Classes in Java^TM technology

The Object class

All classes in Java^TM technology are directly or indirectly derived from the Object class. Some of the subclasses of Object class are - Boolean, Number, Void, Math, String, StringBuffer etc.

Some of the important methods defined in the Object class are given below. These methods are available to all Java classes.

1. boolean equals(Object obj) - The equals method in Object class returns true if two references point to the same object. Some classes like String and Boolean overload this method. The difference between the equals function and the equality operator is covered here.
2. String toString() - The function is used to convert objects to String. If a subclass does not override this method, the method returns a textual representation of the object, which has the following format : <name of the class>@<hash code value of the object>".
3. The following methods related to threads are also defined in Object class -
   void notify()
void notifyall()
void wait(long timeout) throws InteruptedException
void wait(long timeout, int nanosec) throws InteruptedException
void wait() throws InteruptedException


Wrapper classes

Corresponding to all the primitive types Java technology defines wrapper classes. Some examples of these wrapper classes are - Character, Boolean, Integer, Double.

Important methods in the Math class

Some of the methods defined in the Math class are used frequently. These are explained below. Besides the functionality, it is important to understand the arguments and return type of these functions.
static double ceil(double(d)) : The method ceil returns the smallest double value equal to a mathematical integer, that is not less than the argument. For example,

	ceil(3.4) returns 4.0
	ceil(-2.3) returns -2.0
	ceil(3.0) returns 3.0

static double floor(double(d)) : The method floor returns the largest double value equal to a mathematical integer, that is not greater than the argument. For example,

	floor(3.4) returns 3.0
	floor(-2.3) returns -3.0
	floor(3.0) returns 3.0

static int round (float f) and static long round(double d) : The method round returns the integer closest to the argument.

	round(3.7) returns 4
	round(3.2) returns 3
	round(3.0) returns 3
	round(-3.1) returns -3

String class

The String class is used to implement immutable character strings. This means that the character string cannot be changed once it has been created. Some of the important methods are explained below.
int length() - The number of characters in the String class are returned by the length() method.

String substring(int startIndex)
String substring(int startIndex, int endIndex)
The method substring extracts a substring from a string. The method extracts a string from the startIndex to the index endIndex - 1. If endIndex is not specified then string till the end of input string is returned. The example below illustrates this

	String str = "I am a string";
	int len = str.length();
	String str2 = str.substring(2,5);

After the above statements str2 contains the string "am ". The string str still has the same value "I am a string". The variable len has value 13.

StringBuffer class

The StringBuffer class implements mutable strings. This means that the characters stored in the string and the capacity of the string can be changed.

Garbage Collection

Java technology's Garbage collection is complex. In this section I am only giving a brief overview of Garbage Collection. Java technology supports automatic garbage collection. This means that the programmer does not need to free the memory used by objects. Java technology's runtime environment can claim the memory from the objects that are no longer in use. Objects that are not being referred become candidates for garbage collection. It is important to note that these objects are candidates only. Java technology does not guarantee that Garbage collection would happen on these objects. Before actually freeing up the memory, garbage collector invokes the finalize() method of the Object being freed.

The System.gc() method can be invoked by the program to suggest to Java technology that the Garbage Collector be invoked. However there is no guarantee when the garbage collection would be invoked. There is also no guarantee on the order in which objects will be garbage collected.

The example illustrates when a string Object becomes available for Garbage Collection.

public class GCTest {
   public static void main(String args[]) {
      String a,b;
      String c = new String("test");
      a = c;
      c = null; // The String "test" is not yet 
      //available for GC as a still points to "test"
      b = new String("xyz");
      b = c; // String "xyz" is now available for GC.
      a = null; 
      //String "test" is now available for GC.
   }
}