Oracle 1z0-830 - Java SE 21 Developer Professional

In the provided code, a list abc is created containing the strings "a", "b", and "c". The first forEach operation attempts to convert each element to uppercase by assigning x = x.toUpperCase();. However, this assignment only changes the local variable x within the lambda expression and does not modify the elements in the original list abc. Strings in Java are immutable, meaning their values cannot be changed once created. Therefore, the original list remains unchanged.

The second forEach operation iterates over the original list and prints each element. Since the list was not modified, the output will be the concatenation of the original elements: abc.

To achieve the output ABC, you would need to collect the transformed elements into a new list, as shown below:

java

List abc = List.of("a", "b", "c");

List upperCaseAbc = abc.stream()

map(String::toUpperCase)

collect(Collectors.toList());

upperCaseAbc.forEach(System.out::print);

In this corrected version, the map operation creates a new stream with the uppercase versions of the original elements, which are then collected into a new list upperCaseAbc. The forEach operation then prints ABC.

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

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

In Java, FileOutputStream and ObjectOutputStream are used for writing data to files, but they have different purposes and methods. Let's analyze each statement:

fos.write("Today");

The FileOutputStream class is designed to write raw byte streams to files. The write method in FileOutputStream expects a parameter of type int or byte[]. Since "Today" is a String, passing it directly to fos.write("Today"); will cause a compilation error because there is no write method in FileOutputStream that accepts a String parameter.

fos.writeObject("Today");

The FileOutputStream class does not have a method named writeObject. The writeObject method is specific to ObjectOutputStream. Therefore, attempting to call fos.writeObject("Today"); will result in a compilation error.

oos.write("Today");

The ObjectOutputStream class is used to write objects to an output stream. However, it does not have a write method that accepts a String parameter. The available write methods in ObjectOutputStream are for writing primitive data types and objects. Therefore, oos.write("Today"); will cause a compilation error.

oos.writeObject("Today");

The ObjectOutputStream class provides the writeObject method, which is used to serialize objects and write them to the output stream. Since String implements the Serializable interface, "Today" can be serialized. Therefore, oos.writeObject("Today"); is valid and compiles successfully.

In summary, the only statement that compiles without errors is oos.writeObject("Today");.

References:

Java SE 21 & JDK 21 - ObjectOutputStream

Java SE 21 & JDK 21 - FileOutputStream

In this code, we have two interfaces and a class with a main method:

WithDefaultMethod Interface:

Declares a default method print() that outputs "default".

WithStaticMethod Interface:

Extends WithDefaultMethod.

Declares a static method print() that outputs "static".

DefaultAndStaticMethods Class:

Contains the main method, which calls WithStaticMethod.print().

Key Points:

Static Methods in Interfaces:

Static methods in interfaces are not inherited by implementing or extending classes or interfaces. They belong solely to the interface in which they are declared.

Default Methods in Interfaces:

Default methods can be inherited by implementing classes, but they cannot be overridden by static methods in subinterfaces.

Execution Flow:

The main method calls WithStaticMethod.print().

This invokes the static method print() defined in the WithStaticMethod interface, which outputs "static".

Therefore, the program compiles successfully and prints static.

Understanding do-while Execution

A do-while loopexecutes at least oncebefore checking the condition.

++counter < 3 increments counterbeforeevaluating the condition.

Step-by-Step Execution

Iteration 1:counter = 0, print "0", then ++counter becomes 1, condition 1 < 3 istrue.

Iteration 2:counter = 1, print "1", then ++counter becomes 2, condition 2 < 3 istrue.

Iteration 3:counter = 2, print "2", then ++counter becomes 3, condition 3 < 3 isfalse, so loop exits.

Final Output

0 1 2

Thus, the correct answer is:0 1 2

References:

Java SE 21 - Control Flow Statements

Java SE 21 - do-while Loop

Understanding Inner Classes in Java

Aninner class (non-static nested class)requires an instance of the outer classbefore it can be instantiated.

Incorrect instantiationof the inner class at n1:

java

InnerClass innerObject = new InnerClass(); // Compilation error

Since InnerClass is anon-staticinner class, itmust be created from an instance of OuterClass.

Correct Way to Instantiate the Inner Class

java

OuterClass outerObject = new OuterClass();

OuterClass.InnerClass innerObject = outerObject.new InnerClass(); // Correct

Thiscorrectly associatesthe inner class with an instance of OuterClass.

Why Does Compilation Fail?

The error occurs atline n1because InnerClass is beinginstantiated incorrectly.

Thus, the correct answer is:Compilation fails at line n1.

References:

Java SE 21 - Nested and Inner Classes

Java SE 21 - Accessing Outer Class Members

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.

In the provided code, the class ThisCalls has two constructors:

No-Argument Constructor (ThisCalls()):

This constructor calls the boolean constructor with this(true);.

Boolean Constructor (ThisCalls(boolean flag)):

This constructor attempts to call the no-argument constructor with this();.

This setup creates a circular call between the two constructors:

The no-argument constructor calls the boolean constructor.

The boolean constructor calls the no-argument constructor.

Such a circular constructor invocation leads to a compile-time error in Java, specifically "recursiveconstructor invocation." The Java Language Specification (JLS) states:

"It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this."

Therefore, the code will not compile due to this recursive constructor invocation.

In this code, the class Main contains both an instance variable instanceVar and a static variable staticVar. The sequence of initialization and execution is as follows:

Static Variable Initialization:

staticVar is declared and initialized to 666.

Static Block Execution:

The static block executes, updating staticVar to 42.

Instance Variable Initialization:

When a new instance of Main is created, instanceVar is initialized to the current value of staticVar, which is 42.

main Method Execution:

The main method creates a new instance of Main and prints the values of instanceVar and staticVar.

Therefore, the output of the program is 42 42.