Being well-prepared for job interviews and having a strong grasp of fundamental Java programming concepts are essential for intermediate developers. In this page, we will delve into some of the most intriguing interview questions that intermediate developers are likely to encounter in their job search.
You might also be interested to check the following collections of interview questions:
- Java Interview Questions for Junior Developers,
- Java Interview Questions for Senior Developers,
- Most Popular Java Interview Questions.
In this page you will find interview questions that cover various topics, including functional programming, streams API, data structures and other critical areas. By the end of this page, you will have gained a comprehensive understanding of these concepts and be better equipped to succeed in your next Java job interview. Let’s dive in!
What is autoboxing and unboxing in Java?
Autoboxing is the process of converting a primitive type to its corresponding object wrapper class, while unboxing is the opposite process. For example, autoboxing converts an int to an Integer object, and unboxing converts an Integer object to an int.
Learn more at: Autoboxing and Unboxing in Java.
What are the access modifiers in Java, and what is their significance?
Java has four access modifiers: public, private, protected, and default (also known as package-private). These modifiers control the visibility and accessibility of class members (variables, methods, and inner classes).
- Public members can be accessed from anywhere.
- Private members can only be accessed from within the same class.
- Protected members can be accessed within the same package or by subclasses.
- Default members can be accessed within the same package.
Learn more at: Java Access Modifiers: Boost Your Programming Skills.
What is the difference between a static method and a non-static method in Java?
The difference is that a static method belongs to the class and can be called using the class name, while a non-static method belongs to an object and can only be called on an object of that class. Static methods cannot access non-static members (variables or methods) of the class, while non-static methods can access both static and non-static members.
Learn more at: Differences Between Instance and Static Methods.
What is a lambda expression in Java?
A lambda expression is a concise way to express a method that can be used as a functional interface. It allows you to pass behavior as an argument to a method or assign it to a variable, similar to a method pointer in other programming languages.
Learn more at: Master Lambda Expressions and Enhance Your Java Programming.
What is the purpose of the super keyword in Java?
The super keyword is used to refer to the superclass of a class. It can be used to call the constructor of the superclass, access the superclass’s methods or variables, and differentiate between overridden methods.
Learn more at: Keyword super in Java.
What is a varargs parameter in Java?
A varargs parameter is a way to pass a variable number of arguments to a method. It is denoted by an ellipsis (…) after the parameter type in the method signature. The varargs parameter is treated as an array within the method.
What is the purpose of the transient keyword in Java?
The transient keyword is used to indicate that a variable should not be serialized when the object is saved to a file or transferred over a network. Transient variables are typically used for data that is not critical or can be easily recreated.
Learn more at: Transient Keyword in Java (Explained!).
What is the difference between the StringBuilder and StringBuffer classes in Java?
Both StringBuilder and StringBuffer are used to manipulate strings in Java. The main difference between them is that StringBuilder is not thread-safe, while StringBuffer is. This means that StringBuffer can be accessed by multiple threads simultaneously without causing issues, while StringBuilder cannot.
Learn more at: Difference between StringBuilder and StringBuffer classes.
What is the purpose of the final keyword in Java?
The final keyword can be applied to variables, methods, and classes in Java. When applied to a variable, it makes the variable a constant that cannot be changed. When applied to a method, it prevents the method from being overridden in a subclass. When applied to a class, it prevents the class from being subclassed.
Learn more at: Java Final Keyword: A Comprehensive Guide.
What is the purpose of the synchronized keyword in Java?
The synchronized keyword is used to ensure that only one thread at a time can access a block of code or an object. This is important for preventing race conditions and ensuring thread safety. Synchronization can be applied to methods, blocks of code, or entire classes.
What is a static initializer in Java?
A static initializer is a block of code that is executed when a class is loaded into memory. It is used to initialize static variables or perform other initialization tasks that are required before the class can be used.
Learn more at: Mastering Java’s Powerful Static Initializer Block.
What is the difference between the final, finally, and finalize keywords in Java?
The final keyword is used to declare a variable, method, or class as immutable or unchangeable. The finally keyword is used to define a block of code that is executed after a try/catch block, regardless of whether an exception is thrown. The finalize() method is called by the garbage collector when an object is about to be destroyed, allowing it to perform any necessary cleanup tasks.
What is a reflection in Java?
Reflection is a feature in Java that allows you to inspect and modify the behavior of objects at runtime. It provides a way to examine and modify class information, method information, and field information at runtime. This can be useful for building tools that analyze or modify code dynamically.
Learn more at: Using Java Reflection.
What is the difference between the super() and this() constructors in Java?
The difference is that the super() constructor is used to call the constructor of the superclass, while this() is used to call a constructor in the same class. Both super() and this() must be the first statement in the constructor.
What is the difference between an abstract method and a default method in Java interfaces?
The difference is that an abstract method is a method that does not have an implementation and must be implemented by any class that implements the interface. A default method is a method that has a default implementation, but can be overridden by implementing classes if needed.
Default methods were introduced in Java 8 as a way to add functionality to interfaces without breaking existing implementations.
What is the purpose of the extends keyword in Java?
The extends keyword is used to create a subclass that inherits behavior from a superclass. This allows you to reuse code and create hierarchies of related classes.
What is a marker interface in Java?
A marker interface is an interface that does not define any methods, but is used to mark a class as having a particular capability or behavior. Examples of marker interfaces in Java include Serializable, Cloneable, and RandomAccess.
Learn more at: Java Marker Interface.
What is Serialization in Java?
Serialization is the process of converting an object into a stream of bytes so that it can be stored or transmitted over a network. This allows objects to be persisted to disk or sent between applications running on different machines.
Learn more at: Serialization and Deserialization in Java: A Comprehensive Guide.
What is the Serializable interface in Java?
The Serializable interface is a marker interface that indicates that a class can be serialized. When a class implements the Serializable interface, its object can be serialized and deserialized using the ObjectOutputStream and ObjectInputStream classes.
What is the difference between Serializable and Externalizable interfaces in Java?
The difference is that Serializable is a marker interface that is used for default serialization and requires no implementation of methods. Externalizable is an interface that defines two methods, readExternal() and writeExternal(), which must be implemented for custom serialization. Externalizable provides greater control over the serialization process, but requires more work than Serializable.
What are functional interfaces in Java?
Functional interfaces are interfaces that have only one abstract method. They are used to represent functions as objects in Java, and are the basis for lambda expressions and method references. Examples of functional interfaces in Java include Runnable, Callable, and Predicate.
Learn more at: Master functional interfaces in Java for efficient code.
What is the Function interface in Java?
The Function interface is a functional interface that represents a function that takes one argument and returns a result. It is defined in the java.util.function package and has a single abstract method, apply(). The Function interface can be used to pass functions as parameters to methods or to create lambda expressions.
Learn more at: Function Functional Interface in Java.
What is the difference between a Consumer and a Supplier interface in Java?
The difference is that a Consumer is a functional interface that represents a function that takes one argument and returns no result. A Supplier is a functional interface that represents a function that takes no arguments and returns a result. Consumers are used to perform operations on objects, while Suppliers are used to generate objects.
Learn more at: Consumer Functional Interface in Java. / Supplier Functional Interface in Java.
What is the Predicate interface in Java?
The Predicate interface is a functional interface that represents a boolean-valued function of one argument. It is defined in the java.util.function package and has a single abstract method, test(). The Predicate interface can be used to filter collections or to create lambda expressions that return true or false based on some condition.
Learn more at: Predicate Functional Interface in Java.
What is the BiFunction interface in Java?
The BiFunction interface is a functional interface that represents a function that takes two arguments and returns a result. It is defined in the java.util.function package and has a single abstract method, apply(). The BiFunction interface can be used to pass functions as parameters to methods or to create lambda expressions.
Learn more at: BiFunction Functional Interface in Java.
What is the Function.identity() method in Java?
The Function.identity() method is a static method in the java.util.function package that returns a Function that returns its input argument unchanged. This method can be useful when you want to pass a function as an argument, but you don’t need to transform the input argument.
What is the Collections Framework in Java?
The Collections Framework is a set of classes and interfaces that provide reusable data structures, algorithms, and utility methods for working with collections of objects in Java.
Learn more at: Java Collections.
What are some of the key interfaces in the Collections Framework?
Some of the key interfaces in the Collections Framework include List, Set, Map, Queue, and Deque.
What is the difference between List and Set in Java?
List is an ordered collection of elements where duplicates are allowed, whereas Set is an unordered collection of unique elements.
What is the difference between ArrayList and LinkedList in Java?
The difference is that ArrayList is implemented as a resizable array, whereas LinkedList is implemented as a doubly linked list. ArrayList provides constant-time access to its elements, whereas LinkedList provides constant-time insertion and deletion of elements.
Learn more at: ArrayList in Java. / LinkedList in Java.
What is the difference between HashMap and TreeMap in Java?
HashMap is an unordered collection of key-value pairs, whereas TreeMap is a sorted map that maintains its elements in ascending order based on the keys.
Learn more at: Map in Java. / What is a TreeMap in Java?
What is the difference between HashSet and TreeSet in Java?
HashSet is an unordered collection of unique elements, whereas TreeSet is a sorted set that maintains its elements in ascending order based on their natural ordering or a custom Comparator.
Learn more at: What is a HashSet in Java? / What is a TreeSet in Java?
What is the difference between HashMap and Hashtable in Java?
HashMap is not synchronized and allows null values and keys, whereas Hashtable is synchronized and does not allow null values or keys.
What is the purpose of the Comparable interface in Java?
The Comparable interface is used to define a natural ordering of objects. It allows objects to be compared to each other using the compareTo() method, which returns a negative integer, zero, or a positive integer depending on whether the object is less than, equal to, or greater than the specified object.
Learn more at: Comparable in Java.
What is the purpose of the Comparator interface in Java?
The Comparator interface is used to define a custom ordering of objects. It allows objects to be compared to each other using the compare() method, which returns a negative integer, zero, or a positive integer depending on whether the first object is less than, equal to, or greater than the second object.
Learn more at: Comparator interface in Java.
What is the difference between a Collection and a Map in Java?
The difference is that a Collection is a group of objects, whereas a Map is a group of key-value pairs. Collections are used to store and manipulate a group of related objects, whereas Maps are used to store and manipulate associations between keys and values.
What is the purpose of the Iterator interface in Java?
The Iterator interface is used to iterate over a collection of objects one by one. It allows you to remove elements from the underlying collection during the iteration process.
Learn more at: Iterating ArrayList Using Iterator.
What is the purpose of the ListIterator interface in Java?
The ListIterator interface is a specialized Iterator that allows you to traverse a List in both forward and backward directions. It also allows you to modify the List during the iteration process.
What is the difference between a Queue and a Stack in Java?
A Queue is a collection that orders its elements in a first-in, first-out (FIFO) manner, whereas a Stack is a collection that orders its elements in a last-in, first-out (LIFO) manner.
What is the difference between Iterator and ListIterator in Java?
Iterator is used to iterate over a collection in forward direction only, whereas ListIterator is used to iterate over a List in both forward and backward directions.
What is the difference between a HashSet and a LinkedHashSet in Java?
The difference is that HashSet is an unordered collection of unique elements, whereas LinkedHashSet is an ordered collection of unique elements that maintains the order in which they were added to the set.
What is the purpose of the EnumSet class in Java?
The EnumSet class is a specialized Set implementation designed to work with enum types. It provides a compact and efficient way to store and manipulate sets of enum values.
What is the purpose of the BitSet class in Java?
The BitSet class is a specialized collection of bits that can be manipulated using bitwise operations. It is often used to represent sets of boolean values or to perform efficient bitwise operations.
What is the purpose of the SortedSet interface in Java?
The SortedSet interface is a subinterface of the Set interface that guarantees that the elements are stored in a sorted order. The order is determined either by the natural ordering of the elements or by a custom Comparator.
What is the difference between Arrays.asList() and Collections.singletonList() in Java?
The difference is that Arrays.asList() returns a fixed-size List backed by the specified array, whereas Collections.singletonList() returns a List that contains a single element.
What is the purpose of the Arrays.sort() method in Java?
The Arrays.sort() method is used to sort an array of objects in ascending order. It can be used with any array that implements the Comparable interface.
Learn more at: Using the sort() method from ArrayList class.
What is the purpose of the Collections.sort() method in Java?
The Collections.sort() method is used to sort a List of objects in ascending order. It can be used with any List that implements the Comparable interface.
Learn more at: Using the sort() method from Collections class.
What are Java Streams?
Java Streams are a sequence of data elements that can be processed in a declarative manner. They are designed to allow for parallel processing of data and can be used to efficiently process large amounts of data.
Learn more at: Java Stream API Tutorial.
What are the different types of Java Streams?
There are two types of Java Streams: intermediate streams and terminal streams. Intermediate streams allow you to apply operations to transform the data, while terminal streams are used to perform a final action on the data, such as counting or collecting the elements into a list.
Learn more at: What are the Core Stream API Operations?
How do you create a Stream in Java?
You can create a Stream in Java using the stream() or parallelStream() method of a Collection, or by calling the Stream.of() method with a set of elements.
Learn more at: How to create a Stream of Integers?
What are some common Stream operations?
Some common Stream operations include filtering, mapping, sorting, reducing, and collecting. These operations allow you to transform, filter, and process the data in the stream to get the desired output.
How do you perform a parallel operation on a Stream?
To perform a parallel operation on a Stream, you can call the parallel() method on the stream. This allows the processing of data to be split across multiple threads, which can improve performance for large amounts of data.
Learn more at: How to create a Parallel Stream in Java?
What is the difference between a Stream and a Collection in Java?
The difference is that a Stream is not a data structure, but rather a sequence of data elements that can be processed in a declarative manner. A Collection is a data structure that holds a collection of elements. While a Stream can be created from a Collection, they serve different purposes and have different properties.
Learn more at: Streams VS Collections in Java.
What is lazy evaluation in Java Streams?
Lazy evaluation is a feature of Java Streams where the intermediate operations are only executed when a terminal operation is called on the Stream. This can help improve performance by avoiding unnecessary operations on the data.
What is the difference between a stateful and stateless intermediate operation?
The difference is that a stateless intermediate operation does not depend on the order or state of the elements in the Stream, while a stateful intermediate operation does. Stateful operations can have performance implications when used in parallel Streams.
What is a Collector in Java Streams?
A Collector is used to combine the elements of a Stream into a result, such as a List or Map. Collectors can be used to perform a final action on a Stream.
What is the purpose of the flatMap() operation in Java Streams?
The flatMap() operation in Java Streams is used to transform a Stream of collections or arrays into a flat Stream of elements. This is useful when you need to process data in a nested data structure.
Learn more at: Streams – flatMap() Operation.
What is the difference between findFirst() and findAny() in Java Streams?
findFirst() returns the first element in a Stream, while findAny() returns any element in the Stream. When used with parallel Streams, findAny() can be faster because it does not need to return the first element in the Stream.
Learn more at: Streams – findFirst() Operation. / Streams – findAny() Operation.
What is the purpose of the reduce() operation in Java Streams?
The reduce() operation is used to combine the elements of a Stream into a single result. It takes a binary operator that takes two elements and returns a single element, and applies this operator to the elements in the Stream in a cumulative manner.
Learn more at: Streams – reduce() Operation.
What is the purpose of the distinct() operation in Java Streams?
The distinct() operation in Java Streams is used to remove duplicate elements from a Stream. It uses the equals() method of the elements to determine if they are duplicates.
Learn more at: Streams – distinct() Operation.
What is the purpose of the sorted() operation in Java Streams?
The sorted() operation in Java Streams is used to sort the elements in the Stream according to a specified order. It can use a natural ordering of the elements, or a Comparator can be provided to specify a custom order.
Learn more at: Streams – sorted() Operation.
Conclusion
Throughout this page, we have delved into a variety of important Java interview questions that are commonly encountered by intermediate developers during their job search. By now, you should have a solid grasp on essential concepts in Java programming, such as functional programming, streams API, and data structures.
It’s crucial to be well-prepared for job interviews, and having the ability to confidently answer these types of questions can give you a competitive advantage in the job market. Make sure to take advantage of resources like the Java Tutorials page, which provides all the necessary tools to help you prepare effectively. Keep practicing, keep learning, and best of luck in your future interviews!
Frequently asked questions
- How do I take a good Java interview?
To perform well in a Java interview, it’s essential to have a solid understanding of Java programming concepts, data structures, algorithms, and object-oriented programming principles. You should also be familiar with common Java libraries and frameworks, as well as the latest trends and updates in the Java ecosystem. Practicing coding challenges and reviewing frequently asked Java interview questions can help you feel more confident during the interview. Additionally, be prepared to discuss your previous Java development experience and how you’ve solved challenges in your previous projects. Lastly, remember to communicate effectively, be attentive, and ask clarifying questions when necessary. - Is Java OK for coding interviews?
Yes, Java is a popular choice for coding interviews. Many companies, especially larger corporations, use Java as their primary programming language for development. As a result, interviewers are often well-versed in Java and can assess a candidate’s skills and knowledge in the language more effectively. Additionally, Java’s popularity means that there are many resources available to help candidates prepare for Java coding interviews, such as online tutorials, practice problems, and mock interviews. - Are Java coding interviews more difficult than interviews for other programming languages?
It’s difficult to make a general statement on whether Java coding interviews are more difficult than interviews for other programming languages, as the level of difficulty can vary based on several factors such as the company, the position, and the interviewer’s preferences. - How old is an intermediate developer?
The term “intermediate developer” is not typically associated with a specific age range. It is more often used to describe developers who have a moderate level of experience and proficiency in a particular programming language or technology. The level of experience and proficiency required to be considered an intermediate developer can vary depending on the organization and the specific job requirements.
