Oracle 1Z0-830 Online Practice Questions and Exam Preparation

Oracle 1Z0-830 Online Questions & Answers

• Question 61:

  Given:

  java

  Optional optionalName = Optional.ofNullable(null);

  String bread = optionalName.orElse("Baguette");

  System.out.print("bread:" + bread);

  String dish = optionalName.orElseGet(() -> "Frog legs");

  System.out.print(", dish:" + dish);

  try {

  String cheese = optionalName.orElseThrow(() -> new Exception());

  System.out.println(", cheese:" + cheese);

  } catch (Exception exc) {

  System.out.println(", no cheese.");

  }

  What is printed?

  A. bread:Baguette, dish:Frog legs, cheese.
  B. bread:Baguette, dish:Frog legs, no cheese.
  C. bread:bread, dish:dish, cheese.
  D. Compilation fails.
  B. bread:Baguette, dish:Frog legs, no cheese.

  ---

  Explanation

  Understanding Optional.ofNullable(null)

  Optional.ofNullable(null); creates an empty Optional (i.e., it contains no value).

  Optional.of(null); would throw a NullPointerException, but ofNullable(null); safely creates an empty Optional.

  Execution of orElse, orElseGet, and orElseThrow

  orElse("Baguette")

  Since optionalName is empty, "Baguette" is returned.

  bread = "Baguette"

  Output:"bread:Baguette"

  orElseGet(() -> "Frog legs")

  Since optionalName is empty, "Frog legs" is returned from the lambda expression.

  dish = "Frog legs"

  Output:", dish:Frog legs"

  orElseThrow(() -> new Exception())

  Since optionalName is empty, an exception is thrown.

  The catch block catches this exception and prints ", no cheese.".

  Thus, the final output is:

  makefile

  bread:Baguette, dish:Frog legs, no cheese.

  References:

  Java SE 21 and JDK 21 - Optional

  Java SE 21 - Functional Interfaces

• Question 62:

  Given:

  java

  interface SmartPhone {

  boolean ring();

  }

  class Iphone15 implements SmartPhone {

  boolean isRinging;

  boolean ring() {

  isRinging = !isRinging;

  return isRinging;

  }

  }

  Choose the right statement.

  A. Iphone15 class does not compile
  B. Everything compiles
  C. SmartPhone interface does not compile
  D. An exception is thrown at running Iphone15.ring();
  A. Iphone15 class does not compile

  ---

  Explanation

  In this code, the SmartPhone interface declares a method ring() with a boolean return type. The Iphone15 class implements the SmartPhone interface and provides an implementation for the ring() method.

  However, in the Iphone15 class, the ring() method is declared without the public access modifier. In Java, when a class implements an interface, it must provide implementations for all the interface's methods with the same or a more

  accessible access level. Since interface methods are implicitly public, the implementing methods in the class must also be public. Failing to do so results in a compilation error.

  Therefore, the Iphone15 class does not compile because the ring() method is not declared as public.

• Question 63:

  Which of the following java.io.Console methods does not exist?

  A. read()
  B. reader()
  C. readLine()
  D. readLine(String fmt, Object... args)
  E. readPassword()
  F. readPassword(String fmt, Object... args)
  A. read()

  ---

  Explanation

  java.io.Console is used for interactive input from the console.

  Existing Methods in java.io.Console

  reader() # Returns a Reader object.

  readLine() # Reads a line of text from the console.

  readLine(String fmt, Object... args) # Reads a formatted line.

  readPassword() # Reads a password, returning a char[].

  readPassword(String fmt, Object... args) # Reads a formatted password.

  read() Does Not Exist

  Console does not have a read() method .

  If character-by-character reading is required, use:

  java

  Console console = System.console();

  Reader reader = console.reader();

  int c = reader.read(); // Reads one character

  read() is available in Reader , but not in Console .

  Thus, the correct answer is:read() does not exist.

  References:

  Java SE 21 - Console API Java SE 21 - Reader API

• Question 64:

  Given:

  java

  List integers = List.of(0, 1, 2);

  integers.stream()

  .peek(System.out::print)

  .limit(2)

  .forEach(i -> {});

  What is the output of the given code fragment?

  A. Compilation fails
  B. An exception is thrown
  C. 01
  D. 012
  E. Nothing
  C. 01

  ---

  Explanation

  In this code, a list of integers integers is created containing the elements 0, 1, and 2. A stream is then created from this list, and the following operations are performed in sequence:

  peek(System.out::print):

  The peek method is an intermediate operation that allows performing an action on each element as it is encountered in the stream. In this case, System.out::print is used to print each element. However, since peek is intermediate, the printing

  occurs only when a terminal operation is executed.

  limit(2):

  The limit method is another intermediate operation that truncates the stream to contain no more than the specified number of elements. Here, it limits the stream to the first 2 elements.

  forEach(i -> {}):

  The forEach method is a terminal operation that performs the given action on each element of the stream. In this case, the action is an empty lambda expression (i -> {}), which does nothing for each element.

  The sequence of operations can be visualized as follows:

  Original Stream Elements: 0, 1, 2

  After peek(System.out::print): Elements are printed as they are encountered.

  After limit(2): Stream is truncated to 0, 1. After forEach(i -> {}): No additional action; serves to trigger the processing.

  Therefore, the output of the code is 01, corresponding to the first two elements of the list being printed due to the peek operation.

• Question 65:

  Given:

  java

  var now = LocalDate.now();

  var format1 = new DateTimeFormatter(ISO_WEEK_DATE);

  var format2 = DateTimeFormatter.ISO_WEEK_DATE;

  var format3 = new DateFormat(WEEK_OF_YEAR_FIELD);

  var format4 = DateFormat.getDateInstance(WEEK_OF_YEAR_FIELD);

  System.out.println(now.format(REPLACE_HERE));

  Which variable prints 2025-W01-2 (present-day is 12/31/2024)?

  A. format4
  B. format2
  C. format3
  D. format1
  B. format2

  ---

  Explanation

  In this code, now is assigned the current date using LocalDate.now(). The goal is to format this date to the ISO week date format, which represents dates in the YYYY-'W'WW-E pattern, where: YYYY: Week-based year 'W': Literal 'W' character WW: Week number

  E: Day of the week

  Given that the present day is December 31, 2024, this date falls in the first week of the week-based year 2025. Therefore, the ISO week date representation would be 2025-W01-2, where '2' denotes Tuesday.

  Among the provided formatters:

  format1: This line attempts to create a DateTimeFormatter using a constructor, which is incorrect because DateTimeFormatter does not have a public constructor that accepts a pattern directly. This would result in a compilation error.

  format2: This is correctly assigned the predefined DateTimeFormatter.ISO_WEEK_DATE, which formats dates in the ISO week date format.

  format3: This line attempts to create a DateFormat instance using a field, which is incorrect because DateFormat does not have such a constructor. This would result in a compilation error.

  format4: This line attempts to get a DateFormat instance using an integer field, which is incorrect because DateFormat.getDateInstance() does not accept such parameters. This would result in a compilation error.

  Therefore, the only correct and applicable formatter is format2. Using format2 in the now.format() method will produce the desired output: 2025-W01-2.

• Question 66:

  Given:

  java

  try (FileOutputStream fos = new FileOutputStream("t.tmp");

  ObjectOutputStream oos = new ObjectOutputStream(fos)) {

  fos.write("Today");

  fos.writeObject("Today");

  oos.write("Today");

  oos.writeObject("Today");

  } catch (Exception ex) {

  // handle exception

  }

  Which statement compiles?

  A. fos.write("Today");
  B. fos.writeObject("Today");
  C. oos.write("Today");
  D. oos.writeObject("Today");
  D. oos.writeObject("Today");

  ---

  Explanation

  In Java, FileOutputStream and ObjectOutputStream are used for writing data to files, but they have different purposes and methods. Let's analyze each statement:

  fos.write("Today");

  The FileOutputStream class is designed to write raw byte streams to files. The write method in FileOutputStream expects a parameter of type int or byte[]. Since "Today" is a String, passing it directly to fos. write("Today"); will cause a

  compilation error because there is no write method in FileOutputStream that accepts a String parameter.

  fos.writeObject("Today");

  The FileOutputStream class does not have a method named writeObject. The writeObject method is specific to ObjectOutputStream. Therefore, attempting to call fos.writeObject("Today"); will result in a compilation error.

  oos.write("Today");

  The ObjectOutputStream class is used to write objects to an output stream. However, it does not have a write method that accepts a String parameter. The available write methods in ObjectOutputStream are for writing primitive data types

  and objects. Therefore, oos.write("Today"); will cause a compilation error.

  oos.writeObject("Today");

  The ObjectOutputStream class provides the writeObject method, which is used to serialize objects and write them to the output stream. Since String implements the Serializable interface, "Today" can be serialized. Therefore, oos.writeObject

  ("Today"); is valid and compiles successfully.

  In summary, the only statement that compiles without errors is oos.writeObject("Today");.

  References:

  Java SE 21 and JDK 21 - ObjectOutputStream

  Java SE 21 and JDK 21 - FileOutputStream

• Question 67:

  Which two of the following aren't the correct ways to create a Stream?

  A. Stream stream = Stream.of("a");
  B. Stream stream = Stream.ofNullable("a");
  C. Stream stream = Stream.generate(() -> "a");
  D. Stream stream = Stream.of();
  E. Stream stream = new Stream();
  F. Stream stream = Stream.builder().add("a").build();
  G. Stream stream = Stream.empty();
  E. Stream stream = new Stream();
  F. Stream stream = Stream.builder().add("a").build();

  ---

  Explanation

  In Java, the Stream API provides several methods to create streams. However, not all approaches are valid.

• Question 68:

  Given:

  java

  public class SpecialAddition extends Addition implements Special {

  public static void main(String[] args) {

  System.out.println(new SpecialAddition().add());

  }

  int add() {

  return --foo + bar--;

  }

  }

  class Addition {

  int foo = 1;

  }

  interface Special {

  int bar = 1;

  }

  What is printed?

  B. 1
  C. 2
  D. It throws an exception at runtime.
  E. Compilation fails.
  E. Compilation fails.

  ---

  Explanation

  1.Why does the compilation fail?

  The interface Special contains bar as int bar = 1;.

  In Java, all interface fields are implicitly public, static, and final.

  This means that bar is a constant (final variable).

  The method add() contains bar--, which attempts to modify bar.

  Since bar is final, it cannot be modified, causing a compilation error .

  2.Correcting the Code

  To make the code compile, bar must not be final. One way to fix this:

  java

  class SpecialImpl implements Special {

  int bar = 1;

  }

  Or modify the add() method:

  java

  int add() {

  return --foo + bar; // No modification of bar

  }

  Thus, the correct answer is:Compilation fails.

  References:

  Java SE 21 - Interfaces Java SE 21 - Final Variables

• Question 69:

  Which statement about virtual threads (Project Loom, Java 21) is true?

  A. Virtual threads share OS-level stacks.
  B. Virtual threads require explicit pool management.
  C. Virtual threads are created using Thread.startVirtualThread().
  D. Virtual threads block the carrier thread during I/O.
  C. Virtual threads are created using Thread.startVirtualThread().

  ---

  Explanation

  Java 21 introduces lightweight virtual threads created via Thread.startVirtualThread(Runnable), managed by the JVM’s scheduler. They perform non-blocking I/O and do not require explicit pooling.

• Question 70:

  Given:

  java

  StringBuffer us = new StringBuffer("US");

  StringBuffer uk = new StringBuffer("UK");

  Stream stream = Stream.of(us, uk);

  String output = stream.collect(Collectors.joining("-", "=", ""));

  System.out.println(output);

  What is the given code fragment's output?

  A. US-UK
  B. An exception is thrown.
  C. -US=UK
  D. =US-UK
  E. Compilation fails.
  F. US=UK
  D. =US-UK

  ---

  Explanation

  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.