Oracle 1Z0-830 Online Practice Questions and Exam Preparation

Oracle 1Z0-830 Online Questions & Answers

• Question 41:

  Given:

  java

  var sList = new CopyOnWriteArrayList();

  Which of the following statements is correct?

  A. The CopyOnWriteArrayList class is a thread-safe variant of ArrayList where all mutative operations are implemented by making a fresh copy of the underlying array.
  B. The CopyOnWriteArrayList class is not thread-safe and does not prevent interference amongconcurrent threads.
  C. The CopyOnWriteArrayList class's iterator reflects all additions, removals, or changes to the list since the iterator was created.
  D. The CopyOnWriteArrayList class does not allow null elements.
  E. Element-changing operations on iterators of CopyOnWriteArrayList, such as remove, set, and add, are supported and do not throw UnsupportedOperationException.
  A. The CopyOnWriteArrayList class is a thread-safe variant of ArrayList where all mutative operations are implemented by making a fresh copy of the underlying array.

  ---

  Explanation

  The CopyOnWriteArrayList is a thread-safe variant of ArrayList in which all mutative operations (such as add, set, and remove) are implemented by creating a fresh copy of the underlying array. This design allows for safe iteration over the list

  without requiring external synchronization, as iterators operate over a snapshot of the array at the time the iterator was created. Consequently, modifications made to the list after the creation of an iterator are not reflected in that iterator.

  docs.oracle.com

  Evaluation of Options:

  Option A:Correct. This statement accurately describes the behavior of CopyOnWriteArrayList.

  Option B:Incorrect. CopyOnWriteArrayList is thread-safe and is designed to prevent interference among concurrent threads.

  Option C:Incorrect. Iterators of CopyOnWriteArrayList do not reflect additions, removals, or changes made to the list after the iterator was created; they operate on a snapshot of the list's state at the time of their creation.

  Option D:Incorrect. CopyOnWriteArrayList allows null elements.

  Option E:Incorrect. Element-changing operations on iterators, such as remove, set, and add, are not supported in CopyOnWriteArrayList and will throw UnsupportedOperationException.

• Question 42:

  Given:

  java

  var _ = 3;

  var $ = 7;

  System.out.println(_ + $);

  What is printed?

  A. 10
  B. _$
  C. It throws an exception.
  D. Compilation fails.
  D. Compilation fails.

  ---

  Explanation

  The var keyword and identifier rules:

  The var keyword is used for local variable type inference introduced in Java 10 .

  However, Java does not allow _ (underscore) as an identifier since Java 9 .

  If we try to use _ as a variable name, the compiler will throw an error:

  pgsql

  error: as of release 9, '_' is a keyword, and may not be used as an identifier

  The $ symbol as an identifier:

  The $ character is a valid identifier in Java.

  However, since _ is not allowed, the code fails to compile before even reaching $ .

  Thus,the correct answer is "Compilation fails."

  References:

  Java SE 21 - var Local Variable Type Inference

  Java SE 9 - Restrictions on _ Identifier

• Question 43:

  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

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

• Question 44:

  Given:

  java

  var frenchCities = new TreeSet();

  frenchCities.add("Paris");

  frenchCities.add("Marseille");

  frenchCities.add("Lyon");

  frenchCities.add("Lille");

  frenchCities.add("Toulouse");

  System.out.println(frenchCities.headSet("Marseille"));

  What will be printed?

  A. [Paris]
  B. [Paris, Toulouse]
  C. [Lille, Lyon]
  D. Compilation fails
  E. [Lyon, Lille, Toulouse]
  C. [Lille, Lyon]

  ---

  Explanation

  In this code, a TreeSet named frenchCities is created and populated with the following cities: "Paris", "Marseille", "Lyon", "Lille", and "Toulouse". The TreeSet class in Java stores elements in a sorted order according to their natural ordering,

  which, for strings, is lexicographical order.

  Sorted Order of Elements:

  When the elements are added to the TreeSet, they are stored in the following order:

  "Lille"

  "Lyon"

  "Marseille"

  "Paris"

  "Toulouse"

  headSet Method:

  The headSet(E toElement) method of the TreeSet class returns a view of the portion of this set whose elements are strictly less than toElement. In this case, frenchCities.headSet("Marseille") will return a subset of frenchCities containing all

  elements that are lexicographically less than "Marseille".

  Elements Less Than "Marseille":

  From the sorted order, the elements that are less than "Marseille" are:

  "Lille"

  "Lyon"

  Therefore, the output of the System.out.println statement will be [Lille, Lyon].

  Option Evaluations:

  A. [Paris]: Incorrect. "Paris" is lexicographically greater than "Marseille".

  B. [Paris, Toulouse]: Incorrect. Both "Paris" and "Toulouse" are lexicographically greater than "Marseille".

  C. [Lille, Lyon]: Correct. These are the elements less than "Marseille".

  D. Compilation fails: Incorrect. The code compiles successfully.

  E. [Lyon, Lille, Toulouse]: Incorrect. "Toulouse" is lexicographically greater than "Marseille".

• Question 45:

  Given:

  java

  var ceo = new HashMap<>();

  ceo.put("Sundar Pichai", "Google");

  ceo.put("Tim Cook", "Apple");

  ceo.put("Mark Zuckerberg", "Meta");

  ceo.put("Andy Jassy", "Amazon");

  Does the code compile?

  A. True
  B. False
  B. False

  ---

  Explanation

  In this code, a HashMap is instantiated using the var keyword:

  java

  var ceo = new HashMap<>();

  The diamond operator <> is used without explicit type arguments. While the diamond operatorallows the compiler to infer types in many cases, when using var, the compiler requires explicit type information to infer the variable's type.

  Therefore, the code will not compile because the compiler cannot infer the type of the HashMap when both var and the diamond operator are used without explicit type parameters.

  To fix this issue, provide explicit type parameters when creating the HashMap:

  java

  var ceo = new HashMap();

  Alternatively, you can specify the variable type explicitly:

  java

  Map

  contentReference[oaicite:0]{index=0}

• Question 46:

  Which of the following isn't a valid option of the jdeps command?

  A. --check-deps
  B. --generate-open-module
  C. --list-deps
  D. --generate-module-info
  E. --print-module-deps
  F. --list-reduced-deps
  A. --check-deps

  ---

  Explanation

  The jdeps tool is a Java class dependency analyzer that can be used to understand the static dependencies of applications and libraries. It provides several command-line options to customize its behavior.

  Valid jdeps Options:

  --generate-open-module: Generates a module declaration (module-info.java) with open directives for the given JAR files or classes.

  --list-deps: Lists the immediate dependencies of the specified classes or JAR files.

  --generate-module-info: Generates a module declaration (module-info.java) for the given JAR files or classes.

  --print-module-deps: Prints the module dependencies of the specified modules or JAR files.

  --list-reduced-deps: Lists the reduced dependencies, showing only the packages that are directly depended upon.

  Invalid Option:

  --check-deps: There is no --check-deps option in the jdeps tool.

  Conclusion:

  Option A (--check-deps) is not a valid option of the jdeps command.

• Question 47:

  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

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

• Question 48:

  Given:

  java

  StringBuilder result = Stream.of("a", "b")

  .collect(

  () -> new StringBuilder("c"),

  StringBuilder::append,

  (a, b) -> b.append(a)

  );

  System.out.println(result);

  What is the output of the given code fragment?

  A. cbca
  B. acb
  C. cacb
  D. abc
  E. bca
  F. cba
  G. bac
  F. cba

  ---

  Explanation

  In this code, a Stream containing the elements "a" and "b" is processed using the collect method. The collect method is a terminal operation that performs a mutable reduction on the elements of the stream using a Collector. In this case,

  custom implementations for the supplier, accumulator, and combiner are provided.

  Components of the collect Method:

  Supplier:

  () -> new StringBuilder("c")

  This supplier creates a new StringBuilder initialized with the string "c".

  Accumulator:

  StringBuilder::append

  This accumulator appends each element of the stream to the StringBuilder.

  Combiner:

  (a, b) -> b.append(a)

  This combiner is used in parallel stream operations to merge two StringBuilder instances. It appends the contents of a to b.

  Execution Flow:

  Stream Elements:"a", "b"

  Initial StringBuilder:"c"

  Accumulation:

  The first element "a" is appended to "c", resulting in "ca".

  The second element "b" is appended to "ca", resulting in "cab".

  Combiner:

  In this sequential stream, the combiner is not utilized. The combiner is primarily used in parallel streams to merge partial results.

  Final Result:

  The StringBuilder contains "cab". Therefore, the output of the program is:

  nginx

  cab

• Question 49:

  Given:

  java

  import java.io.*;

  class A implements Serializable {

  int number = 1;

  }

  class B implements Serializable {

  int number = 2;

  }

  public class Test {

  public static void main(String[] args) throws Exception {

  File file = new File("o.ser");

  A a = new A();

  var oos = new ObjectOutputStream(new FileOutputStream(file));

  oos.writeObject(a);

  oos.close();

  var ois = new ObjectInputStream(new FileInputStream(file)); B b = (B) ois.readObject();

  ois.close();

  System.out.println(b.number);

  }

  }

  What is the given program's output?

  A. 1
  B. 2
  C. Compilation fails
  D. ClassCastException
  E. NotSerializableException
  D. ClassCastException

  ---

  Explanation

  In this program, we have two classes, A and B, both implementing the Serializable interface, and a Test class with the main method.

  Program Flow:

  Serialization:

  An instance of class A is created and assigned to the variable a.

  An ObjectOutputStream is created to write to the file "o.ser".

  The object a is serialized and written to the file.

  The ObjectOutputStream is closed.

  Deserialization:

  An ObjectInputStream is created to read from the file "o.ser".

  The program attempts to read an object from the file and cast it to an instance of class

  B.

  The ObjectInputStream is closed.

  Analysis:

  Serialization Process:

  The object a is an instance of class A and is serialized into the file "o.ser".

  Deserialization Process:

  When deserializing, the program reads the object from the file and attempts to cast it to class

  B.

  However, the object in the file is of type A, not

  B.

  Since A and B are distinct classes with no inheritance relationship, casting an A instance to B is invalid.

  Exception Details:

  Attempting to cast an object of type A to type B results in a ClassCastException.

  The exception message would be similar to:

  pgsql

  Exception in thread "main" java.lang.ClassCastException: class A cannot be cast to class B

  Conclusion:

  The program compiles successfully but throws a ClassCastException at runtime when it attempts to cast the deserialized object to class

  B.

• Question 50:

  Which of the following statements of local variables declared with var are invalid (Choose 4) ?

  A. var a = 1;(Valid: var correctly infers int)
  B. var b = 2, c = 3.0;
  C. var d[] = new int[4];
  D. var e;
  E. var f = { 6 };
  F. var h = (g = 7);
  B. var b = 2, c = 3.0;
  C. var d[] = new int[4];
  D. var e;
  E. var f = { 6 };

  ---

  Explanation

  1.Valid Use Cases of var

  var is a local variable type inference feature.

  The compiler infers the type from the assigned value .

  Example of valid use:

  java

  var a = 10; // Type inferred as int

  var str = "Hello"; // Type inferred as String

  2.Analyzing the Given Statements

  Statement

  Valid/Invalid

  Reason

  var a = 1;

  Valid

  Type inferred as int.

  var b = 2, c = 3.0;

  #Invalid

  var does not allow multiple declarations in one statement.

  var d[] = new int[4];

  #Invalid

  Array brackets [] are not allowed with var.

  var e;

  #Invalid

  var requires an initializer (cannot be declared without assignment).

  var f = { 6 };

  #Invalid

  { 6 } is an array initializer , which must have an explicit type.

  var h = (g = 7);

  Valid

  g is assigned 7, and h gets its value.

  Thus, the correct answers are:B, C, D, E

  References:

  Java SE 21 - Local Variable Type Inference (var) Java SE 21 - var Restrictions