Immutable classes are Java classes whose objects can not be modified once created. Any modification in Immutable object results in new object. For example String is immutable in Java. Mostly Immutables are also final in Java, in order to prevent subclasses from overriding methods in Java which can compromise Immutability.

Stringbuffer methods are synchronized while StringBuilder is Non Synchronized.

The classloader is a subsystem of JVM that is used to load classes and interfaces. There are many types of classloaders e.g. Bootstrap classloader, Extension classloader, System classloader, Plugin classloader etc.

You can not override private or static method in Java, if you create a similar method with same return type and same method arguments that's called method hiding.

The ThreadLocal class in Java enables you to create variables that can only be read and written by the same thread. Thus, even if two threads are executing the same code, and the code has a reference to a ThreadLocal variable, then the two threads cannot see each other's ThreadLocal variables.

sleep() is a blocking operation that keeps a hold on the monitor / lock of the shared object for the specified number of milliseconds.

wait(), on the other hand, simply pauses the thread until either (a) the specified number of milliseconds have elapsed or (b) it receives a desired notification from another thread (whichever is first), without keeping a hold on the monitor/lock of the shared object.

HashSet is superior in almost every way: O(1) add, remove and contains, vs. O(log(N)) for TreeSet. However, TreeSet is indispensable when you wish to maintain order over the inserted elements or query for a range of elements within the set.

We can pass a lambda expression as an object to a method. This reduces the overhead involved in passing an anonymous class. We can also pass a method as a parameter to another method using lambda expressions.

ThreadFactory is an interface that is meant for creating threads instead of explicitly creating threads by calling new Thread(). Its an object that creates new threads on demand. Using thread factories removes hardwiring of calls to new Thread, enabling applications to use special thread subclasses, priorities, etc.

Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

Generics provide compile-time type safety that allows programmers to catch invalid types at compile time. Java Generic methods and generic classes enable programmers to specify, with a single method declaration, a set of related methods or, with a single class declaration, a set of related types.

Daemon thread is a low priority thread, which runs intermittently in the background doing the garbage collection operation for the java runtime system.

The core abstraction that must be implemented by all servlets is the javax.servlet.Servlet interface. Each servlet must implement it either directly or indirectly, either by extending javax.servlet.GenericServlet or javax.servlet.http.HTTPServlet. Finally, each servlet is able to serve multiple requests in parallel using multithreading.

‘Exception’ and ‘Error’ classes are both subclasses of the Throwable class. The Exception class is used for exceptional conditions that a user’s program should catch. The Error class defines exceptions that are not expected to be caught by the user program.

Connection Pooling facilitates reuse of the same connection object to serve a number of client requests. Every time a client request is received, the pool is searched for an available connection object and it's highly likely that it gets a free connection object. Otherwise, either the incoming requests are queued or a new connection object is created and added to the pool (depending upon how many connections are already there in the pool and how many the particular implementation and configuration can support). As soon as a request finishes using a connection object, the object is given back to the pool from where it's assigned to one of the queued requests.