1500+ Java Developer Interview Questions and Answers

The function sorts an integer array containing only 0s and 1s in linear time complexity.

• Use two pointers, one starting from the beginning and the other from the end of the array.

• Swap the elements at the pointers if the element at the beginning pointer is 1 and the element at the end pointer is 0.

• Continue moving the pointers towards each other until they meet in the middle of the array.

The order of execution will be: parent's run() -> child's run() -> parent's walk()

• When p.run() is called, it will first execute the run() method of the parent class

• Inside the parent's run() method, it will call the walk() method of the parent class

• Since the child class overrides the parent class, the child's run() method will be executed next

• Inside the child's run() method, it will call the run() method of the parent class using super.run()

• Finally, the walk() method of the pa...read more

The task is to convert a Binary Search Tree into a Greater Sum Tree.

• Traverse the BST in reverse inorder (right, root, left) to visit nodes in descending order.

• Keep track of the sum of all greater nodes encountered so far.

• Update the value of each node by adding the sum of greater nodes and update the sum.

• Continue this process until all nodes have been visited.

• Return the modified BST.

Java Developer interview questions covering array list, linkedlist, hashmap, synchronization, abstract vs interface, singleton class, Spring framework, database configuration, and Java 8 features.

• ArrayList and LinkedList differ in terms of implementation, performance, and usage

• HashMap uses hashing to store and retrieve key-value pairs

• Synchronized is used to ensure thread safety and prevent race conditions

• Abstract classes cannot be instantiated and can have both abstract and n...read more

Explaining differences between abstract class and interface, throw and throws, and why we use throws. Also, why String is immutable.

• Abstract class can have both abstract and non-abstract methods, while interface can only have abstract methods.

• A class can implement multiple interfaces, but can only extend one abstract class.

• Throw is used to explicitly throw an exception, while throws is used to declare the exceptions that a method may throw.

• We use throws to inform the caller o...read more

To sort a list of students on the basis of their First name, use the Collections.sort() method with a custom Comparator.

• Create a custom Comparator that compares the first names of the students.

• Implement the compare() method in the Comparator to compare the first names.

• Use the Collections.sort() method to sort the list of students using the custom Comparator.

If hashcode returns a constant, all elements will be stored in the same bucket, resulting in poor performance.

• All elements will be stored in the same bucket of the HashMap.

• This will lead to poor performance as the HashMap will essentially become a linked list.

• Retrieving elements will take longer as the HashMap will have to iterate through the linked list to find the desired element.

Java interview questions on OOPS concepts, polymorphism, exceptions, data structures, and threading.

• OOPS concepts include encapsulation, inheritance, and polymorphism.

• Polymorphism refers to the ability of an object to take on multiple forms.

• Singleton class is a class that can only have one instance, while factory method is a method that creates objects.

• Exception is an error that occurs during program execution, with a hierarchy of exceptions.

• Volatile keyword is used to indica...read more

Code to filter out loans with incomplete status using Java 8 features.

• Use stream() method to convert the list of loans into a stream

• Use filter() method to filter out loans with incomplete status

• Use collect() method to collect the filtered loans into a new list

This program returns the length of the longest word from a string whose length is even.

• Split the string into an array of words using a space as the delimiter.

• Iterate through the array and check if the length of each word is even.

• Keep track of the length of the longest even word encountered.

• Return the length of the longest even word.

The interview questions cover various topics related to Java development, including Singleton pattern, memory management, exception handling, Spring framework, and design patterns.

• Singleton pattern ensures a class has only one instance and provides a global point of access to it.

• Auto Configuration in Spring Boot automatically configures the Spring application based on dependencies present in the classpath.

• @Primary annotation is used to give higher preference to a bean when mu...read more

To handle a mutable object inside an immutable class, make the object private and provide only read-only access methods.

• Make the mutable object private to encapsulate it within the immutable class.

• Provide read-only access methods to retrieve the state of the mutable object.

• Ensure that the mutable object cannot be modified from outside the class.

• If necessary, create defensive copies of the mutable object to prevent unintended modifications.

Yes, we can keep an object of a Class as a key in HashMap.

• The key in a HashMap can be any non-null object.

• The key's class must override the hashCode() and equals() methods for proper functioning.

• If two objects have the same hashCode(), they will be stored in the same bucket and compared using equals().

• If the key is mutable, its state should not be modified after it is used as a key in the HashMap.

Java OOPS concepts include inheritance, polymorphism, encapsulation, and abstraction.

• Inheritance allows a class to inherit properties and methods from another class.

• Polymorphism allows objects to take on multiple forms and behave differently based on their context.

• Encapsulation hides the implementation details of a class and only exposes necessary information.

• Abstraction allows for the creation of abstract classes and interfaces that can be implemented by other classes.

• Exampl...read more

This interview question covers various topics including programming, design patterns, data structures, and exception handling in Java.

• Write a program to generate the Fibonacci series

• Write a program using boolean variables

• Explain the ClassCastException and how to handle it

• Discuss the Singleton design pattern and its implementation with different JDK versions

• Explain abstract classes and interfaces in your current project

• Explain hashing in HashMap and the number of buckets

• Differ...read more

Query to find authors who have written more than 10 books

• Use the GROUP BY clause to group the records by author

• Use the HAVING clause to filter the groups with a count greater than 10

• Join the Bookauthor table with the Author table to get the author names

Yes, Java is platform-independent because of its 'write once, run anywhere' principle.

• Java programs are compiled into bytecode, which can be executed on any platform with a Java Virtual Machine (JVM).

• The JVM acts as an interpreter, translating the bytecode into machine code specific to the underlying platform.

• This allows Java programs to run on different operating systems and hardware architectures without modification.

• For example, a Java program developed on a Windows machin...read more

JDK is a development kit, JRE is a runtime environment, and JVM is a virtual machine for executing Java code.

• JDK includes JRE and development tools like javac and java

• JRE includes JVM and necessary libraries to run Java applications

• JVM is responsible for interpreting Java bytecode and executing it

• Example: JDK 8 includes JRE 8 and development tools like javac and java

• Example: JRE 8 includes JVM 8 and necessary libraries to run Java applications

Private variables provide encapsulation and data hiding.

• Private variables can only be accessed within the class they are declared in, providing encapsulation and preventing direct modification from outside the class.

• Getters and setters provide controlled access to private variables, allowing for validation and manipulation of the data before it is accessed or modified.

• For example, a class may have a private variable for a person's age, but the getter can return a calculated a...read more

Answers to interview questions for a Java Developer position

• Q1. Mention the technologies you have worked on, such as Java, Spring, Hibernate, etc.

• Q2. List any corporate training you have attended, like Java certification courses or workshops.

• Q3. Explain how you handle non-productive team members, emphasizing communication and motivation strategies.

• Q4. Highlight your extraordinary skills that set you apart from other technical associates, such as problem-solving abilities or l...read more

Not overriding equals method can lead to incorrect comparison of objects. Not overriding hashCode method can lead to incorrect behavior in hash-based data structures.

• If equals method is not overridden, the default implementation from Object class will be used which compares object references.

• This can lead to incorrect comparison of objects where two different objects with same content are considered unequal.

• If hashCode method is not overridden, the default implementation from...read more

throw is used to explicitly throw an exception, while throws is used to declare exceptions that a method may throw.

• throw is used within a method to throw an exception explicitly

• throws is used in the method signature to declare the exceptions that the method may throw

• final is a keyword used to make a variable, method, or class unchangeable

• finally is a block that is used to execute code regardless of whether an exception is thrown or not

• finalize is a method that is called by th...read more

Java Developer interview questions on beans, functional interfaces, SQL queries, hibernate, and more.

• Scope of beans refers to the visibility of a bean within an application context.

• Functional interfaces have only one abstract method and can be used as lambda expressions.

• Classes cannot be private, but their constructors can be.

• SQL query to find employees of a particular department table: SELECT * FROM employees WHERE department = 'department_name'

• Cascading in Hibernate refers ...read more

Object-oriented programming (OOP) principles in Java include encapsulation, inheritance, polymorphism, and abstraction.

• Encapsulation: Bundling data and methods together into a single unit (class) to hide implementation details.

• Inheritance: Creating new classes (subclasses) from existing classes (superclasses) to inherit properties and behaviors.

• Polymorphism: The ability of an object to take on many forms, allowing objects of different classes to be treated as objects of a com...read more

The @Component annotation is a generic stereotype annotation that can be used in place of other annotations.

• The @Component annotation is a general-purpose annotation that can be used in place of @Service, @Controller, @Configuration, and @Repository annotations.

• However, using more specific annotations can help in better understanding the role of the annotated class.

• For example, using @Service annotation on a class that provides a service makes it easier to understand the purp...read more

Keywords for thread safe program in Java

• Synchronization using synchronized keyword

• Using volatile keyword for shared variables

• Using atomic classes for thread safe operations

• Using thread safe collections like ConcurrentHashMap

• Using locks and semaphores for synchronization

• Avoiding shared mutable state

• Using immutable objects

• Using thread local variables

Answering questions related to Java interfaces and string manipulation

• Interface is a collection of abstract methods that can be implemented by a class

• Inheritance is supported by Java, but only single inheritance is allowed

• Interfaces are used to achieve abstraction and provide a contract for implementing classes

• Advantages of program to reverse each word in a string include improved readability and searchability

• To reverse each word in a string, split the string into words, reve...read more

Using Java 8, find the sum of squares of all the odd numbers in the arraylist.

• Use the stream() method to convert the ArrayList to a stream

• Use the filter() method to filter out the even numbers

• Use the mapToInt() method to square each odd number

• Use the sum() method to find the sum of the squared odd numbers