Need to interview a Java developer for a freelance project or job? Here are 37 essential Java interview questions and answers provided by some of our top Java experts here at Arc.<\/p>\n\n\n\n
Although technical interviews can\u2019t gauge how well a candidate would perform on a real-life project, this is still an integral part of the hiring process. Here are some Java interview questions that you can ask a developer to evaluate their understanding of the language.<\/p>\n\n\n\n
Looking to hire the best remote developers? Arc can help you:<\/em><\/p>\n\n\n\n \u26a1\ufe0f Get instant candidate matches without searching Try Arc to hire top developers now \u2192<\/strong><\/em><\/a><\/p>\n\n\n\n However, the methods This is one of those Java interview questions where some people will be thinking what!?<\/em> But, I\u2019ve met a lot of Java developers who\u2019ve not run a Java application outside of an IDE for years.<\/p>\n\n\n\n This is a tricky Java interview question\u2026 it doesn\u2019t have to be!<\/p>\n\n\n\n Garbage Collection simply cleans up unused memory when an object goes out of scope and is no longer needed. However, an application could create a huge number of large objects that causes an OutOfMemoryError.<\/p>\n\n\n\n A ClassNotFoundException means the class file for a requested class is not on the classpath of the application. A NoClassDefFoundErrormeans that the class file existed at runtime, but for some reason the class could not be turned into a Class definition. <\/p>\n\n\n\n A common cause is an exception being thrown in static initialization blocks.<\/p>\n\n\n\n Struggling with interview prep? Meet senior developers from Amazon, Microsoft, and Google now on Codementor. They\u2019ll help you tackle coding challenges, practice interviews, and sharpen your skills in live 1:1 sessions. Explore our other software development interview questions and answers to prep for your next remote job.<\/p>\n\n\n\n It isn\u2019t accurate because it will only account for the number of characters within the String. In other words, it will fail to account for code points outside of what is called the BMP (Basic Multilingual Plane), that is, code points with a value of The reason is historical: when Java was first defined, one of its goal was to treat all text as Unicode; but at this time, Unicode did not define code points outside of the BMP. By the time Unicode defined such code points, it was too late for char to be changed.<\/p>\n\n\n\n This means that code points outside the BMP are represented with two chars in Java, in what is called a surrogate pair<\/strong>. Technically, a char in Java is a UTF-16 code unit.<\/p>\n\n\n\n The correct way to count the real numbers of characters within a String, i.e. the number of code points, is either:<\/p>\n\n\n\n or, with Java 8:<\/p>\n\n\n\n Because of This code:<\/p>\n\n\n\n will print The most accurate way to tell whether two double values are equal to one another is to use There are, in fact, two problems:<\/p>\n\n\n\n While the second problem is rarely a concern, the first certainly is a concern.<\/p>\n\n\n\n For instance, in most Windows installations, the default charset is As such, such a simple string as \u201c\u00e9\u201d will give a different result for this operation depending on whether this code is run on Windows or Linux.<\/p>\n\n\n\n The solution is to always specify a Charset, as in, for instance:<\/p>\n\n\n\n The what is the problem with this code?<\/em> question is one of the most popular Java interview questions, but it’s not necessarily going to be this one above, of course. Be prepared to do some detective work to identify the issue.<\/p>\n\n\n\n Also, keep in mind: while the problem may be exception handling, method overloading, an access specifier issue, or something else, it could also be nothing at all! This is one of those trick Java interview questions where the answer will rely on your gut that everything is perfect with the code already.<\/p>\n\n\n\n The JIT is the JVM\u2019s mechanism by which it can optimize code at runtime.<\/p>\n\n\n\n JIT means Just In Time. It is a central feature of any JVM. Among other optimizations, it can perform code inlining, lock coarsening or lock eliding, escape analysis etc.<\/p>\n\n\n\n The main benefit of the JIT is on the programmer\u2019s side: code should be written so that it just works; if the code can be optimized at runtime, more often than not, the JIT will find a way.<\/p>\n\n\n\n (On a more advanced note: the JIT is such a complex piece of machinery that it makes it complicated to do accurate performance benchmarks for JVM code; this is why such frameworks as JMH exist.)<\/p>\n\n\n\n This code:<\/strong><\/p>\n\n\n\n prints The problem here is that this expression:<\/p>\n\n\n\n has integer literals on both sides of the operator: In order for the result to be a double as expected, at least one operand of the operation needs to be a double. For instance:<\/p>\n\n\n\n or:<\/p>\n\n\n\n In this code:<\/strong><\/p>\n\n\n\n what is the inferred type of the method reference It is an The problem is that the Stream returned by This should be used instead:<\/p>\n\n\n\n Consider the following piece of code:<\/strong> What will be the contents of the list after this operation and why?<\/strong><\/p>\n\n\n\n The contents will be:<\/p>\n\n\n\n The reason is that there are two removal operations on a List:<\/p>\n\n\n\n The JVM will always select the most specific overload of a method; and here we pass an int as an argument, the code therefore removes the element at index 2.<\/p>\n\n\n\n To remove the _element_ 2 from the list, the following needs to be written:<\/p>\n\n\n\n This is my go-to first interview question. It helps me gauge a candidate\u2019s ability to understand a problem and write an algorithm to solve it.<\/p>\n\n\n\n If someone has not solved the problem before, I expect to see some code with loops and if\/then\u2019s. Maybe some The naive solution is often to loop through the letters of the first string and see if they\u2019re all in the second string. The next thing to look for is that the candidate should also do that in reverse too (check string 1 for string 2\u2019s letters)? The next thing to look for is, what about strings with duplicate letters, like VASES?<\/p>\n\n\n\n If you can realize that these are all required and create a functional, non-ridiculous solution, I am happy.<\/p>\n\n\n\n Of course, one can solve it trivially by sorting and comparing both strings. If someone catches this right away, they usually have seen the problem before. But that\u2019s a good sign that someone cares enough to do prep work. Then we can tackle a harder problem.<\/p>\n\n\n\n The details of the implementation are not important; what\u2019s important is that the candidate understands what they need to do, and also understands why their solution works or doesn\u2019t work. If the candidate can demonstrate this, they’re on the right track.<\/p>\n\n\n\n Here is one way to implement a better solution, comparing sorted strings:<\/p>\n\n\n\n
\u26a1\ufe0f Identify top applicants from our network of 350,000+
\u26a1\ufe0f Hire 4x faster with vetted candidates (qualified and interview-ready)<\/em><\/p>\n\n\n\nBasic Java Interview Questions<\/h2>\n\n\n\n
1. What\u2019s the difference between
String<\/code>, StringBuffer<\/code>, and StringBuilder<\/code>?<\/h3>\n\n\n\nString<\/code> is an immutable class. In older JDKs the recommendation when programmatically building a String was to use StringBuffer<\/code> since this was optimized to concatenate multiple Strings together. <\/p>\n\n\n\nStringBuffer<\/code> were marked as synchronized, which meant that there was a performance penalty, hence StringBuilder<\/code> was introduced to provide a non-synchronized way to efficiently concatenate and modify Strings<\/code>.<\/p>\n\n\n\n2. How do you run a Java application on the command line and set the
classpath<\/code> with multiple jars?<\/h3>\n\n\n\njava -cp \/dev\/myapp.jar:\/dev\/mydependency.jar com.arc.MyApp<\/code><\/pre>\n\n\n\n3. What is the difference between
final<\/code>, finalize<\/code> and finally<\/code>?<\/h3>\n\n\n\nfinal<\/code> is a Java keyword used to indicate that either a method can not override in a subclass, or a class can not be extended or a field can not be modified. finalize<\/code> is a method that gets called on an instance of an Object when it is garbage collected. finally<\/code> is a Java keyword used in exception handling to indicate a block of code that should always be run whether an exception is thrown or not.<\/p>\n\n\n\n4. How does Garbage Collection prevent a Java application from going out of memory?<\/h3>\n\n\n\n
5. What\u2019s the difference between a
ClassNotFoundException<\/code> and NoClassDefFoundError<\/code>?<\/h3>\n\n\n\n
\n\n\n\n
Book a session with our interview prep tutors<\/a> today! Your first 15 minutes are free.<\/strong><\/p>\n\n\n\n\n
\n
\n\n\n\n6. Why isn\u2019t
String<\/code>\u2018s .length() accurate?<\/h3>\n\n\n\nU+10000<\/code> or greater.<\/p>\n\n\n\nsomeString.codePointCount(0, someString.length())<\/code><\/pre>\n\n\n\nsomeString.codePoints().count()<\/code><\/pre>\n\n\n\n7. Given two double values
d1<\/code>, d2<\/code>, why isn\u2019t it reliable to test their equality using:<\/h3>\n\n\n\nd1 == d2<\/code><\/pre>\n\n\n\nDouble.NaN<\/code> (literally: \u201cNot a Number\u201d).<\/p>\n\n\n\nfinal<\/strong> double<\/strong> d1 = Double.NaN;\nfinal<\/strong> double<\/strong> d2 = Double.NaN;\n\nSystem.out.println(d1 == d2);<\/code><\/pre>\n\n\n\nfalse<\/code>.<\/p>\n\n\n\nDouble.compare()<\/code> and test against 0, as in:<\/p>\n\n\n\nSystem.out.println(Double.compare(d1, d2) == 0);<\/code><\/pre>\n\n\n\n8. What is the problem with this code:<\/h3>\n\n\n\n
final<\/strong> byte<\/strong>[] bytes = someString.getBytes();<\/code><\/pre>\n\n\n\nCP1252<\/code>; but on Linux installations, the default charset will be UTF-8.<\/p>\n\n\n\nfinal byte[] bytes = someString.getBytes(StandardCharsets.UTF_8);<\/code><\/pre>\n\n\n\n9. What is the JIT?<\/h3>\n\n\n\n
10. How do you make this code print
0.5<\/code> instead of 0<\/code>?<\/h3>\n\n\n\nfinal<\/strong> double<\/strong> d = 1 \/ 2;\n\nSystem.out.println(d);<\/code><\/pre>\n\n\n\n0<\/code>. Why? How do you make this code print 0.5<\/code> instead?<\/strong><\/p>\n\n\n\n1 \/ 2<\/code><\/pre>\n\n\n\n1<\/code> and 2<\/code>. As a consequence, an integer division will be performed, and the result of 1<\/code> divided by 2<\/code> in an integer division is 0<\/code>.<\/p>\n\n\n\nfinal<\/strong> double<\/strong> d = 1 \/ 2.0;<\/code><\/pre>\n\n\n\nfinal<\/strong> double<\/strong> d = 1.0 \/ 2;<\/code><\/pre>\n\n\n\n11. What is the inferred type of the method reference
System.out::println?<\/code><\/h3>\n\n\n\nIntStream.range(0, 10).forEach(System.out::println);<\/code><\/pre>\n\n\n\nSystem.out::println?<\/code><\/strong><\/p>\n\n\n\nIntConsumer<\/code>.<\/p>\n\n\n\nIntStream.range(0, 10)<\/code> returns an IntStream<\/code>, and IntStream<\/code> defines a .forEach() method accepting an IntConsumer as an argument, whose prototype is:<\/p>\n\n\n\nvoid<\/strong> accept<\/strong>(int<\/strong> value);\n<\/code><\/pre>\n\n\n\nSystem.out<\/code> is a PrintStream<\/code>, and a PrintStream<\/code> has a method named println<\/code> which takes an int as an argument and returns void. This matches the signature of an IntConsumer<\/code>, hence the result.<\/p>\n\n\n\n12. What is the problem with this code?<\/h3>\n\n\n\n
final<\/strong> Path path = Paths.get(...);\n\nFiles.lines(path).forEach(System.out::println);<\/code><\/pre>\n\n\n\nFiles.lines()<\/code> is not closed.<\/p>\n\n\n\ntry<\/strong> (\n final<\/strong> Stream<String> stream = Files.lines(path);\n) {\n stream.forEach(System.out::println);\n}<\/code><\/pre>\n\n\n\nStream<\/code> extends BaseStream<\/code>, and BaseStream<\/code> extends AutoCloseable<\/code>. While this has no influence on streams you obtain from collections for instance, the stream returned by Files.lines()<\/code> is I\/O bound. Neglecting to close it correctly may lead to a resource leak if an error occurs while processing the stream.<\/p>\n\n\n\n13. What will be the contents of a list after a given operation and why?<\/h3>\n\n\n\n
(Question provided by Francis Galiegue)<\/em><\/p>\n\n\n\nfinal<\/strong> List<Integer> list = new<\/strong> ArrayList<>();\n\nlist.add(1);\nlist.add(2);\nlist.add(3);\n\nlist.remove(2);<\/code><\/pre>\n\n\n\n[ 1, 2 ]<\/code><\/pre>\n\n\n\nlist.remove(Integer.valueOf(2));<\/code><\/pre>\n\n\n\n14. Write a function to detect if two strings are anagrams (for example, SAVE and VASE)<\/h3>\n\n\n\n
HashMaps<\/code>. I look for the ability to break down the problem to see what you need to check, what the edge cases are, and whether the code meets those criteria.<\/p>\n\n\n\npublic<\/strong> static<\/strong> boolean<\/strong> isAcronym<\/strong>(String s1, String s2) {\n\n if<\/strong> (s1.length() != s2.length()) return<\/strong> false<\/strong>;\n\n HashMap<Character, Integer> charCounts = new<\/strong> HashMap<>();\n\n \/\/ Calculate chracter counts<\/em>\n\n for<\/strong> (int<\/strong> i = 0; i < s1.length(); i++) {\n if<\/strong> (charCounts.containsKey(s1.charAt(i))) {\n charCounts.put(s1.charAt(i), charCounts.get(s1.charAt(i)) + 1);\n } else<\/strong> {\n charCounts.put(s1.charAt(i), 1);\n }\n }\n\n \/\/ Compare counts with characters in s2<\/em>\n\n for<\/strong> (int<\/strong> i = 0; i < s2.length(); i++) {\n if<\/strong> (charCounts.containsKey(s2.charAt(i))) {\n charCounts.put(s2.charAt(i), charCounts.get(s2.charAt(i)) - 1);\n } else<\/strong> {\n return<\/strong> false<\/strong>;\n }\n }\n\n \/\/ Check all letters matched<\/em>\n for<\/strong> (int<\/strong> count : charCounts.values()) {\n if<\/strong> (count != 0) return<\/strong> false<\/strong>;\n }\n\n return<\/strong> true<\/strong>;\n}<\/code><\/pre>\n\n\n\npublic<\/strong> static<\/strong>