All topics Glossary

Java Core interview questions

54 Java Core interview questions with answers, from junior to mid level.

Practice this topic

Foundations

What is Java?
What happens between writing Java code and running it?
What is the JVM (Java Virtual Machine)?
What do you install to write, compile and run Java programs?
What does LTS mean for Java versions, and which versions are LTS?
In Java, what are a class and an object?
In a Java class, what are fields and methods?
What does this code do?
What is public static void main(String[] args)?
What are a package and an import in Java?
What is a JAR file?
What is a Java interface?
What does null mean in Java, and what is a NullPointerException?
What is an IDE?

Core

What is the difference between the JDK, the JRE and the JVM?
"Write once, run anywhere" in Java is possible mainly because:
Which statement about int and Integer is correct?
What does this print?
Why should you avoid s += x for building a long string inside a big loop?
What does the keyword static mean on a method or field?
What does final do?
Which access modifier is used when you write NO keyword in front of a field or method?
What is the main difference between an interface and an abstract class in Java?
Which collection should you choose to store unique items with no duplicates?
In practice, which is the better default for a general-purpose list: ArrayList or LinkedList?
Which of these is a CHECKED exception?
What is the difference between method overloading and overriding?
Explain the four pillars of OOP with Java examples.

Intermediate

If you override equals() in a class, what else must you override, and why?
What does this print?
What is try-with-resources for?
What is type erasure in Java generics?
What does this print?
What does this print?
What is the recommended way to use Optional<T>?
What do records (Java 16+) give you with record Point(int x, int y) {}?
Where do objects live in the JVM, and what lives on the stack?
What does this print?
Why does this code throw ConcurrentModificationException?
Which is true about HashMap, Hashtable and ConcurrentHashMap?
What is the difference between synchronized and volatile?
Why use an ExecutorService (thread pool) instead of new Thread() for every task?
When would you use Comparator instead of Comparable?
Explain how a HashMap works internally.
Explain the Java memory model: heap, stack, metaspace and garbage collection.
What changed in modern Java (8 to 21) that every developer should know?

Advanced

Java has a garbage collector. How can a Java program still have a memory leak?
What problem do virtual threads (Java 21) solve?
Why is this counter NOT thread-safe even though the field is volatile?
What is the practical difference between thenApply and thenCompose in CompletableFuture?
How do you make a class truly immutable in Java?
A class overrides equals but NOT hashCode. What is the likely result?
What does a "stop-the-world" pause in garbage collection mean, and how do modern collectors reduce it?
What concurrency tools does Java give you, and when do you use each?

How it works Privacy GitHub