Java interviews can be tough to pass, especially if you're going in with no experience. That's why it's so important to study and prepare ahead of time for your interview. To help with that, here are 20 of the top most frequently asked Core Java Interview Questions and answers, which will really help you prepare for a successful interview no matter what language you're currently fluent in. These are questions you want to be familiar with, because they're probably the one question you'll be answering the most in your interview.
One of the questions you'll likely be asked during your interview is about threading and serialization. This means asking how your current code fits with the Java serialization protocol. The serialization protocol defines how multiple Java objects can be serialized into a single bytecode, making it possible for a single Java application to communicate with multiple external systems. Without understanding this, you may not be able to write efficient Java code and may even end up having to pay a company to fix your Java code due to missing or incorrect serialization parameters. Discover more facts about programming at https://en.wikipedia.org/wiki/Computer_code.
The second question you'll likely be asked is about static types and reference counts. Java has two fundamental data types: instance variables and reference variables. Instance variables are what you'll use inside a Java program, and references are additional variables that refer to instances of a specific object. Understanding these will help you when writing your Java code, since incorrectly implemented references could crash your program without your even realizing it.
Core Java Interview Questions that you may be asked during a Java interview is about the difference between concurrency and parallelism. Concurrent programming refers to a way to create a large number of threads operating in parallel, without the worry of blocking other processes. Parallelism is another way to think of it, but concurrency only refers to the ability to execute one thread at a time. For example, a Java application that uses a synchronized variable for the purposes of blocking calls on a background thread is considered to be running on a single-threaded program. However, if multiple threads were used, Java would behave differently since it would be trying to use a synchronized variable on a non-threaded program and would not be able to complete the job. Java programmers have two goals when designing a Java application: concurrency and parallelism.
The third question that you may be asked is about the use of references, or rather, how to write them. This is a trickier question, as there are actually two ways to write references - a local variable and a static variable. A local variable simply refers to any variable within a Java program, while a static variable is a named location within a Java program where a certain value can be stored. Java programmers have to follow a few rules in order to use references correctly, and a knowledgeable applicant would be able to explain those rules clearly to the interviewer.
Java also features a useful tool called a Thread Local Storage, which allows a thread to save information to a different place than where it would normally be stored in a Java program. For example, say that you have a large number of long strings stored into a database. If your program needs to look up each and every one of these strings, the Thread Local Storage will make it possible to do so in a single location, without having to look inside of multiple different files. While this type of memory optimization is not commonly seen in everyday Java applications, it is still important to be aware of it when you're preparing for interviews with potential employers. Java programmers need to know just how to use Thread Local Storage, as well as what kind of situations it is best employed in.
