Oracle Java SE 21 Developer

Which keyword is used to define a class in Java?

• Astruct
• Bclass
• Cdefine
• Dobject

What is the correct way to declare the main method in Java?

• Apublic void main(String[] args)
• Bpublic static void main(String[] args)
• Cstatic void main()
• Dpublic static int main(String[] args)

What is encapsulation in Java?

• AInheriting properties from a parent class
• BBundling data and methods that operate on that data within a single unit (class) and rest…
• CConverting objects to strings
• DRunning multiple threads simultaneously

Which access modifier makes a member accessible only within the same class?

• Apublic
• Bprotected
• Cprivate
• Ddefault

What is a record in Java 21?

• AA type of logging mechanism
• BA compact, immutable data carrier class that auto-generates constructor, accessors, equal…
• CA database table reference
• DA type of annotation

What is a sealed class in Java 21?

• AA class that cannot be instantiated
• BA class that restricts which other classes can extend or implement it using the 'permits'…
• CA class stored in a sealed JAR file
• DA final class with no methods

What is a lambda expression in Java?

• AA named method in a class
• BA concise representation of an anonymous function that can be passed as an argument or st…
• CA type of loop construct
• DA debugging tool

What does the Stream API in Java provide?

• AFile I/O streaming
• BA functional-style API for processing sequences of elements with operations like filter,…
• CNetwork socket streaming
• DAudio/video streaming

What is the purpose of the 'extends' keyword in Java?

• ATo extend the length of an array
• BTo establish an inheritance relationship where a subclass inherits from a superclass
• CTo extend the timeout of a method
• DTo increase memory allocation

What is the purpose of the 'module-info.java' file in Java?

• ATo store module documentation
• BTo define a Java module's dependencies, exported packages, and service declarations
• CTo configure the JVM runtime
• DTo list module test cases

What are virtual threads in Java 21?

• AThreads that run in a virtual machine only
• BLightweight threads managed by the JVM that enable high-throughput concurrent application…
• CThreads used in virtual reality applications
• DSimulated threads for testing

What is polymorphism in Java?

• AThe ability to create multiple constructors
• BThe ability of objects of different types to be treated as instances of a common supertyp…
• CThe ability to have multiple packages
• DThe ability to run on multiple platforms

Which interface must a class implement to allow its objects to be compared for ordering?

• ASerializable
• BComparable<T>
• CIterable<T>
• DCloneable

What is the purpose of the 'var' keyword introduced in Java 10?

• ATo declare a variable-length array
• BTo enable local variable type inference, where the compiler determines the type from the…
• CTo declare a variable as volatile
• DTo create a varargs parameter

What is a functional interface in Java?

• AAn interface with no methods
• BAn interface with exactly one abstract method, which can be implemented using a lambda ex…
• CAn interface used only for functions
• DAn interface that extends Function

What is the purpose of the Optional class in Java?

• ATo make method parameters optional
• BA container object that may or may not contain a non-null value, used to avoid NullPointe…
• CTo create optional threads
• DTo define optional module dependencies

What does the 'synchronized' keyword do in Java?

• ASynchronizes the system clock
• BEnsures that only one thread can execute a block of code or method at a time, preventing…
• CSynchronizes two databases
• DSynchronizes file contents

What is the difference between an interface and an abstract class in Java?

• AThey are identical concepts
• BAn interface defines a contract with abstract methods (and default/static methods), while…
• CInterfaces are faster than abstract classes
• DAbstract classes cannot have methods

What is the purpose of the 'try-with-resources' statement in Java?

• ATo try different resource configurations
• BTo automatically close resources (like streams, connections) that implement AutoCloseable…
• CTo retry failed resource allocations
• DTo test resource availability

What is pattern matching for instanceof in Java 21?

• AA regex matching feature
• BAn enhancement where instanceof checks can include a pattern variable that is automatical…
• CMatching design patterns
• DPattern recognition for image processing

What is a text block in Java?

• AA block of commented-out text
• BA multi-line string literal delimited by triple quotes (""") that preserves formatting an…
• CA text input field
• DA block of encrypted text

Given the code: List.of(1,2,3,4,5).stream().filter(n -> n > 2).map(n -> n * 2).toList() — what is the result?

• A[2, 4, 6, 8, 10]
• B[6, 8, 10]
• C[3, 4, 5]
• D[1, 2, 3, 4, 5]

What is the difference between method overloading and method overriding in Java?

• AThey are the same thing
• BOverloading defines multiple methods with the same name but different parameters in the s…
• COverloading is for static methods, overriding is for instance methods
• DOverloading changes the return type only

How do you create a virtual thread in Java 21?

• Anew VirtualThread()
• BThread.ofVirtual().start(runnable) or Thread.startVirtualThread(runnable)
• CThread.createVirtual()
• Dnew Thread(runnable, VIRTUAL)

What is the purpose of the Collectors.groupingBy() method?

• AGrouping files in a directory
• BClassifying stream elements into groups based on a classification function, returning a M…
• CGrouping threads in a pool
• DGrouping modules together

What is the difference between checked and unchecked exceptions in Java?

• AChecked are faster than unchecked
• BChecked exceptions must be declared or handled (extends Exception), while unchecked excep…
• CChecked exceptions are verified at runtime
• DUnchecked exceptions cannot be caught

What is the 'permits' clause used for with sealed classes?

• AGranting file permissions
• BSpecifying the exhaustive list of classes that are allowed to extend or implement the sea…
• CPermitting module access
• DAllowing method access

What does the 'requires' directive do in a module-info.java file?

• ARequires a specific Java version
• BDeclares a dependency on another module, making that module's exported packages available
• CRequires user authentication
• DRequires a minimum memory allocation

What is the CompletableFuture class used for in Java?

• ACompleting unfinished code
• BRepresenting a future result of an asynchronous computation with support for chaining, co…
• CCompleting forms automatically
• DFuture date calculations

What is the purpose of the 'exports' directive in a Java module?

• AExporting files to another server
• BMaking a package accessible to other modules that require this module
• CExporting data to CSV
• DExporting configuration settings

How does pattern matching work in switch expressions in Java 21?

• AUsing regex patterns in switch cases
• BSwitch cases can use type patterns, guarded patterns (with when clause), and null cases t…
• CMatching design patterns in code
• DPattern-based file matching

What is the difference between Stream.map() and Stream.flatMap()?

• Amap() is faster than flatMap()
• Bmap() transforms each element to one output element, while flatMap() transforms each elem…
• CflatMap() only works with flat collections
• Dmap() works with primitives only

What is a record pattern in Java 21?

• AA pattern for recording audio
• BA destructuring pattern that matches a record type and extracts its components in a singl…
• CA logging pattern
• DA database record query pattern

What is the difference between Runnable and Callable in Java?

• AThey are identical interfaces
• BRunnable's run() method returns void and cannot throw checked exceptions, while Callable'…
• CRunnable is newer than Callable
• DCallable cannot be used with threads

What is the purpose of the 'opens' directive in a Java module?

• AOpening a file for reading
• BAllowing deep reflection access to a package at runtime, which is needed by frameworks li…
• COpening a network connection
• DOpening a database cursor

What is the purpose of the @Override annotation?

• ATo override the JVM settings
• BTo indicate that a method is intended to override a superclass method, enabling compile-t…
• CTo override access modifiers
• DTo override final methods

How do you create an unmodifiable list in Java using the List.of() factory method?

• AList.of() creates a mutable list
• BList.of(elements) creates an immutable list that throws UnsupportedOperationException on…
• CList.of() creates an empty array
• DList.of() is not a valid method

What is the ExecutorService interface used for in Java concurrency?

• AExecuting SQL queries
• BManaging a pool of threads and providing methods to submit, execute, and manage asynchron…
• CExecuting system commands
• DManaging service executables