Noor Kumar

String is immutable, once you create a String object you can not modify that object. StringBuffer and StringBuilder are mutable, after creating the object you can modify that. The difference between StringBuffer and StrinngBuilder is, The methods in StringBuilder are synchronized methods and StringBuffer methods are not synchronized . So, if you are concerned about thread safety go for StringBuilder otherwise go for StringBuffer.

Difference between Externalizable and Serializable In case of Serializable, default serialization process is used. while in case of Externalizable custom Serialization process is used which is implemented by application. JVM gives call back to readExternel() and writeExternal() of java.io.Externalizalbe interface for restoring and writing objects into persistence. Externalizable interface provides complete control of serialization process to application. readExternal() and writeExternal() supersede any specific implementation of writeObject and readObject methods. Though Externalizable provides complete control, it also presents challenges to serialize super type state and take care of default values in case of transient variable and static variables in Java. If used correctly Externalizable interface can improve performance of serialization process.

finally block is used to handle important code in Java for example closing you database connections or if there are any input streams and output streams then it would be better to close all such things in the finally block. For example, if you are doing Database connection to perform any query execution and it is important to close all your database connection stuff once you done with our operation and what happens if you get any exception or if your code throws any exception in between. It is very dangerous if you won't the connection, it will cause memory leakage and at one point of time your connection pool will be crate database connection and your application can not execute any query. As finally blocks always execute in java whether it will get exception or not, we can close all database connection or we can write any important in finally block.