Latest Oracle Java Developer Interview Questions (May 2025)

If you’re applying for a Java developer role—especially at Oracle or any big tech company—you better be ready to face a mix of core Java, OOP concepts, frameworks, and even some tricky logic-based questions. But don’t stress, I’ve got you covered with this list of Oracle Java Developer interview questions, plus super simple explanations to help you prep smarter.

What’s the difference between JDK, JRE, and JVM?

This one is super common. Here’s how to remember:

• JDK (Java Development Kit): It’s the full package. Contains everything you need to write and run Java code (compiler, debugger, tools, etc).
• JRE (Java Runtime Environment): Only needed to run Java apps. No tools for writing them.
• JVM (Java Virtual Machine): This guy actually runs your Java code. Converts bytecode into machine code.

TL;DR: JDK = JRE + development tools. JVM is where the magic happens.

What is the difference between == and .equals() in Java?

• == checks if two references point to the same object in memory.
• .equals() checks if the values inside the objects are the same (depends on how it’s overridden).

String a = new String("hello");
String b = new String("hello");
System.out.println(a == b); // false
System.out.println(a.equals(b)); // true

What are the main OOP principles in Java?

Oracle loves this one.

• Encapsulation: Hiding data with private fields + public getters/setters.
• Inheritance: One class inherits another.
• Polymorphism: One method, different behavior.
• Abstraction: Hiding implementation, showing only what’s necessary.

Explain with your own words + real-life example if asked.

What is the difference between abstract class and interface?

• Abstract class: Can have method implementations + abstract methods.
• Interface: Only method signatures (though from Java 8+, you can have default/static methods).

Use interface when you just want to define capability (like Runnable, Comparable).

What’s a checked vs unchecked exception?

• Checked Exception: Caught or declared (like IOException, SQLException).
• Unchecked Exception: Runtime errors (like NullPointerException, ArrayIndexOutOfBounds).

Oracle may ask which one is better to use—depends on use case. Checked forces you to handle errors.

What is the use of the final keyword?

It can be used with:

• Variable: makes it a constant
• Method: cannot be overridden
• Class: cannot be subclassed

Helps write safe and secure code.

Explain the Java Memory Model (Heap, Stack, etc.)

• Heap: Stores objects
• Stack: Stores method calls + local variables
• Method Area: Stores class definitions
• GC (Garbage Collector): Frees memory automatically

Oracle might go deep here, especially on garbage collection strategies.

What are Lambda Expressions?

Introduced in Java 8. Shorter way to write anonymous methods.

(int a, int b) -> a + b

Helps with writing cleaner code for functional programming, especially with Streams API.

What is a Stream in Java?

Part of Java 8. Used to process collections easily (map, filter, reduce, etc.).

Example:

List<String> names = Arrays.asList("Tom", "Jerry", "Bob");
names.stream().filter(name -> name.startsWith("T")).forEach(System.out::println);

Expect follow-up questions if you bring this up.

Explain multithreading in Java

Java supports multithreading using:

• Thread class
• Runnable interface

Let them know you understand synchronization, deadlock, and thread lifecycle. If you can explain ExecutorService or Future, that’s bonus points.

How is HashMap different from Hashtable?

• HashMap: Not synchronized, faster, allows one null key
• Hashtable: Synchronized, slower, no null keys allowed

You can also mention ConcurrentHashMap for thread-safe performance.

What is the difference between ArrayList and LinkedList?

• ArrayList: Faster in accessing elements (index-based), slower in insertion/removal.
• LinkedList: Slower access, faster insertion/deletion (because it’s doubly linked).

Great question to bring up Big-O complexity.

What are design patterns used in Java?

Common ones:

• Singleton: One instance only
• Factory: Returns object based on input
• Observer: Notify observers when a change occurs
• Builder: Create complex objects step by step

What are generics in Java?

Generics allow code reusability and type safety.

List<String> list = new ArrayList<>();

Without generics, you’d need typecasting. Talk about how generics reduce runtime errors.

What’s the use of synchronized keyword?

Used to prevent race conditions by locking a block or method so only one thread can access it at a time.

Example:

synchronized void increment() {
  counter++;
}

Expect follow-ups like: how is it different from ReentrantLock?

What is garbage collection and how does it work?

Java automatically removes unused objects from memory. It uses algorithms like:

• Mark and Sweep
• Generational GC
• G1 (Garbage First)

Talk briefly about GC logs, System.gc(), and how memory leaks can still happen.

What’s the difference between static and instance variables?

• Static: Shared by all objects of the class
• Instance: Separate copy for each object

Same goes for methods—static methods can be called without creating objects.

Final Tips for Oracle Java Interview

• Know Java 8+ features like streams, lambdas, method references
• Brush up on core Java, OOP, collections, and multithreading
• Be ready for code writing and logic building
• If frameworks like Spring or Hibernate come up, be honest—say what you know

Need more help with technical Java interviews, coding problems, or even mock interviews? Just ask—I’m here for it.