ORACLE 1Z0-830 EXAM BOOK: JAVA SE 21 DEVELOPER PROFESSIONAL - TESTPASSKING ONE OF 10 LEADING PLANFORM

Oracle 1z0-830 Exam Book: Java SE 21 Developer Professional - TestPassKing One of 10 Leading Planform

Oracle 1z0-830 Exam Book: Java SE 21 Developer Professional - TestPassKing One of 10 Leading Planform

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Oracle Java SE 21 Developer Professional Sample Questions (Q80-Q85):

NEW QUESTION # 80
Given:
java
List<String> l1 = new ArrayList<>(List.of("a", "b"));
List<String> l2 = new ArrayList<>(Collections.singletonList("c"));
Collections.copy(l1, l2);
l2.set(0, "d");
System.out.println(l1);
What is the output of the given code fragment?

  • A. [d]
  • B. [a, b]
  • C. [d, b]
  • D. [c, b]
  • E. An IndexOutOfBoundsException is thrown
  • F. An UnsupportedOperationException is thrown

Answer: D

Explanation:
In this code, two lists l1 and l2 are created and initialized as follows:
* l1 Initialization:
* Created using List.of("a", "b"), which returns an immutable list containing the elements "a" and
"b".
* Wrapped with new ArrayList<>(...) to create a mutable ArrayList containing the same elements.
* l2 Initialization:
* Created using Collections.singletonList("c"), which returns an immutable list containing the single element "c".
* Wrapped with new ArrayList<>(...) to create a mutable ArrayList containing the same element.
State of Lists Before Collections.copy:
* l1: ["a", "b"]
* l2: ["c"]
Collections.copy(l1, l2):
The Collections.copy method copies elements from the source list (l2) into the destination list (l1). The destination list must have at least as many elements as the source list; otherwise, an IndexOutOfBoundsException is thrown.
In this case, l1 has two elements, and l2 has one element, so the copy operation is valid. After copying, the first element of l1 is replaced with the first element of l2:
* l1 after copy: ["c", "b"]
l2.set(0, "d"):
This line sets the first element of l2 to "d".
* l2 after set: ["d"]
Final State of Lists:
* l1: ["c", "b"]
* l2: ["d"]
The System.out.println(l1); statement outputs the current state of l1, which is ["c", "b"]. Therefore, the correct answer is C: [c, b].


NEW QUESTION # 81
Which of the following statements are correct?

  • A. None
  • B. You can use 'private' access modifier with all kinds of classes
  • C. You can use 'protected' access modifier with all kinds of classes
  • D. You can use 'public' access modifier with all kinds of classes
  • E. You can use 'final' modifier with all kinds of classes

Answer: A

Explanation:
1. private Access Modifier
* The private access modifiercan only be used for inner classes(nested classes).
* Top-level classes cannot be private.
* Example ofinvaliduse:
java
private class MyClass {} // Compilation error
* Example ofvaliduse (for inner class):
java
class Outer {
private class Inner {}
}
2. protected Access Modifier
* Top-level classes cannot be protected.
* protectedonly applies to members (fields, methods, and constructors).
* Example ofinvaliduse:
java
protected class MyClass {} // Compilation error
* Example ofvaliduse (for methods/fields):
java
class Parent {
protected void display() {}
}
3. public Access Modifier
* Atop-level class can be public, butonly one public class per file is allowed.
* Example ofvaliduse:
java
public class MyClass {}
* Example ofinvaliduse:
java
public class A {}
public class B {} // Compilation error: Only one public class per file
4. final Modifier
* finalcan be used with classes, but not all kinds of classes.
* Interfaces cannot be final, because they are meant to be implemented.
* Example ofinvaliduse:
java
final interface MyInterface {} // Compilation error
Thus,none of the statements are fully correct, making the correct answer:None References:
* Java SE 21 - Access Modifiers
* Java SE 21 - Class Modifiers


NEW QUESTION # 82
Given:
java
StringBuffer us = new StringBuffer("US");
StringBuffer uk = new StringBuffer("UK");
Stream<StringBuffer> 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. -US=UK
  • C. An exception is thrown.
  • D. Compilation fails.
  • E. US=UK
  • F. US-UK

Answer: A

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.


NEW QUESTION # 83
Given:
java
Integer frenchRevolution = 1789;
Object o1 = new String("1789");
Object o2 = frenchRevolution;
frenchRevolution = null;
Object o3 = o2.toString();
System.out.println(o1.equals(o3));
What is printed?

  • A. A NullPointerException is thrown.
  • B. A ClassCastException is thrown.
  • C. false
  • D. Compilation fails.
  • E. true

Answer: E

Explanation:
* Understanding Variable Assignments
java
Integer frenchRevolution = 1789;
Object o1 = new String("1789");
Object o2 = frenchRevolution;
frenchRevolution = null;
* frenchRevolution is an Integer with value1789.
* o1 is aString with value "1789".
* o2 storesa reference to frenchRevolution, which is an Integer (1789).
* frenchRevolution = null;only nullifies the reference, but o2 still holds the Integer 1789.
* Calling toString() on o2
java
Object o3 = o2.toString();
* o2 refers to an Integer (1789).
* Integer.toString() returns theString representation "1789".
* o3 is assigned "1789" (String).
* Evaluating o1.equals(o3)
java
System.out.println(o1.equals(o3));
* o1.equals(o3) isequivalent to:
java
"1789".equals("1789")
* Since both areequal strings, the output is:
arduino
true
Thus, the correct answer is:true
References:
* Java SE 21 - Integer.toString()
* Java SE 21 - String.equals()


NEW QUESTION # 84
Given:
java
sealed class Vehicle permits Car, Bike {
}
non-sealed class Car extends Vehicle {
}
final class Bike extends Vehicle {
}
public class SealedClassTest {
public static void main(String[] args) {
Class<?> vehicleClass = Vehicle.class;
Class<?> carClass = Car.class;
Class<?> bikeClass = Bike.class;
System.out.print("Is Vehicle sealed? " + vehicleClass.isSealed() +
"; Is Car sealed? " + carClass.isSealed() +
"; Is Bike sealed? " + bikeClass.isSealed());
}
}
What is printed?

  • A. Is Vehicle sealed? false; Is Car sealed? false; Is Bike sealed? false
  • B. Is Vehicle sealed? true; Is Car sealed? false; Is Bike sealed? false
  • C. Is Vehicle sealed? true; Is Car sealed? true; Is Bike sealed? true
  • D. Is Vehicle sealed? false; Is Car sealed? true; Is Bike sealed? true

Answer: B

Explanation:
* Understanding Sealed Classes in Java
* Asealed classrestricts which other classes can extend it.
* A sealed classmust explicitly declare its permitted subclassesusing the permits keyword.
* Subclasses can be declared as:
* sealed(restricts further extension).
* non-sealed(removes the restriction, allowing unrestricted subclassing).
* final(prevents further subclassing).
* Analyzing the Given Code
* Vehicle is declared as sealed with permits Car, Bike, meaning only Car and Bike can extend it.
* Car is declared as non-sealed, which means itis no longer sealedand can have subclasses.
* Bike is declared as final, meaningit cannot be subclassed.
* Using isSealed() Method
* vehicleClass.isSealed() #truebecause Vehicle is explicitly marked as sealed.
* carClass.isSealed() #falsebecause Car is marked non-sealed.
* bikeClass.isSealed() #falsebecause Bike is final, and a final class isnot considered sealed.
* Final Output
csharp
Is Vehicle sealed? true; Is Car sealed? false; Is Bike sealed? false
Thus, the correct answer is:"Is Vehicle sealed? true; Is Car sealed? false; Is Bike sealed? false" References:
* Java SE 21 - Sealed Classes
* Java SE 21 - isSealed() Method


NEW QUESTION # 85
......

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