A Singletone class;
Only one time in JVM per class loader would be instantiated. Same instance would be use for repeated calls.
public class OnlyOne {
private static OnlyOne one = new OnlyOne();
// private constructor, so this class cannot be instantiated from outside and prevents
//subclassing.
private OnlyOne();
//a global point of access
public static OnlyOne getInstance() {
return one;
}
}
To use it:
OnlyOne myOne = OnlyOne.getInstance();
| Class | Package | Sub Classes | World | |
| public | ✓ | ✓ | ✓ | ✓ |
| protected | ✓ | ✓ | ✓ | |
| package | ✓ | ✓ | ||
| private | ✓ |
If we don’t use any modifier, it uses default one. The default modifier is package.
| str1==str2 | str1.equals(str2) | |
| String | Compares Objs location | Compares Objs String contents |
| StringBuffer, StringBuilder | Compares Objs location | If and only if str1==str2 would be True. |
So if we need to compare or sort in StringBuffer / StringBuilder we need to turn them into String first. For example:
if (str1.ToString() = str2.ToString()) {
System.out.println(“True”);
} else {
System.out.println(“False”);
}
This chart* perfectly captures different features of App Servers versus Web Servers:
| Application Server | Web Server | |
| What is it? | A server that exposes business logic to client applications through various protocols including HTTP. | A server that handles HTTP protocol. |
| Job | Application server is used to serve web based applications and enterprise based applications(i.e servlets, jsps and ejbs…). Application servers may contain a web server internally. | Web server is used to serve web based applications.(i.e servlets and jsps) |
| Functions | To deliver various applications to another device, it allows everyone in the network to run software off of the same machine. | Keeping HTML, PHP, ASP, etc files available for the web browsers to view when a user accesses the site on the web, handles HTTP requests from clients. |
| Supports | distributed transaction and EJB’s | Servlets and JSP |
| Resource utilization | High | Low |
Question: Apache Tomcat is a Web Server or an Application Server?
Answer: Tomcat is a web server (can handle HTTP requests/responses) and web container (implements Java Servlet API, also called servletcontainer) in one. Some may call it an application server, but it is definitely not an fullfledged Java EE application server (it does not implement the whole Java EE API).**
————————————————————————–
* source: http://www.diffen.com/difference/Application_Server_vs_Web_Server
**http://stackoverflow.com/questions/2469949/tomcat-is-web-server-or-application-server
| ArrayList | Array | List | Vector | LinkedList | HashTable | HashMap | Set | Map | |
| Unlimited size | ✓ | × | ✓ | ||||||
| Index based | ✓ | ✓ | ✓ | ||||||
| Allow Nulls | ✓ | ✓ | × | ✓ | |||||
| Synchronized | × | ✓ | ✓ | × | |||||
| Allow duplicate | ✓ | × | |||||||
| Ordered data | × |
|
Abstract class |
Interface |
| Have abstract & non-abstract methods | Only abstract methods |
| Not support multiple inheritance | Supports multiple inheritance |
| Can have final/non-final/statis/non-static variables | Only final & static variables |
| Can have:
· static methods · main method · constructor |
Cannot have
|
| Can provide implementation of interface | Cannot provide implementation of abstract class |
| When we want to add new methods | When we want to have multiple inheritance |
| We extend it | We implement it |
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