1z0-830 Exam Dumps - Java SE 21 Developer Professional

In Java, method overloading allows multiple methods with the same name to exist in a class, provided they have different parameter lists (i.e., different number or types of parameters). However, having two methods with the exact same parameter list and only differing in return type is not permitted.

In the provided code, the FetchService class contains two fetch methods:

public String fetch()

public LocalDate fetch()

Both methods have identical parameter lists (none) but differ in their return types (String and LocalDate, respectively). This leads to a compilation error because the Java compiler cannot distinguish between the two methods based solely on return type.

The Java Language Specification (JLS) states:

"It is a compile-time error to declare two methods with override-equivalent signatures in a class."

In this context, "override-equivalent" means that the methods have the same name and parameter types, regardless of their return types.

Therefore, the code will fail to compile due to the duplicate method signatures, and the correct answer is B: Compilation fails.

In this code, two LocalDate instances are created representing May 4, 2023, and May 4, 2024. The Period.between() method is used to calculate the period between these two dates, and the Duration.between() method is used to calculate the duration between them.

Period Calculation:

The Period.between() method calculates the amount of time between two LocalDate objects in terms of years, months, and days. In this case, the period between May 4, 2023, and May 4, 2024, is exactly one year. Therefore, p is P1Y, which stands for a period of one year. Printing p will output P1Y.

Duration Calculation:

The Duration.between() method is intended to calculate the duration between two temporal objects that have time components, such as LocalDateTime or Instant. However, LocalDate represents a date without a time component. Attempting to use Duration.between() with LocalDate instances will result in an UnsupportedTemporalTypeException because Duration requires time-based units, which LocalDate does not support.

Exception Details:

The UnsupportedTemporalTypeException is thrown when an unsupported unit is used. In this case, Duration.between() internally attempts to access time-based fields (like seconds), which are not supported by LocalDate. This behavior is documented in the Java Bug System underJDK-8170275.

Correct Usage:

To calculate the duration between two dates, including time components, you should use LocalDateTime or Instant. For example:

java

LocalDateTime start = LocalDateTime.of(2023, Month.MAY, 4, 0, 0);

LocalDateTime end = LocalDateTime.of(2024, Month.MAY, 4, 0, 0);

Duration d = Duration.between(start, end);

System.out.println(d); // Outputs: PT8784H

This will correctly calculate the duration as PT8784H, representing 8,784 hours (which is 366 days, accounting for a leap year).

Conclusion:

The output of the given code will be:

pgsql

P1Y

Exception in thread "main" java.time.temporal.UnsupportedTemporalTypeException: Unsupported unit: Seconds

Therefore, the correct answer is D:

nginx

P1Y

UnsupportedTemporalTypeException

In Java, array initialization can be performed in several ways, but certain syntaxes are invalid and will cause compilation errors. Let's analyze each declaration:

var array1 = new String[]{ "foo", "bar", "buz" };

This is a valid declaration. The var keyword allows the compiler to infer the type from the initializer. Here, new String[]{ "foo", "bar", "buz" } creates an anonymous array of String with three elements. The compiler infers array1 as String[]. This syntax is correct and compiles successfully.

var array2[] = { "foo", "bar", "buz" };

This declaration is invalid. While var can be used for type inference, appending [] after var is not allowed. The correct syntax would be either String[] array2 = { "foo", "bar", "buz" }; or var array2 = new String[]{ "foo", "bar", "buz" };. Therefore, this line will cause a compilation error.

var array3 = new String[3] { "foo", "bar", "buz" };

This declaration is invalid. In Java, when specifying the size of the array (new String[3]), you cannot simultaneously provide an initializer. The correct approach is either to provide the size without an initializer (new String[3]) or to provide the initializer without specifying the size (new String[]{ "foo", "bar", "buz" }). Therefore, this line will cause a compilation error.

var array4 = { "foo", "bar", "buz" };

This declaration is invalid. The array initializer { "foo", "bar", "buz" } can only be used in an array declaration when the type is explicitly provided. Since var relies on type inference and there's no explicit type provided here, this will cause a compilation error. The correct syntax would be String[] array4 = { "foo", "bar", "buz" };.

String array5[] = new String[]{ "foo", "bar", "buz" };

This is a valid declaration. Here, String array5[] declares array5 as an array of String. The initializer new String[]{ "foo", "bar", "buz" } creates an array with three elements. This syntax is correct and compiles successfully.

Therefore, the declarations that compile successfully are array1 and array5.

References:

Java SE 21 & JDK 21 - Local Variable Type Inference

Java SE 21 & JDK 21 - Arrays

Understanding Constructors vs. Methods in Java

In Java, aconstructormustnot have a return type.

The followingis NOT a constructorbut aregular method:

java

void Versailles() { // This is NOT a constructor!

Correct way to define a constructor:

java

public Versailles() { // Constructor must not have a return type

Since there isno constructor explicitly defined,Java provides a default no-argument constructor, which does nothing.

Why Does Compilation Fail?

void Versailles() is interpreted as amethod,not a constructor.

This means the default constructor (which does nothing) is called.

Since the method Versailles() is never called, the object fields remain uninitialized.

If the constructor were correctly defined, the output would be:

nginx

Hall of Mirrors has 17 mirrors.

The gardens cover 800 hectares.

How to Fix It

java

public Versailles() { // Corrected constructor

this.mirrorsCount = 17;

this.gardensHectares = 800;

System.out.println("Hall of Mirrors has " + mirrorsCount + " mirrors.");

System.out.println("The gardens cover " + gardensHectares + " hectares.");

}

Thus, the correct answer is:Compilation fails at line n1.

References:

Java SE 21 - Constructors

Java SE 21 - Methods vs. Constructors

Understanding TreeMap.subMap(fromKey, fromInclusive, toKey, toInclusive)

TreeMap.subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) returns aportion of the mapthat falls within the specified key range.

Thefirst boolean parameter(fromInclusive) determines if the fromKey should be included.

Thesecond boolean parameter(toInclusive) determines if the toKey should be included.

Given TreeMap Contents

CopyEdit

{1=Moulin Rouge, 2=Crazy Horse, 3=Paradis Latin, 4=Le Lido, 5=Folies Bergère}

Applying subMap(2, true, 5, false)

Includeskey 2 ("Crazy Horse")✅(fromInclusive = true)

Includeskey 3 ("Paradis Latin")✅

Includeskey 4 ("Le Lido")✅

Excludes key 5 ("Folies Bergère")❌(toInclusive = false)

Final Output

CopyEdit

{2=Crazy Horse, 3=Paradis Latin, 4=Le Lido}

Thus, the correct answer is:✅{2=Crazy Horse, 3=Paradis Latin, 4=Le Lido}

References:

Java SE 21 - TreeMap.subMap()

Java SE 21 - NavigableMap

In this code, two StringBuffer objects, us and uk, are created with the values "US" and "UK", respectively. A stream is then created from these objects using Stream.of(us, uk).

The collect method is used with Collectors.joining("-", "=", ""). The joining collector concatenates the elements of the stream into a single String with the following parameters:

Delimiter ("-"):Inserted between each element.

Prefix ("="):Inserted at the beginning of the result.

Suffix (""):Inserted at the end of the result.

Therefore, the elements "US" and "UK" are concatenated with "-" between them, resulting in "US-UK". The prefix "=" is added at the beginning, resulting in the final output =US-UK.

Understanding ArrayDeque Behavior

ArrayDequeis a double-ended queue (deque), working as aFIFO (queue) and LIFO (stack).

Thedefault behaviorisqueue-like (FIFO)unless explicitly used as a stack.

Step-by-Step Execution

java

var deque = new ArrayDeque<>();

deque.add(1);

deque.add(2);

deque.add(3);

deque.add(4);

deque.add(5);

Deque after additions→ [1, 2, 3, 4, 5]

Operations Breakdown

deque.peek()→ Returns thehead(first element)without removal.

makefile

Output: 1

deque.poll()→ Removes and returns thehead.

go

Output: 1, Deque after poll → `[2, 3, 4, 5]`

deque.pop()→Same as removeFirst(); removes and returns thehead.

perl

Output: 2, Deque after pop → `[3, 4, 5]`

deque.element()→ Returns thehead(same as peek(), but throws an exception if empty).

makefile

Output: 3

Final Output

1 1 2 3

Thus, the correct answer is:1 1 2 3

References:

Java SE 21 - ArrayDeque

Java SE 21 - Queue Operations

In this code, there is a type mismatch between the DoubleStream and the Predicate.

DoubleStream: A sequence of primitive double values.

Predicate<Double>: A functional interface that operates on objects of type Double (the wrapper class), not on primitive double values.

The DoubleStream class provides a method anyMatch(DoublePredicate predicate), where DoublePredicate is a functional interface that operates on primitive double values. However, in the code, a Predicate is used instead of a DoublePredicate. This mismatch leads to a compilation error because anyMatch cannot accept a Predicate when working with a DoubleStream.

To correct this, the predicate should be defined as a DoublePredicate to match the primitive double type:

java

DoubleStream doubleStream = DoubleStream.of(3.3, 4, 5.25, 6.66);

DoublePredicate doublePredicate = d -> d < 5;

System.out.println(doubleStream.anyMatch(doublePredicate));

With this correction, the code will compile and print true because there are elements in the stream (e.g., 3.3 and 4.0) that are less than 5.