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Oracle Java SE 21 Developer Professional Sample Questions (Q45-Q50):

NEW QUESTION # 45
How would you create a ConcurrentHashMap configured to allow a maximum of 10 concurrent writer threads and an initial capacity of 42?
Which of the following options meets this requirement?

A. None of the suggestions.
B. var concurrentHashMap = new ConcurrentHashMap();
C. var concurrentHashMap = new ConcurrentHashMap(42);
D. var concurrentHashMap = new ConcurrentHashMap(42, 0.88f, 10);
E. var concurrentHashMap = new ConcurrentHashMap(42, 10);

Answer: D

Explanation:
In Java, the ConcurrentHashMap class provides several constructors that allow for the customization of its initial capacity, load factor, and concurrency level. To configure a ConcurrentHashMap with an initial capacity of 42 and a concurrency level of 10, you can use the following constructor:
java
public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Parameters:
* initialCapacity: The initial capacity of the hash table. This is the number of buckets that the hash table will have when it is created. In this case, it is set to 42.
* loadFactor: A measure of how full the hash table is allowed to get before it is resized. The default value is 0.75, but in this case, it is set to 0.88.
* concurrencyLevel: The estimated number of concurrently updating threads. This is used as a hint for internal sizing. In this case, it is set to 10.
Therefore, to create a ConcurrentHashMap with an initial capacity of 42, a load factor of 0.88, and a concurrency level of 10, you can use the following code:
java
var concurrentHashMap = new ConcurrentHashMap<>(42, 0.88f, 10);
Option Evaluations:
* A. var concurrentHashMap = new ConcurrentHashMap(42);: This constructor sets the initial capacity to 42 but uses the default load factor (0.75) and concurrency level (16). It does not meet the requirement of setting the concurrency level to 10.
* B. None of the suggestions.: This is incorrect because option E provides the correct configuration.
* C. var concurrentHashMap = new ConcurrentHashMap();: This uses the default constructor, which sets the initial capacity to 16, the load factor to 0.75, and the concurrency level to 16. It does not meet the specified requirements.
* D. var concurrentHashMap = new ConcurrentHashMap(42, 10);: This constructor sets the initial capacity to 42 and the load factor to 10, which is incorrect because the load factor should be a float value between 0 and 1.
* E. var concurrentHashMap = new ConcurrentHashMap(42, 0.88f, 10);: This correctly sets the initial capacity to 42, the load factor to 0.88, and the concurrency level to 10, meeting all the specified requirements.
Therefore, the correct answer is option E.

NEW QUESTION # 46
Given:
java
public class Test {
class A {
}
static class B {
}
public static void main(String[] args) {
// Insert here
}
}
Which three of the following are valid statements when inserted into the given program?

A. A a = new A();
B. B b = new Test.B();
C. B b = new B();
D. B b = new Test().new B();
E. A a = new Test.A();
F. A a = new Test().new A();

Answer: B,C,F

Explanation:
In the provided code, we have two inner classes within the Test class:
* Class A:
* An inner (non-static) class.
* Instances of A are associated with an instance of the enclosing Test class.
* Class B:
* A static nested class.
* Instances of B are not associated with any instance of the enclosing Test class and can be instantiated without an instance of Test.
Evaluation of Statements:
A: A a = new A();
* Invalid.Since A is a non-static inner class, it requires an instance of the enclosing class Test to be instantiated. Attempting to instantiate A without an instance of Test will result in a compilation error.
B: B b = new Test.B();
* Valid.B is a static nested class and can be instantiated without an instance of Test. This syntax is correct.
C: A a = new Test.A();
* Invalid.Even though A is referenced through Test, it is a non-static inner class and requires an instance of Test for instantiation. This will result in a compilation error.
D: B b = new Test().new B();
* Invalid.While this syntax is used for instantiating non-static inner classes, B is a static nested class and does not require an instance of Test. This will result in a compilation error.
E: B b = new B();
* Valid.Since B is a static nested class, it can be instantiated directly without referencing the enclosing class.
F: A a = new Test().new A();
* Valid.This is the correct syntax for instantiating a non-static inner class. An instance of Test is created, and then an instance of A is created associated with that Test instance.
Therefore, the valid statements are B, E, and F.

NEW QUESTION # 47
Given:
java
public class Test {
static int count;
synchronized Test() {
count++;
}
public static void main(String[] args) throws InterruptedException {
Runnable task = Test::new;
Thread t1 = new Thread(task);
Thread t2 = new Thread(task);
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(count);
}
}
What is the given program's output?

A. It's either 1 or 2
B. It's either 0 or 1
C. It's always 2
D. Compilation fails
E. It's always 1

Answer: D

Explanation:
In this code, the Test class has a static integer field count and a constructor that is declared with the synchronized modifier. In Java, the synchronized modifier can be applied to methods to control access to critical sections, but it cannot be applied directly to constructors. Attempting to declare a constructor as synchronized will result in a compilation error.
Compilation Error Details:
The Java Language Specification does not permit the use of the synchronized modifier on constructors.
Therefore, the compiler will produce an error indicating that the synchronized modifier is not allowed in this context.
Correct Usage:
If you need to synchronize the initialization of instances, you can use a synchronized block within the constructor:
java
public class Test {
static int count;
Test() {
synchronized (Test.class) {
count++;
}
}
public static void main(String[] args) throws InterruptedException {
Runnable task = Test::new;
Thread t1 = new Thread(task);
Thread t2 = new Thread(task);
t1.start();
t2.start();
t1.join();
t2.join();
System.out.println(count);
}
}
In this corrected version, the synchronized block within the constructor ensures that the increment operation on count is thread-safe.
Conclusion:
The original program will fail to compile due to the illegal use of the synchronized modifier on the constructor. Therefore, the correct answer is E: Compilation fails.

NEW QUESTION # 48
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. false
B. Compilation fails.
C. A NullPointerException is thrown.
D. A ClassCastException is thrown.
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 # 49
Given:
java
Object myVar = 0;
String print = switch (myVar) {
case int i -> "integer";
case long l -> "long";
case String s -> "string";
default -> "";
};
System.out.println(print);
What is printed?

A. integer
B. nothing
C. string
D. long
E. Compilation fails.
F. It throws an exception at runtime.

Answer: E

Explanation:
* Why does the compilation fail?
* TheJava switch statement does not support primitive type pattern matchingin switch expressions as of Java 21.
* The case pattern case int i -> "integer"; isinvalidbecausepattern matching with primitive types (like int or long) is not yet supported in switch statements.
* The error occurs at case int i -> "integer";, leading to acompilation failure.
* Correcting the Code
* Since myVar is of type Object,autoboxing converts 0 into an Integer.
* To make the code compile, we should use Integer instead of int:
java
Object myVar = 0;
String print = switch (myVar) {
case Integer i -> "integer";
case Long l -> "long";
case String s -> "string";
default -> "";
};
System.out.println(print);
* Output:
bash
integer
Thus, the correct answer is:Compilation fails.
References:
* Java SE 21 - Pattern Matching for switch
* Java SE 21 - switch Expressions

NEW QUESTION # 50
......

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