If youâre interviewing for a Java programming role, then your coding skills will probably be tested. This article has some common Java interview questions and answers that can help you get ready, no matter how much you know about the language.
There is no reverse() utility method in the String class. You can, however, turn the string into a character array and then read it from beginning to end. You can append the characters to a string builder and finally return the reversed string.
Bonus points for adding null check in the method and using StringBuilder for appending the characters. Note that the indexing in Java starts from 0, so you need to start at chars. length – 1 in the for loop.
Swapping numbers without using a third variable is a three-step process thatâs better visualized in code:
The following example code shows how to use a regular expression to check whether the string contains vowels:
You can make a program that takes the given number n, divides it by any number from 2 to n/2, and then checks the result. If the remainder is 0, then itâs not a prime number. The following example code shows one way to check if a given number is a Prime number:
Although this program works, itâs not very memory and time-efficient. There is a prime number M between 2 and â¢N (square root of N) that divides a given number N evenly. This means that N is not a prime number.
A Fibonacci sequence is one in which each number is the sum of the two previous numbers. In this example, the sequence begins with 0 and 1. The following example code shows how to use a for loop to print a Fibonacci sequence:
You can also print a Fibonacci sequence with recursion, since the Fibonacci number is made by adding the two numbers that came before it:
You can use recursion to figure out a 10-number Fibonacci sequence in the following example class:
If the list is long, you can use parallel stream to process it faster, as shown in the code below:
Go to Wikipedia and look up the modulo operation to learn more about the math behind finding out if an integer is odd.
A palindrome string is the same string backwards or forwards. Check for a palindrome by putting the string backwards and seeing if the result is the same as the string you put in. The following example code shows how to use the String charAt(int index) method to check for palindrome strings:
The following example code shows one way to remove spaces from a string using with the Character.isWhitespace() method:
The String class contains two methods to remove leading and trailing whitespaces: trim() and strip(). The strip() method was added to the String class in Java 11. The strip() method uses the Character. isWhitespace() method to check if the character is a whitespace. This method works with Unicode code points, while the trim() method sees as a whitespace character any character whose codepoint value is less than or equal to U 0020.
The strip() method is the recommended way to remove whitespaces because it uses the Unicode standard. The following example code shows how to use the strip() method to remove whitespaces:
The Arrays utility class has many overloaded sort() methods to sort primitive and to object arrays. If you are sorting a primitive array in the natural order, then you can use the Arrays. sort() method, as shown in the following example:
However, if you want to sort an array of objects, then the object must implement the Comparable interface. If you want to specify the sorting criteria, then you can pass the Comparator for the sorting logic. Learn more about Comparable and Comparator in Java.
Deadlock is a scenario in a multi-threaded Java environment where two or more threads are blocked forever. The deadlock situation arises with at two or more threads. The following example code creates a deadlock scenario:
All three threads will be able to acquire a lock on the first object. They are, however, using shared resources and are set up in a way that means they will never be able to get the lock on the second object. You can use the Java thread dump to detect the deadlocks. Learn more about deadlock in Java.
The factorial of an integer is calculated by multiplying all the numbers from 1 to the given number:
The following example code shows how to use recursion to find the factorial of an integer:
LinkedList descendingIterator() returns an iterator that iterates over the element in reverse order. This iterator can be used to make a new Linked List with elements listed in the opposite order, as shown in the code below:
The array elements must be sorted to implement binary search. The binary search algorithm is based on the following conditions:
Merge sort is one of the most efficient sorting algorithms. It works on the principle of âdivide and conquerâ. It works by splitting a list into smaller lists until each list only has one item, then joining those smaller lists together in a way that makes the main list sorted. The following example code shows one way to use merge sort:
Pattern programs are a very popular interview topic. This type of question is used to understand the logical thinking abilities of the interviewee. Refer to Pyramid Pattern Programs in Java for examples of different ways to create pyramid patterns.
To find out if two arrays have the same elements, you must first make a set of elements from each array. Then, you must compare these sets to see if there is an element that is missing from both sets. The following example code shows how to check if two arrays only contain common elements:
Here’s how to use a for loop to go through the elements of an array and add them all up:
There are many ways to solve this problem. You can sort the array in natural ascending order and take the second last value. However, sorting is an expensive operation. You can also use two variables together to find the second biggest number in a single loop, as this example shows:
The code below shows how to use the Random class to mix up the elements and make random index numbers:
To read the file’s contents line by line with the Scanner class, use the code below. Then, use the String contains() method to see if the string is in the file:
Keep in mind that the code example assumes that the string you’re looking for in the file doesn’t have any newline characters in it.
The following example code shows how to use the SimpleDateFormat class to format the date string:
The following example code shows how to use the addAll() method to merge multiple lists in Java:
HashMap is not an ordered collection. Here’s some code that sorts the entries by value and stores them in a LinkedHashMap that remembers the order in which they were added:
The String class doesnât have a method to remove characters. The code below shows how to use the replace() method to make a new string that doesn’t have the given character:
String is immutable in Java. All methods that work with strings return a new string, which is why you need to link it to another variable. Learn more about removing characters from a string in Java.
You can create the character array from the string. Then iterate over it and create a HashMap with the character as key and their count as value. The following example code shows how to extract and count the characters of a string:
The code below shows how to show that a String object can’t be changed, and the comments in the code explain each step:
The code below shows how to use the keyword “extends” to make a child class of the Animal class. The Animal class’s variable is passed on to the new Cat class, which also adds some code that is unique to Cat.
The diamond problem happens when a class inherits from more than one class, and ambiguity happens when it’s not clear which class’s method to run. Java doesnât allow extending multiple classes to avoid the diamond problem illustrated by the following example:
As of Java 7, you can also catch more than one exception in the same catch block, as this example shows. Itâs useful when you have the same code in all the catch blocks.
A NullPointerException will be thrown if you call a function on null, as shown in the code below:
You should have null check in place for early validation, as shown in the following example code:
Records was added as a standard feature in Java 16. Records enable you to create a POJO class with minimal code. Records automatically generates hashCode(), equals(), getter methods, and toString() method code for the class. Records are final and implicitly extend the java. lang. Record class. The following example code shows one way to cerate a record:
Learn more about records in Java. For details about POJO, refer to Plain old Java object on Wikipedia.
Java 15 added the text blocks feature. You can create multiline strings using text blocks. That’s right, the multiline string needs to be written between two triple-double quotes, like this:
The switch expressions were added as a standard feature in Java 14. The following examples show switch expressions as well as multi-label case statements:
In newer versions, the java command will also compile the program even if the class file is missing. If the class is in a package, such as com. example, then it should be inside the folder com/example. The command to compile and run is:
If your class requires some additional JARs to compile and run, you can use the java -cp option. For example:
ThreadStates is the enum with fixed constants fields START, RUNNING, WAITING, and DEAD. All enums implicitly extend the java. lang. Enum class and implement the Serializable and Comparable interfaces. Enum can have methods also. Learn more about enums in Java.
The forEach() method provides a shortcut to perform an action on all the elements of an iterable. The following example code shows how to iterate over the list elements and print them:
To make the code smaller, you can use the forEach() method with a lambda expression, like in the code below:
Java 8 introduced default and static methods in interfaces. This bridged the gap between interfaces and abstract classes. The following example code shows one way to write an interface with the default and static method:
An interface with exactly one abstract method is called a functional interface. The best thing about functional interfaces is that you don’t have to use big anonymous classes to set them up; instead, you can use lambda expressions. The @FunctionalInterface annotation indicates a functional interface, as shown in the following example code:
Runnable is an excellent example of a functional interface. You can use lambda expressions to create a runnable, as shown in the following example code:
When a class has two or more methods with the same name, they are called overloaded methods. The following example code shows as overloaded method called print:
When a superclass method is also implemented in the child class, itâs called overriding. The following example code shows how to annotate the printname() method thatâs implemented in both classes:
The given statement’s result is false because the operator comes before the == operator. So the given expression is evaluated to âs1 == s2 is:abcâ == âabcâ, which is false.
The output of the given statement is ourn. The first character is automatically type cast to int. Then, since the first character index is 0, it will start from o and print until n. Keep in mind that the String substring method makes a substring that starts at index start and goes to index end -1.
The size of the shortSet is 100. Because Java has autoboxing, the expression i, which has the primitive type short, turns into a Short object. Similarly, the expression i – 1 has the primitive type int and is autoboxed to an Integer object. Since there is no Integer object in the HashSet, nothing is removed and the size is 100.
No output. If the flag is true, this code will create an endless loop. If the flag is false, the program will run. The finally block will never be reached.
The given print statement will throw a java. lang. NullPointerException because the OR logical operator evaluates both the literals before returning the result. Since str is null, the . equals() method will throw an exception. It is always best to use short-circuit logical operators like || and In this case, since the first literal would return true, it would skip the second literal evaluation.
The x. concat(y) creates a new string but is not assigned to x, so the value of x is not changed.
It might look like this question is about the order in which the math operators are run, but it’s really about noticing that the main method wasn’t declared static.
This code results in a compile time error. The exception IOException is already caught by the alternative Exception.
You can use this list of 50 Java programming interview questions, which range from easy to hard, to help you prepare for your interview.
Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.
Methods are an integral part of programming across languages like Java, Python, C++ and more They encapsulate reusable logic and allow developers to break complex problems into smaller, more manageable pieces Given their ubiquity in software development, methods programming is a common area of focus in technical interviews for developer roles.
This comprehensive guide explores some of the most frequently asked methods programming interview questions that assess your conceptual knowledge as well as hands-on coding skills. Mastering these questions is key to excelling in your next big coding interview.
Basic Method Interview Questions
Here are some common introductory level questions on methods that interviewers often use to gauge your core understanding:
- What is the difference between a method and a function?
A function and a method may seem identical at first glance but they have nuanced differences based on their usage in procedural vs object oriented programming. A function is standalone logic that can be invoked independently. A method is tied to a class, allowing objects to perform certain actions.
- How do methods improve code reusability?
Methods bundle logic into reusable sections instead of repeating code everywhere. Parameterization allows customization of method behavior based on arguments. Generalized methods work across different scenarios. These factors enhance reusability.
- What are some principles for good method design?
Single responsibility principle – do one thing well Interface segregation principle – don’t force clients to depend on unused interfaces Dependency inversion principle – high level modules shouldn’t depend on low level ones.
- How are errors handled within a method?
Using try-catch blocks. Place risky code in try block and handle errors in catch block. The optional finally block runs regardless of exception.
Method Types
You need clarity on the different categories of methods like:
- What is method overloading and method overriding?
Overloading is same method name but different parameters. Overriding is subclasses providing their own implementation of superclass methods.
- What are the differences between static and instance methods?
Static methods belong to the class. Instance methods belong to objects and can access state. Use static for state-independent logic. Use instance for state-dependent logic.
- When would you use a virtual method?
When subclass behavior needs to override superclass behavior. For example, different types of accounts having their own withdrawal logic.
Method Parameters
Knowing how to correctly handle method parameters is a must-have skill:
- How do you pass parameters by reference vs by value?
Pass by reference sends the address, allowing changes to impact original variable. Pass by value sends a copy, so changes are local.
- What are some best practices for parameter passing?
Limit side effects by avoiding stateful parameters when possible. Validate arguments at start of method to fail fast. Document expectations for parameters in docstrings.
- How can you use method parameters to modify data?
Pass in data as a parameter, operate on it within method logic, return modified data. For example, doubling values in a number array.
Method Access and Scope
Brush up on your knowledge of method visibility and encapsulation:
- What is the difference between public and private methods?
Public methods can be called from anywhere. Private methods can only be called within the same class. Private limits external dependencies.
- When would you use a protected method?
When subclass needs access to superclass method but we want to limit widespread access. Protected enables controlled sharing with subclasses only.
- How do getters and setters enable encapsulation?
Getters access properties and setters change properties. By making fields private and using getters/setters, we control access. This is encapsulation.
Inheritance and Polymorphism
Object oriented programming concepts like inheritance and polymorphism rely heavily on methods:
- How does method overriding allow polymorphism?
Subclass overrides the superclass method signature with its own implementation. The appropriate version is called based on object type enabling polymorphism.
- Explain method resolution order in inheritance hierarchies.
MRO ensures subclasses override before superclasses. For multiple inheritance, it linearizes hierarchy while respecting superclass precedence.
- When does Python use magic methods like init?
Magic methods customize classes. init initializes objects, str returns string representation, add overloads + operator etc.
Asynchronous Methods
Asynchronous programming is powered by callback methods:
- When would you use a callback method?
Pass a callback to be executed after an async operation finishes. For example, handling results of file reading after read completes.
- What are the differences between synchronous and asynchronous methods?
Synchronous methods block further execution until completion. Asynchronous allows concurrency and doesn’t block.
Best Practices
Follow these method best practices to write optimized code:
- How can you improve method performance?
Reduce network calls. Minimize object allocations. Reuse objects instead of recreating. Use efficient data structures and algorithms. Cache when possible.
- What are some method debugging tips?
Log key info. Use debugger breakpoints. Refactor into smaller methods. Validate arguments and outputs. Add useful comments. Handle errors properly.
- How do you document a method effectively?
Explain method behavior clearly. Define parameters and return value. Give usage examples. Note edge cases. Specify performance and restrictions.
This comprehensive guide covers the key methods programming concepts assessed during interviews. Studying these questions and answers will ensure you have the right method knowledge to tackle any curveballs thrown your way and land your dream dev job!
Get our biweekly newsletter
Sign up for Infrastructure as a Newsletter.
Try DigitalOcean for free
Click below to sign up and get $200 of credit to try our products over 60 days!
Please complete your information!
Whiteboard Coding Interviews: 6 Steps to Solve Any Problem
FAQ
What kind of coding questions are asked in an interview?
What is the STAR method in interviewing?
What to expect in a 1 hour coding interview?
What are beginner Java interview questions?
Beginner Java interview questions are designed to test your knowledge of basic Java principles and methods. Here are some common questions and their answers. What makes Java different from some other programming languages? Java is: What is “inheritance” in Java? Inheritance is a core principle in object-oriented programming.
How do I prepare for a Java interview?
We also recommend you brush up on your Java skills with this Java Cheat Sheet before starting your Java interview preparation. We have divided these interview questions into several sections. Check the breakdown below to review basic, advanced, OOPs, exception handling, and programming Java interview questions.
What are intermediate Java interview questions?
Intermediate Java interview questions are meant to see if you both understand and know how to apply some of the essential Java principles and tools. Below are some of the more common mid-level Java interview questions. The Spring framework is used to make enterprise applications using Java.
What does a programming interview entail?
Programming interviews test your knowledge of computer science fundamentals and assess your knowledge of the foundations of programming logic and problem-solving skills. Knowledge of data structures and algorithms is essential, as well as familiarity with the programming language of your choice.
