Understanding Sealed Classes in Java

A sealed class restricts which other classes can extend it.

A sealed class must explicitly declare its permitted subclasses using 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 it is no longer sealed and can have subclasses.

Bike is declared as final, meaning it cannot be subclassed .

Using isSealed() Method

vehicleClass.isSealed() → true because Vehicle is explicitly marked as sealed.

carClass.isSealed() → false because Car is marked non-sealed.

bikeClass.isSealed() → false because Bike is final, and a final class is not 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

In the provided code, four StringBuilder instances are being created using different constructors:

stringBuilder1 : new StringBuilder()

This constructor creates an empty StringBuilder with an initial capacity of 16 characters.

stringBuilder2 : new StringBuilder(10)

This constructor creates an empty StringBuilder with a specified initial capacity of 10 characters.

stringBuilder3 : new StringBuilder("Java")

This constructor creates a StringBuilder initialized to the contents of the specified string "Java".

stringBuilder4 : new StringBuilder(new char[]{'J', 'a', 'v', 'a'})

This line attempts to create a StringBuilder using a char array. However, the StringBuilder class does not have a constructor that accepts a char array directly. The available constructors are:

StringBuilder()

StringBuilder(int capacity)

StringBuilder(String str)

StringBuilder(CharSequence seq)

Since a char array does not implement the CharSequence interface, and there is no constructor that directly accepts a char array, this line will cause a compilation error.

To initialize a StringBuilder with a char array, you can convert the char array to a String first:

java

var stringBuilder4 = new StringBuilder(new String(new char[]{'J', 'a', 'v', 'a'}));

This approach utilizes the String constructor that accepts a char array, and then passes the resulting String to the StringBuilder constructor.

In this code, there is a type mismatch between the DoubleStream and the Predicate < Double > .

DoubleStream : A sequence of primitive double values.

Predicate <</b> Double > : A functional interface that operates on objects of type Double (the wrapper class), not on primitive double values.

The DoubleStream class provides a method anyMatch(DoublePredicate predicate), where DoublePredicate is a functional interface that operates on primitive double values. However, in the code, a Predicate < Double > is used instead of a DoublePredicate. This mismatch leads to a compilation error because anyMatch cannot accept a Predicate < Double > when working with a DoubleStream.

To correct this, the predicate should be defined as a DoublePredicate to match the primitive double type:

java

DoubleStream doubleStream = DoubleStream.of(3.3, 4, 5.25, 6.66);

DoublePredicate doublePredicate = d - > d < 5;

System.out.println(doubleStream.anyMatch(doublePredicate));

With this correction, the code will compile and print true because there are elements in the stream (e.g., 3.3 and 4.0) that are less than 5.