Strings - Java and the Web - Lecture Slides, Slides of Java Programming

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Strings

Strings

• Strings are fundamental part of all computing languages.
• At the basic level, they are just a data structure that can hold a series of characters
• However, strings are not implemented as a character array in Java as in other languages.

Basic String Methods

• length() returns the length of the string
• toLowerCase() converts the string to lower case
• toUpperCase() converts the string to upper case
• replace(char, char) replaces occurrences of one character with another character

Basic Strings continued

• Basic strings are not meant to change frequently so there are no add or append methods
• However the concat(String) method does allow two strings to be concatenated together

Searching a string

• Methods for searching strings
  • indexOf( x ) searches for the first occurrence of x
  • indexOf( x , y ) searches for the first occurrence of x after the offset of y
  • lastIndexOf( x ) searches backwards for the first occurrence of x
  • lastIndexOf( x , y ) searches backwards for the first occurrence of x after the offset of y

Example of string search

• indexOf( x ) and indexOf( x , y ) can find all occurrences of a character(s) in a string public void paint(Graphics g) { String str = new String("Wish You Were Here"); int count = 0; int fromIndex = 0; while(fromIndex != -1) { fromIndex = str.indexOf("er", fromIndex); if (fromIndex != -1) { count++; fromIndex++; } } g.drawString(String.valueOf(count), 10, 10); }

Parsing Strings continued

• Different default constructors are provided
  • Tokenize the string based on the default delimiters
  • Tokenize the string based on a specified set of delimiters
  • Tokenize the string based on a specified set of delimiters with a boolean flag to specify whether the delimiters should also be returned as tokens

StringBuffer class

• The StringBuffer class is provided for strings that need may need to be changed
• The StringBuffer class contains methods for both inserting and appending text
• An object created as a StringBuffer can easily be converted to an object of the String class if needed

More on StringBuffer Class

• StringBuffer objects are mutable and capacity & length affect performance
• If the StringBuffer object needs to be expanded during an append or insert, a new array is created and the old data copied to it
• Use capacity() and ensureCapacity(int) methods to minimize expansions

Length v. Capacity

• The length() method returns the length of the string in the StringBuffer
• The capacity() method returns the total “space” in a StringBuffer
• The ensureCapacity(int) method insures the StringBuffer has at least the specified amount of capacity remaining