1Z0-830 Java SE 21 Developer - Practice Test 5

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Question 1 MEDIUM

Which of the following local variable declarations using var are invalid? (Choose two.)

Select all that apply

var a = 10;
var b;
var c = null;
var d = new String("Java");

var requires an initializer because the compiler must infer the type at compile time. var a = 10; -> Valid (int inferred) var b; -> Invalid (no initializer) var c = null; -> Invalid (null has no type information) var d = new String("Java"); -> Valid (String inferred) Exam Tip: var cannot be used without initialization and cannot infer from null. See more: Data Types & Variables

Question 2 MEDIUM

What is printed? int x = 5; int y = ++x + x++; System.out.println(y);

11
12
13
Compilation fails

Evaluation is left-to-right. ++x -> x becomes 6, returns 6 x++ -> returns 6, then x becomes 7 6 + 6 = 12 Final x = 7. See more: Data Types & Variables

Question 3 MEDIUM

What is printed? int i = 0; do { System.out.print(i); } while(++i < 3);

012
123
01
Compilation fails

Iteration steps: Print 0 -> i becomes 1 Print 1 -> i becomes 2 Print 2 -> i becomes 3 -> condition fails Output: 012 See more: Control Flow

Question 4 MEDIUM

Which statement about method overriding is correct?

Return type must be identical
Return type may be covariant
Access modifier can be more restrictive
Static methods are overridden

Overriding allows covariant return types (subclass return allowed). Access cannot be more restrictive. Static methods are hidden, not overridden. See: Object-Oriented Approach -> Object-Oriented Approach (Overriding Rules)

Question 5 EASY

What is printed? try { throw new RuntimeException(); } catch(Exception e) { System.out.print("A"); } finally { System.out.print("B"); }

A
B
AB
Compilation fails

Exception is caught -> prints A. finally always executes -> prints B. Output: AB See more: Exception Handling

Question 6 EASY

Which collection maintains natural sorted order?

HashSet
TreeSet
LinkedHashSet
ArrayList

TreeSet keeps elements sorted using natural ordering or Comparator. See: Arrays & Collections -> Arrays & Collections (Sorted Collections)

Question 7 MEDIUM

What is printed? Stream.of(1,2,3) .filter(i -> i > 1) .map(i -> i * 2) .forEach(System.out::print);

123
46
24
Compilation fails

Filter keeps 2 and 3. Map multiplies -> 4 and 6. Output: 46 See more: Streams & Lambdas

Question 8 EASY

Which directive exports a package in module-info.java?

requires
opens
exports
provides

exports makes a package accessible to other modules. See: Packaging & Deployment -> Packaging & Deployment (Modules)

Question 9 EASY

Which keyword prevents a method from being accessed by multiple threads simultaneously?

volatile
transient
synchronized
final

synchronized enforces mutual exclusion on a monitor lock. See more: Concurrency

Question 10 EASY

Which class is used to serialize objects?

FileWriter
ObjectOutputStream
BufferedWriter
DataOutputStream

ObjectOutputStream writes Serializable objects. See more: Java I/O API

Question 11 EASY

Which JDBC interface represents a database connection?

Statement
Connection
ResultSet
Driver

Connection represents a session with the database. See more: JDBC

Question 12 EASY

Which class is used for locale-specific formatting of numbers?

DecimalFormat
NumberFormat
Formatter
Locale

NumberFormat handles locale-aware number formatting. See more: Localization

Question 13 EASY

Which feature allows pattern matching in switch statements?

Records
Text Blocks
Pattern Matching for switch
Modules

Java 21 supports pattern matching for switch. See more: Java 21 New Features

Question 14 EASY

What is printed? String s = " Java "; System.out.print("[" + s.strip() + "]");

[ Java ]
[Java]
[ Java ]
Compilation fails

strip() removes leading and trailing Unicode whitespace. Original: " Java " After strip(): "Java" Output: [Java] Exam Tip: strip() is Unicode-aware; trim() is not. See more: Data Types & Variables

Question 15 EASY

What is printed? int a = 10; int b = 20; System.out.println(a > b ? "A" : "B");

A
B
Compilation fails
Runtime exception

Ternary operator evaluates condition a > b. 10 > 20 is false -> returns "B". See more: Data Types & Variables

Question 16 MEDIUM

Which statement about switch expressions is correct?

break is mandatory
yield is used to return a value from a block
Switch expressions cannot return values
Switch expressions require default

Switch expressions can return values. When using a block, yield returns the value. break is not used in expression form. See more: Control Flow

Question 17 EASY

Which keyword prevents a class from being extended?

static
abstract
final
sealed

final prevents subclassing completely. sealed restricts but does not fully prevent extension. See: Object-Oriented Approach -> Class Design

Question 18 MEDIUM

Which of the following compile? (Choose two.)

Select all that apply

List<? extends Number> l = new ArrayList<Integer>();
List<? super Integer> l = new ArrayList<Number>();
List<Number> l = new ArrayList<Integer>();
List<Integer> l = new ArrayList<Number>();

Generics are invariant. ? extends allows subclasses (Integer extends Number). ? super allows supertypes. Direct List<Number> = new ArrayList<Integer>() does NOT compile. See: Object-Oriented Approach -> Generics & Wildcards

Question 19 EASY

Which exception is unchecked?

IOException
SQLException
NullPointerException
InterruptedException

Unchecked exceptions extend RuntimeException. NullPointerException is unchecked. See more: Exception Handling

Question 20 EASY

Which method retrieves but does not remove the head of a Queue?

poll()
remove()
peek()
pop()

peek() retrieves without removing. poll() removes. See: Arrays & Collections -> Queue Interface

Question 21 EASY

Which intermediate operation returns a Stream?

forEach()
count()
map()
reduce()

map() is intermediate and returns a Stream. forEach(), count(), reduce() are terminal operations. See more: Streams & Lambdas

Question 22 EASY

What is printed? Optional<String> o = Optional.of("Java"); System.out.println(o.orElse("Default"));

Java
Default
null
Compilation fails

orElse returns contained value if present. See more: Streams & Lambdas

Question 23 EASY

Which directive declares a dependency on another module?

exports
requires
opens
uses

requires declares dependency. See: Packaging & Deployment -> Module Dependencies

Question 24 EASY

Which interface represents a task returning a value?

Runnable
Callable
Supplier
Executor

Callable returns a value and can throw checked exceptions. See more: Concurrency

Question 25 MEDIUM

Which class allows thread-safe iteration without ConcurrentModificationException?

ArrayList
HashMap
CopyOnWriteArrayList
TreeSet

CopyOnWriteArrayList uses snapshot iteration. See more: Concurrency

Question 26 EASY

Which class is part of NIO?

File
Path
FileReader
BufferedWriter

Path is part of java.nio.file package. See more: Java I/O API

Question 27 EASY

Which method executes a SELECT statement?

executeUpdate()
executeQuery()
execute()
runQuery()

executeQuery() returns a ResultSet. See more: JDBC

Question 28 EASY

Which object holds the results of a query?

Connection
Statement
ResultSet
DriverManager

ResultSet stores query results row-by-row. See more: JDBC

Question 29 EASY

Which class represents a specific geographical region for formatting?

ZoneId
Locale
Currency
DateFormat

Locale defines language and region. See more: Localization

Question 30 MEDIUM

Which is true about records?

They can extend another class
They implicitly extend java.lang.Record
They cannot implement interfaces
They allow mutable components

Records implicitly extend java.lang.Record. They cannot extend other classes. Components are final. See more: Java 21 New Features

Question 31 MEDIUM

What is printed? int a = 4; int b = a++ * 2; System.out.println(b + " " + a);

8 4
8 5
10 5
Compilation fails

a++ uses 4, then increments to 5. 4 * 2 = 8. Final a = 5. Output: 8 5 See more: Data Types & Variables

Question 32 EASY

Which keyword skips the current loop iteration?

break
continue
return
yield

continue skips remaining code in the current iteration and moves to the next. See more: Control Flow

Question 33 EASY

Which of the following about constructors is correct?

Constructors can have return types
Constructors can be abstract
Constructors have no return type
Constructors must be public

Constructors do NOT have return types (not even void). They cannot be abstract. They may have any access modifier. See: Object-Oriented Approach -> Constructors

Question 34 EASY

Which block always executes?

try
catch
finally
throw

finally executes regardless of exception handling (except JVM termination). See more: Exception Handling

Question 35 MEDIUM

Which List implementation is synchronized?

ArrayList
LinkedList
Vector
CopyOnWriteArrayList

Vector is legacy synchronized. CopyOnWriteArrayList is concurrent but not fully synchronized. See: Arrays & Collections -> Collections

Question 36 EASY

Which terminal operation returns an Optional?

count()
findFirst()
forEach()
map()

findFirst() returns Optional<T>. See more: Streams & Lambdas

Question 37 MEDIUM

Which keyword allows reflective access in modules?

exports
requires
opens
provides

opens allows reflection at runtime. See: Packaging & Deployment -> Modules

Question 38 EASY

Which method waits for a thread to finish?

sleep()
join()
wait()
yield()

join() waits for thread completion. See more: Concurrency

Question 39 EASY

Which method reads all lines from a file?

Files.read()
Files.readAllLines()
File.readLines()
Path.read()

Files.readAllLines(Path) returns List<String>. See more: Java I/O API

Question 40 EASY

Which JDBC method modifies database data?

executeQuery()
executeUpdate()
executeSelect()
fetch()

executeUpdate() is used for INSERT, UPDATE, DELETE. See more: JDBC

Question 41 EASY

Which class loads JDBC drivers?

Connection
DriverManager
Statement
Database

DriverManager manages JDBC drivers and connections. See more: JDBC

Question 42 MEDIUM

Which method formats currency based on locale?

NumberFormat.getCurrencyInstance()
Currency.getInstance()
Formatter.currency()
Locale.currency()

getCurrencyInstance() formats according to locale. See more: Localization

Question 43 MEDIUM

Which keyword restricts subclasses to specific types?

final
sealed
abstract
static

sealed restricts permitted subclasses. See more: Java 21 New Features

Question 44 MEDIUM

Which modifier must permitted subclasses declare?

static
volatile
final/sealed/non-sealed
abstract only

Subclasses must declare final, sealed, or non-sealed. See more: Java 21 New Features

Question 45 MEDIUM

Which collector groups elements?

Collectors.toList()
Collectors.groupingBy()
Collectors.joining()
Collectors.counting()

groupingBy groups stream elements by classifier. See more: Streams & Lambdas

Question 46 EASY

Which functional interface takes two arguments and returns a result?

Function
BiFunction
Predicate
Supplier

BiFunction<T,U,R> accepts two inputs and produces result. See more: Streams & Lambdas

Question 47 EASY

Which access modifier is most restrictive?

public
protected
default
private

private restricts access to the same class only. See: Object-Oriented Approach -> Access Modifiers

Question 48 EASY

Which keyword allows inheritance?

implements
extends
inherits
super

extends is used for class inheritance. See: Object-Oriented Approach -> Inheritance

Question 49 EASY

Which loop guarantees at least one execution?

for
while
do-while
foreach

do-while executes body before condition check. See more: Control Flow

Question 50 MEDIUM

Which is a valid text block declaration?

String s = """Hello""";
String s = """ Hello """;
String s = """Hello;
Text s = "Hello";

Text blocks require line break after opening delimiter. See more: Data Types & Variables

Question 51 MEDIUM

Which method sorts a List?

Collections.sort()
List.sort()
Both
None

Both Collections.sort() and List.sort() can sort lists. See: Arrays & Collections -> Collections Utility

Question 52 EASY

Which method pauses a thread?

wait()
join()
sleep()
yield()

Thread.sleep pauses execution for specified time. See more: Concurrency

Question 53 EASY

Which is a terminal operation?

filter()
map()
sorted()
collect()

collect() triggers stream processing. See more: Streams & Lambdas

Question 54 MEDIUM

Which class handles date formatting by locale?

DateTimeFormatter
DateFormat
LocalDate
ZoneId

DateFormat formats dates using locale settings. See more: Localization

Question 55 EASY

Which interface represents file path abstraction?

File
Path
Stream
URI

Path represents file/directory path in NIO. See more: Java I/O API

Question 56 EASY

Which keyword refers to superclass?

this
parent
super
base

super references superclass members. See: Object-Oriented Approach -> Inheritance

Question 57 EASY

Which feature reduces boilerplate for data carriers?

Enums
Records
Sealed classes
Modules

Records auto-generate constructor, getters, equals, hashCode. See more: Java 21 New Features

Question 58 MEDIUM

Pattern matching for instanceof allows?

Automatic casting
Multiple inheritance
Checked exceptions
Parallel execution

instanceof pattern matching combines type check and cast. See more: Java 21 New Features

Question 59 EASY

Which concurrent map implementation exists?

HashMap
ConcurrentHashMap
TreeMap
WeakHashMap

ConcurrentHashMap is thread-safe without full synchronization. See more: Concurrency

Question 60 MEDIUM

Which interface allows LIFO operations?

Queue
Deque
Set
Map

Deque supports stack (LIFO) and queue (FIFO) operations. See: Arrays & Collections -> Deque

Question 61 EASY

Which statement throws an exception explicitly?

try
throw
catch
finally

throw is used to explicitly throw an exception. See more: Exception Handling

Question 62 EASY

Which keyword exits a method?

break
continue
exit
return

return exits a method and optionally returns value. See more: Control Flow

Question 63 EASY

Which primitive type is 64-bit floating point?

float
double
int
long

double is 64-bit IEEE 754 floating point. See more: Data Types & Variables

Question 64 EASY

Which keyword is used to define an interface?

class
interface
abstract
implements

interface defines a contract in Java. See: Object-Oriented Approach -> Interfaces

Question 65 EASY

Which functional interface returns boolean?

Supplier
Predicate
Function
Consumer

Predicate<T> returns boolean. See more: Streams & Lambdas

Question 66 MEDIUM

Which tool packages modules into runtime images?

javac
jar
jlink
javadoc

jlink creates custom runtime images for modular applications. See: Packaging & Deployment -> Deployment Tools

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