🛣️☕2️⃣1️⃣ Dear followers, let's prepare for Java 21 certification together!
1️⃣ How would you answer this question:
Given:
What is printed?
- A) Chanel
- B) Chanel Dior Louis Vuitton
- C) An ArrayIndexOutOfBoundsException is thrown at runtime.
- D) Compilation fails.
#PathToJava21 #java #certification
💡 Solution
Correct Answer:
A) Chanel
Detailed Explanation:
Initialization:
var hauteCouture = new String[]{ "Chanel", "Dior", "Louis Vuitton" };
var i = 0;
The hauteCouture array contains: ["Chanel", "Dior", "Louis Vuitton"].
i is initialized to 0.
do-while Loop Execution:
The do-while loop ensures that the body executes at least once.
First Iteration:
Body Execution:
System.out.print(hauteCouture[i] + " ");
i = 0, so hauteCouture[0] is accessed, and "Chanel " is printed.
Condition Check:
i++ > 0
The post-increment operator (i++) means:
First, the current value of i (which is 0) is compared with 0 in the condition.
The comparison evaluates to false because 0 > 0 is false.
After the condition is evaluated, i is incremented to 1.
Loop Ends:
Since the condition i++ > 0 evaluated to false, the loop terminates after the first iteration.
Final Output:
Only "Chanel " is printed.
Why Other Options Are Incorrect:
B) Chanel Dior Louis Vuitton: The loop terminates after the first iteration, so only "Chanel" is printed.
C) An ArrayIndexOutOfBoundsException is thrown at runtime: The loop does not run out of bounds because it terminates before accessing an invalid index.
D) Compilation fails: The code compiles successfully without any syntax errors.
Nested Classes: https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html
[JLS] Postfix Increment Operator ++: https://docs.oracle.com/javase/specs/jls/se16/html/jls-15.html#jls-15.15.2
Assignment, Arithmetic, and Unary Operators: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html
(Controlling Program Flow)
2️⃣ How would you answer this question:
Given:
What is printed?
A)
Inner class:
------------
Outer field
B) Nothing
C) An exception is thrown at runtime.
D) Compilation fails at line n1.
E) Compilation fails at line n2.
#PathToJava21 #java #certification
💡 Solution
The correct answer is:
D) Compilation fails at line n1.
Explanation:
In Java, an inner (non-static) class is associated with an instance of its enclosing class.
To create an instance of the inner class, you must first create an instance of the outer class.
This is because the inner class requires an enclosing instance of the outer class to access its members.
Key Points in the Code:
Inner Class Definition:
InnerClass is a non-static inner class of OuterClass.
It can access the members of the enclosing OuterClass instance (e.g., outerField).
The Problem at Line n1:
InnerClass innerObject = new InnerClass();
This line attempts to instantiate the inner class without specifying an enclosing OuterClass instance.
In Java, the correct way to instantiate an inner class is:
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
Here, outerObject is an instance of the OuterClass created earlier.
Why Compilation Fails:
Since InnerClass is not static, the compiler requires an explicit enclosing instance to create the inner class.
The provided code does not specify an instance of OuterClass, so it results in a compilation error at line n1.
Corrected Code:
To fix the issue, update line n1 as follows:
OuterClass.InnerClass innerObject = outerObject.new InnerClass();
Output for Corrected Code:
If the code is fixed, the program will print:
Inner class:
------------
Outer field
Why Other Options Are Incorrect:
A) "Outer field": This would only happen if the code were correct.
B) Nothing: The code doesn't run due to the compilation error.
C) An exception is thrown at runtime: The error is at compile time, not runtime.
E) Compilation fails at line n2: Line n2 is correct and would work if line n1 were fixed
Nested Classes: https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html
(Using Object-Oriented Concepts in Java)
3️⃣ How would you answer this question:
Given:
What is printed?
- A) 1 1 1 1
- B) 1 5 5 1
- C) 1 1 2 3
- D) 1 1 2 2
- E) 5 5 2 3
#PathToJava21 #java #certification
💡 Solution
The correct answer is:
C) 1 1 2 3
Explanation:
Initialization:
The deque contains: [1, 2, 3, 4, 5] (elements are added at the end).
First Operation: peek():
System.out.print(deque.peek() + " ");
peek() retrieves, but does not remove, the head of the deque (FIFO behavior by default for ArrayDeque).
The head is 1.
Output so far: 1
Deque state: [1, 2, 3, 4, 5]
Second Operation: poll():
System.out.print(deque.poll() + " ");
poll() retrieves and removes the head of the deque.
The head is 1, so it is removed.
Output so far: 1 1
Deque state: [2, 3, 4, 5]
Third Operation: pop():
System.out.print(deque.pop() + " ");
pop() is equivalent to removeFirst() for a deque. It retrieves and removes the head of the deque.
The head is 2, so it is removed.
Output so far: 1 1 2
Deque state: [3, 4, 5]
Fourth Operation: element():
System.out.print(deque.element() + " ");
element() retrieves, but does not remove, the head of the deque. It is similar to peek() but throws an exception if the deque is empty (not the case here).
The head is 3.
Output so far: 1 1 2 3
Deque state: [3, 4, 5]
Final Output:
The printed result is: 1 1 2 3.
Incorrect Options:
A) 1 1 1 1: The values change after poll() and pop().
B) 1 5 5 1: This does not match the deque behavior.
D) 1 1 2 2: The fourth output is 3, not 2.
E) 5 5 2 3: The initial head is 1, not 5.
(Working with Arrays and Collections)
4️⃣ How would you answer this question:
Given:
What is printed?
- A)0 1 2 3
- B) 0 1 2
- C) 1 2 3 4
- D) 1 2 3
- E) An exception is thrown.
- F) Compilation fails.
#PathToJava21 #java #certification
💡 Solution
Explanation:
Initialization:
var counter = 0;
The variable counter is initialized to 0.
do-while Loop:
The do-while loop will execute the block of code at least once, regardless of the condition, because the condition is checked after the loop body executes.
First Iteration:
System.out.print(counter + " ");
Prints 0 (value of counter is 0).
++counter < 3
The ++counter increments counter to 1.
The condition 1 < 3 evaluates to true, so the loop continues.
Second Iteration:
System.out.print(counter + " ");
Prints 1 (value of counter is 1).
++counter < 3
The ++counter increments counter to 2.
The condition 2 < 3 evaluates to true, so the loop continues.
Third Iteration:
System.out.print(counter + " ");
Prints 2 (value of counter is 2).
++counter < 3
The ++counter increments counter to 3.
The condition 3 < 3 evaluates to false, so the loop ends.
Final Output:
The loop has printed: 0 1 2.
Incorrect Options:
- A) 0 1 2 3: The number 3 is not printed because the condition ++counter < 3 becomes false when counter is incremented to 3.
- C) 1 2 3 4: This is incorrect; the output starts at 0, and the loop terminates before printing 3.
- D) 1 2 3: This is incorrect; the output includes 0 as the first printed value.
- E) An exception is thrown: No exception is thrown in this code.
- F) Compilation fails: The code compiles successfully.
[JLS] Prefix Increment Operator ++: https://docs.oracle.com/javase/specs/jls/se16/html/jls-15.html#jls-15.15.1
Assignment, Arithmetic, and Unary Operators: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html
(Controlling Program Flow)
5️⃣ How would you answer this question:
Which of the following java.io.Console does not exist?
- read()
- reader()
- readLine()
- readLine(String fmt, Object... args)
- readPassword()
- readPassword(String fmt, Object... args)
#PathToJava21 #java #certification
💡 Solution
The correct answer is:
read() does not exist in java.io.Console.
Here’s a breakdown of the methods mentioned:
1. read()
Does not exist in java.io.Console. This method is not part of the Console class.
2. reader()
Exists: This method returns a Reader object associated with the console, allowing low-level character input.
3. readLine()
Exists: This method reads a single line of text from the console.
4. readLine(String fmt, Object... args)
Exists: This method displays a formatted prompt (based on the format string and arguments) and reads a line of text from the console.
5. readPassword()
Exists: This method reads a password or a confidential text from the console, masking the input for security.
6. readPassword(String fmt, Object... args)
Exists: This method displays a formatted prompt (based on the format string and arguments) and reads a password or confidential text, masking the input for security.
Final Answer:
read() is the method that does not exist in java.io.Console.
(Using Java I/O API)
6️⃣ How would you answer this question:
Which of the following statements are correct?
- A) You can use 'private' access modifier with all kinds of classes
- B) 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
- F) None
#PathToJava21 #java #certification
💡 Solution
The correct answer is: F) None.
Here’s why the individual options are incorrect:
A) You can use 'private' access modifier with all kinds of classes
Explanation: The private access modifier cannot be applied to top-level classes (classes declared directly in a file). It can only be applied to inner classes (classes declared inside another class).
B) You can use 'protected' access modifier with all kinds of classes
Explanation: The protected access modifier is not applicable to top-level classes. It can only be used for members (fields, methods, and constructors) of a class.
D) You can use 'public' access modifier with all kinds of classes
Explanation: The public access modifier can be applied to top-level classes, but it cannot be used with local classes (classes declared inside a method) or anonymous classes.
E) You can use 'final' modifier with all kinds of classes
Explanation: The final modifier can be applied to top-level classes and inner classes, but it cannot be applied to abstract classes since final prevents inheritance, and abstract classes are designed to be extended.
F) None
None of the options are universally correct for all types of classes. Thus, this is the correct answer.
[JLS] Class Modifiers:
The access modifier public (§6.6) pertains only to top level classes (§7.6) and member classes (§8.5, §9.5), not to local classes (§14.3) or anonymous classes (§15.9.5).
The access modifiers protected and private pertain only to member classes.
The modifier static pertains only to member classes and local classes.
It is a compile-time error if the same keyword appears more than once as a modifier for a class declaration, or if a class declaration has more than one of the access modifiers public, protected, and private.
It is a compile-time error if a class declaration has more than one of the modifiers sealed, non-sealed, and final.