java notes

  Java Package A java package is a group of similar types of classes, interfaces and sub-packages. Package in java can be categorized in two form, built-in package and user-defined package. There are many built-in packages such as java, lang, awt, javax, swing, net, io, util, sql etc.  Advantage of Java Package Java package is used to categorize the classes and interfaces so that they can be easily maintained. 2) Java package provides access protection. 3) Java package removes naming collision.  s imple example of java package package mypack;   public class Simple{     public static void main(String args[]) {       System.out.println("Welcome to package");      }   } How to access package from another package? There are three ways to access the package from outside the package. import package.*; import package.classname; fully qualified name.      1) Using package name.* If you use package.* then all the classes and interfaces of this package will be accessible but not

 Abstraction in Java   Data Abstraction is the property by virtue of which only the essential details are displayed to the user.The trivial or the non-essentials units are not displayed to the user. Ex: A car is viewed as a car rather than its individual components. Ways to achieve Abstraction There are two ways to achieve abstraction in java Abstract class  Interface Abstract class in Java:- A class which is declared with the abstract keyword is known as an abstract class in java. It can have abstract and non-abstract methods (method with the body). A class which is declared as abstract is known as an abstract class . It can have abstract and non-abstract methods. It needs to be extended and its method implemented. It cannot be instantiated. Points to Remember An abstract class must be declared with an abstract keyword. It can have abstract and non-abstract methods. It cannot be instantiated. It can have constructor and static methods also. It can have final methods which will f

  Java StringBuffer class StringBuffer is a peer class of String that provides much of the functionality of strings. String represents fixed-length, immutable character sequences while StringBuffer represents growable and writable character sequences. Syntax: StringBuffer s = new StringBuffer(“text"); Important Constructors of StringBuffer class:- Constructor Description StringBuffer() creates an empty string buffer with the initial capacity of 16. StringBuffer(String str) creates a string buffer with the specified string. StringBuffer(int capacity) creates an empty string buffer with the specified capacity as length.   Important methods of StringBuffer class Modifier and Type Method Description public synchronized StringBuffer append(String s)