String and StringBuilder

1
String and StringBuilder

• Part I String

2
About Strings

• Strings are objects, but there is a special
  syntax for writing String literals
• "Hello"
• Strings, unlike most other objects, have a
  defined operation (as opposed to a method)
• " This " "is String " "concatenation""
• Strings can contain any character, but some of
  them must be escaped in order to write them in
  a literal
• \" stands for the double-quote (") character
• \n stands for the newline character
• \\ stands for the backslash (\)character
• Each of these is written as a two-character
  sequence, but represents a single character in
  the string

3
Useful String methods I

• char charAt(int index)
• Returns the character at the given index position
  (0-based)
• boolean startsWith(String prefix)
• Tests if this String starts with the prefix
  String
• boolean endsWith(String suffix)
• Tests if this String ends with the suffix String

4
Useful String methods II

• boolean equals(Object obj)
• Tests if this String is the same as the obj
  (which may be any type false if its not a
  String)
• boolean equalsIgnoreCase(String other)
• Tests if this String is equal to the other
  String, where case does not matter
• int length()
• Returns the length of this string note that this
  is a method, not an instance variable

5
Useful String methods III

• int indexOf(char ch)
• Returns the position of the first occurrence of
  ch in this String, or -1 if it does not occur
• int indexOf(char ch, int fromIndex)
• Returns the position of the first occurrence of
  ch, starting at (not after) the position
  fromIndex
• There are two similar methods that take a String
  instead of a char as their first argument

6
Useful String methods IV

• int lastIndexOf(char ch)
• Returns the position of the last occurrence of ch
  in this String, or -1 if it does not occur
• int lastIndexOf(char ch, int fromIndex)
• Returns the position of the last occurrence of
  ch, searching backward starting at position
  fromIndex
• There are two similar methods that take a String
  instead of a char as their first argument

7
Useful String methods V

• String substring(int beginIndex)
• Returns a new string that is a substring of this
  string, beginning with the character at the
  specified index and extending to the end of this
  string.
• String substring(int beginIndex, int endIndex)
• Returns a new string that is a substring of this
  string, beginning at the specified beginIndex and
  extending to the character at index endIndex - 1.
  Thus the length of the substring is
  endIndex-beginIndex

8
Understanding index

• With charAt(index), indexOf(x), and
  lastIndexOf(x), just count characters (starting
  from zero)

• With substring(from) and substring(from, to), it
  works better to count positions between characters

• So, for example, substring(4, 8) is "said",
  andsubstring(8, 12) is ", \"H"

• If indexOf(',') is 8, then substring(0,
  indexOf(',')) is "She said"and
  substring(indexOf(',') 1) is " \"Hi\""

9
Useful String methods VI

• String toUpperCase()
• Returns a new String similar to this String, in
  which all letters are uppercase
• String toLowerCase()
• Returns a new String similar to this String, in
  which all letters are lowercase
• String trim()
• Returns a new String similar to this String, but
  with whitespace removed from both ends

10
Useful String methods VII

• String split(String regex)
• Breaks the string up into an array of strings
• The parameter is a regular expression that
  defines what separates the strings
• For example, String s "one, two, three"
  String ss s.split(", ")
• This assigns the array "one", "two", "three" to
  ss
• Regular expressions are complex expressions that
  assign meanings to many common punctuation marks,
  such as , , period, and
• Hence, regular expressions are powerful, but can
  be treacherous if you arent very familiar with
  them

11
Finally, a useless String method

• String toString()
• Returns this String
• Why do we have this method?
• Consistency--Every Object has a toString() method

12
Strings are immutable

• A String, once created, cannot be changed
• None of the preceding methods modify the String,
  although several create a new String
• Statements like this create new Strings
• myString myString anotherCharacter
• Creating a few extra Strings in a program is no
  big deal
• Creating a lot of Strings can be very costly

13
More about equals

• If you write String s "abc" String t
  "abc"the compiler only creates the string
  "abc" once, and makes s and t both refer to this
  one string
• It can do this because strings are immutable
• Hence, the test s t will be true
• However, if you now write String u "a"
  "bc"the test s u will be false
• This is because they are different strings
• Moral Use equals for strings, not

14
Still more about equals

• Suppose you want to test whether a variable name
  has the value "Dave"
• Heres the obvious way to do it if
  (name.equals("Dave")) ...
• But you could also do it this way if
  ("Dave".equals(name)) ...
• It turns out that the second way is usually
  better
• Why?
• If name null, the first way will cause a
  NullPointerException, but the second way
  will just return false

15
Strings, Etc.

• Part II StringBuilder

16
About StringBuilders

• A StringBuilder has a capacity (the number of
  characters it can hold) and a length (the number
  of characters it is currently holding)
• If the capacity is exceeded, the StringBuilder is
  copied to a new location with more room
• StringBuilder is a reimplementation of
  StringBuffer
• The API (collection of methods) is the same
• StringBuffers are threadsafe, but StringBuilders
  are more efficient
• StringBuilders are used to implement String
  concatenation
• Whenever you say String s "ab" "cd", Java
  creates a StringBuffer containing the characters
  a and b, appends the characters c and d to it,
  and converts the result back to a String
• As you might guess, this isnt terribly
  efficient, but its fine if you dont overdo it

17
StringBuilder constructors

• StringBuilder()
• Constructs a StringBuilder with a capacity of 16
  characters
• StringBuilder(int capacity)
• Constructs a StringBuilder with the requested
  capacity
• StringBuilder(String str)
• Constructs a StringBuilder containing the String
  str

18
Useful StringBuilder methods I

• StringBuilder append(X)
• Appends X to the end of this StringBuilder also
  (as a convenience) returns this StringBuilder
• The append method is so heavily overloaded that
  it will work with any argument if the argument
  is an object, its toString() method is used

19
Useful StringBuilder methods II

• int length()
• Returns the number of characters in this
  StringBuilder
• void setLength(int newLength)
• Sets the number of characters in this
  StringBuilder this may result in truncation of
  characters at the end, or addition of null
  characters

20
Useful StringBuilder methods III

• char charAt(int index)
• Returns the character at the location index
• void setCharAt(int index, char ch)
• Sets the character at location index to ch
• StringBuilder reverse()
• The sequence of characters in this StringBuilder
  is replaced by the reverse of this sequence, and
  also returned as the value of the method

21
Useful StringBuilder methods IV

• StringBuilder insert(int offset, X)
• Insert X starting at the location offset in this
  StringBuilder, and also return this StringBuilder
  as the value of the method. Like append, this
  method is heavily overloaded
• StringBuilder deleteCharAt(int index)
• Deletes the character at location index
• StringBuilder delete(int start, int end)
• Deletes chars at locations start through end-1

22
Useful StringBuilder methods V

• String substring(int start)
• Returns a new String of characters from this
  StringBuilder, beginning with the character at
  the specified index and extending to the end of
  this string.
• String substring(int start, int end)
• Returns a new String of characters from this
  StringBuilder, beginning at location start and
  extending to the character at index end-1. Thus
  the length of the substring is end-begin
• String toString()
• Returns the characters of this StringBuilder as a
  String

23
When to use StringBuilders

• If you make a lot (thousands) of changes or
  additions to a String, it is much more efficient
  to use a StringBuilder
• If you are simply examining the contents of a
  String, then a String is at least as efficient as
  a StringBuilder
• For incidental use (such as creating output
  lines), use Strings they are more convenient

24
Strings, etc.

• Part III Characters

25
The Character class

• char is a primitive type, not an object,
  therefore
• there are no methods you can call on a char
• This is why we need a Character class!
• There are a lot of methods in the Character class
• They are all static
• Do you see why?

26
Some Character methods

• static boolean isDigit(char ch)
• static boolean isLetter(char ch)
• static boolean isLetterOrDigit(char ch)
• static boolean isLowerCase(char ch)
• static boolean isUpperCase(char ch)
• static boolean isWhitespace(char ch)
• static char toLowerCase(char ch)
• static char toUpperCase(char ch)
• For more methods, see java.lang.Character

27
The End