Which OOP principle is best described as hiding internal state and exposing behavior through methods?

Encapsulation, by keeping fields private and using accessors.

Inheritance, because a subclass hides its parent.

Polymorphism, because one method has many forms.

Abstraction, because it removes all implementation.

What does the @Override annotation give you when redefining a method from a superclass?

It makes the compiler verify the method actually overrides something.

It makes the method run faster at the JVM level.

It forces the method to be called before the parent method.

It automatically calls the superclass version for you.

Which statement about abstract classes versus interfaces in modern Java is correct?

A class can implement multiple interfaces but extend only one class.

A class can extend multiple abstract classes.

Interfaces can never contain any method body.

Abstract classes cannot have constructors.

What is autoboxing in Java?

The automatic conversion between a primitive and its wrapper class.

The JVM wrapping every object in a try-catch block.

Packaging classes into a JAR at build time.

Copying a primitive onto the heap for garbage collection.

Why does new String("hi") == "hi" evaluate to false?

== compares object references, and new creates a distinct object outside the pool.

The two strings contain different characters.

String literals are always null until assigned.

== on strings always returns false in Java.

What is the average time complexity of get(key) on a well-distributed HashMap?

O(n) because it scans every entry.

O(n log n) due to sorting on access.

Which interface should a class implement so its instances can be stored as keys in a HashMap and found reliably?

It must override both equals() and hashCode().

It must implement Comparable only.

It must implement Serializable.

What does the bounded wildcard List allow?

Reading elements as Number, but not adding elements (except null).

Adding any Number subtype freely.

Changing the list to hold any type at runtime.

What is type erasure in Java generics?

Generic type parameters are removed at compile time and not present in bytecode.

The JVM deletes unused classes to save memory.

Generics are checked at runtime through reflection.

The compiler erases the whole class if it is never instantiated.

What is the key difference between a checked and an unchecked exception?

Checked exceptions must be declared or handled; unchecked ones need not be.

Unchecked exceptions cannot be caught.

Checked exceptions only occur in the JVM, not user code.

There is no real difference; the terms are interchangeable.

In a try-with-resources statement, when is a resource closed?

Automatically at the end of the try block, in reverse order of creation.

Only if you call .close() in a finally block yourself.

When the garbage collector runs.

Never; the resource stays open until the program exits.

What does the Java compiler (javac) produce from a .java source file?

Platform-independent bytecode in a .class file.

Native machine code for the current CPU.

An executable binary you can run directly on the OS.

Optimized assembly tuned for the local machine.

What is the primary job of the JIT compiler in the JVM?

To compile frequently executed bytecode into native code at runtime.

To translate Java source into bytecode.

To free unused objects from the heap.

To load .class files from disk.

Which statement about garbage collection in the JVM is accurate?

The JVM reclaims objects that are no longer reachable from any live reference.

You must manually free every object with a delete call.

Calling System.gc() guarantees immediate collection.

Objects are collected the instant they leave scope.

What is a functional interface in Java?

An interface with exactly one abstract method.

Any interface annotated with @Override.

An interface that only contains static methods.

An interface that extends Function by name.

What does the intermediate Stream operation map do?

Transforms each element into a new value, producing a new stream.

Removes elements that fail a predicate.

Collects the stream into a Map.

Immediately runs the pipeline and returns a list.

Why are most Stream intermediate operations described as "lazy"?

They do no work until a terminal operation is invoked.

They always run on a background thread.

They cache every result forever.

They skip elements at random to save time.

What does the synchronized keyword guarantee for a block of code?

Mutual exclusion on the monitor lock plus visibility of changes across threads.

That the method runs faster under load.

That exceptions inside the block are swallowed.

That the block runs on a dedicated thread.

What does declaring a field volatile guarantee?

Reads and writes go to main memory, so other threads see the latest value.

Compound operations like count++ become atomic.

The field can never be modified after initialization.

The field is stored on disk instead of memory.

Why is an ExecutorService preferred over creating a new Thread() for each task?

It reuses a managed pool of threads instead of spawning unbounded new ones.

It runs tasks without using any threads at all.

It makes every task run in a guaranteed order.

It disables the garbage collector during execution.

What does a Java record primarily give you?

A concise immutable data carrier with generated constructor, accessors, equals, hashCode, and toString.

A mutable class with public fields and setters.

A logging utility for writing to disk.

An interface for database rows.

What does a sealed class or interface control?

Exactly which classes are permitted to extend or implement it.

Whether the class can be garbage collected.

Whether fields are encrypted at runtime.

That the class cannot be instantiated at all.

What does the local variable var keyword do in var list = new ArrayList ();?

Infers the static type from the initializer at compile time.

Makes the variable dynamically typed like in JavaScript.

Declares a field usable across the whole class.

Forces the variable to be final.

How does a Java switch expression (arrow form) differ from a classic switch statement?

It returns a value and does not fall through between arms.

It can only switch on int values.

It requires a break after every case.

It runs every matching case in sequence.

Quizzes

Java: Core Language & JVM Fundamentals

Java JVM Programming

Java: Core Language & JVM Fundamentals

Mohammad Abu Mattar20 QuestionsIntermediatePublished: 01 Aug, 2026Updated: 01 Aug, 2026

Java has quietly powered banks, Android, and countless backend systems for decades, and the language keeps evolving with records, sealed classes, and virtual threads. This quiz walks through the core language, the JVM, collections, generics, streams, and modern features. Read each question carefully, pick the single best answer, and use the explanations to sharpen your mental model.

Java: Core Language & JVM Fundamentals

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