1) // wash the BMW

25
Aggregate Array Parameters (1)

• Sometimes, we need to operate on entire arrays.
  Methods can be defined to take entire arrays as
  arguments. Assume the following class
  definition
• public class CarWash
• public void washAll(Car c)
• And the following usage
• CarWash cw new CarWash()
• Car myCars new Car3
• myCars0 new Car(Porshe)
• myCars1 new Car(BMW)
• myCars2 new Car(Benz)
• cw.washAll(myCars) // wash all my cars

26
Aggregate Array Parameters (2)

• Since arrays are also objects, they are passed
  into methods using call-by-reference.
• A pointer to the array is passed into the method
  (like in C).
• The contents of the array pointed to by an actual
  parameter can be changed by the method.
• static void shiftValuesLeft(int x)
• int i, temp
• tempx0
• for (i0 ilt(x.length-1) i)
• xixi1
• xx.length-1temp
• // in main method of same class
• int a 3,5,7,8
• shiftValuesLeft(a) // the content of array a
  will be changed

27
Aggregate Assignment

• One array can be assigned to another in a single
  statement
• double array1 new double6
• double array2 new double5
• array1 array2
• array1 becomes bound to whatever array2 was bound
  to. The array to which array1 was originally
  bound is eligible for garbage collection.

28
Boundary Violations

• Assume the following code
• Animal animals new Animal4
• animals4 new Animal()
• The above code would generate a run-time
  exception of type ArrayIndexOutOfBoundsException

29
Array Examples

• Reading values of an int array
• int x new int100
• for (i0 iltx.length i)
• System.out.println(Enter an integer )
• xi Integer.parseInt(stdin.readLine().trim())
  
• Shifting values of an int array
• int temp x0
• for (i0 iltx.length i)
• xixi1
• xx.length-1temp

30
Array Examples (cont.)

• Finding the location of the maximum value in an
  int array
• int loc 0
• for (i1 iltx.length i)
• if(xigtxloc)
• loci
• Finding the first location of a given value in an
  int array
• int loc, i0
• while ((iltxlength) (xi!item))
• ii1
• if (igtx.length) loc-1 // not found
• else loci // found

31
Array Examples (cont.)

• Finding summation of the values in an int array
• int sum0
• for (i0 iltx.length i)
• sumsumxi
• Reversing the contents of an int array
• int temp,i,j
• for (i0,jx.length-1 iltj i, j--)
• tempxi
• xixj
• xjtemp

32
Multi-Dimensional Arrays

• Allocation of two-dimensional array
• int x new int53
• Accessing elements of a two-dimensional array
• x00 5
• Initializing a two-dimensional array
• int a 3,5,7,8,6,9

33
Arrays of Varying Length

• The size of each row of a two-dimensional array
  can be different
• int x
• x new int5
• x0 new int1
• x1 new int2
• x2 new int3
• x3 new int4
• x4 new int5
• x.length ? 5
• x0.length ? 1
• x1.length ? 2
• ? x1 new int10

x











34
Strings and char Arrays

• Strings in Java are not char arrays (in C and C
  strings are char arrays).
• But, we convert a char array into a string or a
  string into a char array.
• char array into string
• char name m,a,r,k
• String aname new String(name
• string into char array
• String aname mary
• char name aname.toCharArray()