Lecture on Thursday, 01 April 2004

String Class

Finding Substrings

int indexOf ( String str ) : returns the index of the first occurrence of the substring str or -1 if the substring does not exist.
int indexOf ( String str, int fromIndex ) : returns the first occurrence of the substring starting from the fromIndex.
int lastIndexOf ( String str ) : returns the index of the last occurrence of the substring str.
int lastIndexOf ( String str, int fromIndex ) : returns the index of the last occurrence of the specified substring searching backward starting at the specified index.

Converting Character Arrays to Strings and vice versa

char[] charArray = { 'H', 'e', 'l', 'l', 'o' };
String hello = new String ( charArray );

String world = new String ( "World" );
char[] worldArray = world.toCharArray();

Converting Numeric values to Strings

double x = 2.512;
String xStr = String.valueOf ( x );

Test Prefix and Suffix of Strings

String str = "polymorphism";
str.startsWith ("poly") => true
str.endsWith ("ism") => true

Splitting a String

String line = "This is a test";
String[] result = line.split ( "\\s" );
for ( int i = 0; i < result.length; i++ )
  System.out.println ( result[i] );

StringBuffer Class

A String object is immutable. A StringBuffer object is like a String object but can be modified. A string buffer is a sequence of characters but the length and content of the sequence can be changed through certain method calls.

The principal operations on a StringBuffer are append() and insert() methods. The append() method adds characters at the end of the buffer and the insert() method adds the characters at a specified location.

Every string buffer has a capacity. As long as the length of the character sequence does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If there is an overflow, the string buffer is automatically made larger.

Creating a StringBuffer Object

StringBuffer strBuf1 = new StringBuffer();
StringBuffer strBuf2 = new StringBuffer ( 100 );
StringBuffer strBuf3 = new StringBuffer ( "Hello World" );

Getting the capacity of a String Buffer

int x = strBuf2.capacity();

Adding Characters or Strings to a String Buffer

strBuf2.append ("put");
strBuf2.insert (0, "out");

Delete or Replace Characters in a String Buffer

strBuf3.delete(6,11);
strBuf2.replace(3,6,"look");
strBuf2.setCharAt (6, 'p');

Reverse the Character Sequence

StringBuffer strBuf4 = strBuf2.reverse();

Get length of the character sequence in a String Buffer

int x = strBuf4.length();

Convert a String Buffer to a String

String str = strBuf4.toString();

Command Line Arguments
You can pass parameters to the method main() from the command line. The command line parameters are stored in the String array args. Supposing you start the program as follows:

java prog 1 2 3

The numbers 1, 2, and 3 will be stored as strings "1", "2" and "3" in the array args. For example, the string "1" will be in args[0] and the string "3" will be in args[2].