Here are some important interview questions

Q:What is the difference between Serializable and Externalizable in Java?

Ans:By implementating java.io.Serializable, you get "automatic" serialization capability for objects of your class. No need to implement any other logic, it'll just work. The Java runtime will use reflection to figure out how to marshal and unmarshal your objects.

In earlier version of Java, reflection was very slow, and so serializaing large object graphs (e.g. in client-server RMI applications) was a bit of a performance problem. To handle this situation, the java.io.Externalizable interface was provided, which is like java.io.Serializable but with custom-written mechanisms to perform the marshalling and unmarshalling functions (you need to implement readExternal and writeExternal methods on your class). This gives you the means to get around the reflection performance bottleneck.

In recent versions of Java (1.3 onwards, certainly) the performance of reflection is vastly better than it used to be, and so this is much less of a problem. I suspect you'd be hard-pressed to get a meaningful benefit from Externalizable with a modern JVM.

Also, the built-in Java serialization mechanism isn't the only one, you can get third-party replacements, such as JBoss Serialization, which is considerably quicker, and is a drop-in replacement for the default.

A big downside of Externalizable is that you have to maintain this logic yourself - if you add, remove or change a field in your class, you have to change your writeExternal/readExternalmethods to account for it.

Q:What is Daemon thread.

Ans:

When a new thread is created it inherits the daemon status of its parent.

Normal thread and daemon threads differ in what happens when they exit. When the JVM halts any remaining daemon threads are abandoned:

finally blocks are not executed,

stacks are not unwound - the JVM just exits.

Due to this reason daemon threads should be used sparingly and it is dangerous to use them for tasks that might perform any sort of I/O.