Oracle 1Z0-830 Online Practice Questions and Exam Preparation

Oracle 1Z0-830 Online Questions & Answers

• Question 1:

  What is the output?

  var map = new HashMap();

  map.put("A", 1);

  map.merge("A", 2, Integer::sum);

  map.merge("B", 3, Integer::sum);

  System.out.println(map);

  A. {A=3, B=3}
  B. {A=2, B=3}
  C. {A=1, B=3}
  D. {A=3}
  A. {A=3, B=3}

  ---

  explanation:

  Explanation

  merge(key,val,remappingFn) adds a new key or combines values. "A"

• Question 2:

  Given:

  var list = List.of(1,2,3,4,5);

  System.out.println(list.stream().dropWhile(i -> i < 3)

  .takeWhile(i -> i < 5)

  .toList());

  What is printed?

  A. [3, 4]
  B. [1, 2, 3, 4]
  C. [4, 5]
  D. []
  A. [3, 4]

  ---

  explanation:

  Explanation

  dropWhile(i < 3) skips 1 and 2, leaving [3, 4, 5]; takeWhile(i < 5) then takes 3 and 4 until predicate fails. Final list [3, 4].

• Question 3:

  Consider:

  List names = List.of("Bob","Alice","Eve");

  var sorted = names.stream()

  .sorted(Comparator.comparing(String::length).reversed()

  .thenComparing(Comparator.naturalOrder()))

  .toList();

  System.out.println(sorted);

  What is printed?

  A. [Eve, Bob, Alice]
  B. [Alice, Eve, Bob]
  C. [Alice, Bob, Eve]
  D. [Bob, Eve, Alice]
  B. [Alice, Eve, Bob]

  ---

  explanation:

  Explanation

  Comparator first sorts by length (descending 3 > 3 > 5

• Question 4:

  Given:

  record Box(T value) {}

  public static void main(String[] args) {

  Box b = new Box(10);

  System.out.println(b.value().doubleValue());

  }

  What is printed?

  A. 10.0
  B. Compilation fails
  C. A ClassCastException
  D. null
  A. 10.0

  ---

  explanation:

  Explanation

  The wildcard ? extends Number allows reading as Number; value() returns Integer

• Question 5:

  Which statement about sealed interfaces and records is correct?

  A. A record can never implement a sealed interface.
  B. A record must explicitly be listed in the permits clause to implement a sealed interface.
  C. A record implements sealed interfaces implicitly without permits.
  D. Sealed interfaces cannot be extended by records.
  B. A record must explicitly be listed in the permits clause to implement a sealed interface.

  ---

  explanation:

  Explanation

  When an interface is sealed, any implementing class or record must appear in its permits clause unless the interface and the record are in the same compilation unit.

• Question 6:

  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:

  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 7:

  Which of the following text block declarations compiles?

  A. String s = """Hello "Java";""";
  B. String s = """Hello Java""";
  C. String s = """Hello Java";
  D. String s = """"Hello Java"""";
  B. String s = """Hello Java""";

  ---

  explanation:

  Explanation

  A text block starts and ends with triple quotes """. Option B uses correct delimiters and line breaks; the others have mismatched or extra quotes.

• Question 8:

  What is the output?

  record R(int x) implements AutoCloseable {

  public void close() { System.out.print("close "); }

  }

  try (R r = new R(1)) {

  System.out.print("run ");

  } finally {

  System.out.print("finally ");

  }

  A. run finally
  B. close run finally
  C. run close finally
  D. run finally close
  C. run close finally

  ---

  explanation:

  Explanation

  The try-with-resources block invokes AutoCloseable.close() after the try body completes but before finally. Output order: run close finally.

• Question 9:

  What is the output?

  record R(int x) implements AutoCloseable {

  public void close() { System.out.print("close "); } } try (R r = new R(1)) {

  System.out.print("run "); } finally { System.out.print("finally "); }

  A. run finally
  B. close run finally
  C. run close finally
  D. run finally close
  C. run close finally

  ---

  explanation:

  Explanation

  The try-with-resources block invokes AutoCloseable.close() after the try body completes but before finally. Output order: run close finally.

• Question 10:

  Given:

  sealed interface Shape permits Circle, Square {} final class Circle implements Shape { double radius; } final class Square implements Shape { double side; }

  static double area(Shape s) {

  return switch (s) {

  case Circle c -> Math.PI * c.radius * c.radius;

  case Square sq -> sq.side * sq.side;

  }; }

  Which statement is true?

  A. The code does not compile because the switch must have a default.
  B. The switch expression exhaustively handles all permitted subtypes.
  C. Pattern variables c and sq must be declared final.
  D. The method area must declare throws Exception.
  B. The switch expression exhaustively handles all permitted subtypes.

  ---

  explanation:

  Explanation

  Because Shape is a sealed interface permitting only Circle and Square, the compiler can verify exhaustive coverage, so no default label is required.