Built classes in Java and their usage in coding.

Sun-Microsystems have provided many built classes that can be used while developing an application. Some of important built packages are:

·        java.lang

·        java.io

·        java.awt

·        java.util

·        java.sql

·        javax.swing

·        java.rmi

In article we will discuss features and methods provided by some of these important built in packages in Java:

v java.lang: This is most important built in package provided by Sun-Microsystems which contains java.lang.Object class and this java.lang.Object class will be parent of every class that you define in your source either directly or indirectly. Also this class and whole java.lang package along with its features and method is available to use with any Java program and you need not provide any ‘import’ statement or package information to use the features and methods in this package. It is internally linked to every Java class by JVM internally.

Classes available in java.lang are:

java.lang.Object/System/Runtime/String/Integer/Float/Double/Character/Long/Thread/ThreadGroup etc.

For ex. to see the available methods and features in Object class you can use following syntax at command prompt:

Ex: javap java.lang.Object [Enter]

Interfaces available in java.lang are:

java.lang.Cloneable/CharSequence/Runnable/Comparable

v java.lang.Object class: As said in above paragraph it is at the highest level of Java class hierarchy and every class written in Java will be child to this class. In other words java.lang.Object is parent to every class in Java. If class is not extending any other class then it is said to be direct child of java.lang.Object class and if a class is extending the other class then it is said to be indirect child of java.lang.Object.

Ex:

class A //A is direct subclass of java.lang.Object.

{

}

Class B extends A //B is indirect subclass of java.lang.Object

{

}

As java.lang.Object class is super class for all the classes in Java, inside the object class reference variable, a reference of any type can be stored. You can see all the methods available with java.lang.Object class by typing ‘javap’ followed by ‘java.lang.Object’ at command prompt and you can also use these methods with your classes.

Some important methods of java.lang.Object class are:

                                i.            getClass(): getClass() is an instance method in Object class and it can be invoked by using the instance of any class and it will return java.lang.class instance of type using which it is invoked.

To get type information about an instance you can invoke this method, which will return instance of type with which it is invoked.

The getClass() method is implemented in non-Java language and it cannot be overridden as it has modifier ‘final’.

                              ii.            hashCode(): Internally JVM assigns integer values to every instance created by it, and these integer values are called as hashCode.

JVM will use these unique integer values to differentiate between the instances. This method is also implemented in non-Java language and it is non-final method which means it can be overridden by a programmer to use it effectively. If you are overriding this method then you must follow some rules:

ü  The hasCode method should return same value for multiple invocations using the same object.

ü  If equals() method of object class is returning true value for two objects then their hashCode value must be same.

ü  If hasCode value for two objects is returning true then it is not mandatory that their hashCode value must be distinct.

                            iii.            equals(Object): This method is commonly used to compare the two objects in Java, on the basis of reference. This method internally uses ‘==’ operator to compare the two objects. It should be used for the comparison of content so it is recommended to override the equals() method and to provide the implementation for comparison of content. If both objects are same on the basis of content then it should return ‘true’ otherwise ‘false’ value should be returned.

                             iv.            toString():  This method returns the string representation of the object for which it is invoked.

The string representation includes class name followed by ‘@’ symbol and it is followed by HexStrignRepresentation of hashcode of object with which ‘toString() ‘ is invoked.

toString() can be overridden to return the state of object. In collection framework container classes overrides this method and returns elements present inside the collection.