98 Java interview questions you need to know

Examples of Java Interview Tasks and Questions

Speaking to developers in the community, these are some of their favourite interview questions from personal experience and how they dealt with them:

Alan Mellor, a Java developer since 2001, and currently a Consultant Software Engineer at BJSS, said his favourite interview was being asked to do a 30-45 minute observed pair-programming session with a senior developer.

The task was simply to read some data, pick some data out, and to write it to console.

Alan continues to highlight how this was a great opportunity to show off how he thinks about stuff, how he writes his tests, how he splits his code, how he names his variables, how he applies dry and solid principles, and what sort of interactions he has with a programming pair.

In his experience, Alan believes this was much better than being asked some ‘weird quirk’ of the Java language specification.

Jevin Menezes, an Open Source software enthusiast who learned Java fundamentals during his studies of engineering, comments on the types of interview questions he recommends for newcomers in Java.

The two questions he experienced tested his knowledge of core concepts.

94. Is String a datatype in Java?

The interviewer wanted an explanation which stated that String is not a primitive data type in Java; it is one of the most extensively used objects in Java, which is an instance of String class.

95. String class is defined under which package in Java?

The answer to this Java question is java.lang package, and this will test the very basics of core Java knowledge.

David Owens, who is a freelancing Java developer also provides interview questions about the core fundamentals of Java knowledge.

The following questions are what he believes is fair to expect during an interview:

96. What occurs when an object is constructed?

Answer: Several things happen in a specific order to guarantee the object is constructed properly:

1. Memory is distributed from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implementation-specific data consist of point to class and method data.
2. The instance variables of the objects are initialised to their default values.
3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its uppercase. This process continues until the constructor is for java.lang.Object is called, as java.lang.Object is the base class for all objects in Java.
4. Before the body of the constructor is executed, all instance variable initialisers and initialisation blocks are executed. Then the body of the constructor is executed. This, the constructor for the base class completes first and constructor for the most derived class completes last.

A question like this will indicate a good understanding of processes with the core Java language.

97. Explain the difference between the public, private, final, protected, and default modifiers.

Answer: These keywords – public, private, final, protected, and default modifiers – are to specify access levels for member variables, or for member functions (methods).

• Public variables: these are variables that are noticeable to every class.
• Private variables: these are variables that are noticeable only to the class to which they fit in.
• Protected variables: these are variables that are noticeable only to the class to which they fit in, as well as any other relevant subclasses.

The decision to use private, protected, or public variables can be difficult. It is a question of whether or not an external object (or program), actually needs direct access to the information.

If you prefer to have other objects to access internal data, but want to control it at the same time, you would make it either private or protected. However, be mindful of delivering functions which can affect the data in an organised way.

Although David mentions it isn’t fair to place too much emphasis on the protected and default modifiers and exactly what they mean. Though extra credit could be assumed for why and which circumstances you would use these.

98. Explain why Java does not have multiple inheritance.

Answer: Multiple inheritance is a feature of some object-oriented computer programming languages. This is where an object or class can come into characteristics and features from one or more parent object or parent class.

Java does not support multiple inheritance because it can cause ambiguity in various scenarios, mainly because of the ‘diamond problem’. Java is designed to be a simple programming language.

The problem is that is a class extended two other classes, and both superclasses had, for example, a doStuff() method, which version of doStuff() would the subclass inherit? This type of scenario is typically known as ‘Diamond Problem’ also known as ‘Deadly Diamond of Death’, and because of the shape of the class diagram that can be created in a multiple inheritance design.

The diamond is formed when classes B and C both extend A, and both C and C have overridden the method in A. Class D has, in principle, inherited two different implementations of the identical method.

When drawn as a class diagram, the shape of the four classes looks the same as a diamond.

After the introduction of Default Methods in Java 8, the interfaces can also have the method bodies. In Java 8 interface, it can have method definition using Default Methods, then clearly it should also result in ambiguity? Yes, but Java 8 can handle this type of compatible problem.

In Java 8, you cannot implement multiple interfaces having the same signature, without explicitly overriding the methods in the Child class. It is possible to call the Parent interface method explicitly from the Child class.

Explaining the inheritance exception for Java 8 may not be necessary but David believes this may show extra credibility knowing this extra bit of information.

David also discusses his love for snippets of code being projected onto a wall or are shown to you in a reasonably easy-to-read form, and you are asked to describe what they mean, what they print, and what value of a variable would be.

However, the types of questions he isn’t too fond of are coding exercises in which you need to type. Especially, if it is in an editor platform or IDE that is unfamiliar, as he believes this is not a test of knowledge.

David’s advice for candidates going through the Java interview process is to understand that when they are asking questions and evaluating you on your answers, it is entirely appropriate to evaluate them on the content of those questions and how they react to your answers. His indication of a good company is: ‘if this company was public, would I buy stock in them?’

And that is our round-up of the popular Java interview questions you may come across. These questions have been outlined on how to best answer during an interview, however, it is advised to do as much research as possible to ensure you answer the question as best as you can.

Have you experienced any interesting Java interview questions? Let us know In the comments and explain how you answered the question.