Oracle 1Z0-830 - Questions & Answers

(BufferedWriter writer = new BufferedWriter("file.txt") is incorrect.)

The incorrect statement option B is because BufferedWriter does not have a constructor that accepts a String (file name) directly. The correct way to use BufferedWriter is to wrap it around a FileWriter, like this:

java

try (BufferedWriter writer = new BufferedWriter(new FileWriter("file.txt"))) { writer.write("Hello");

}

Evaluation of Other Options:

Option A (Files.write)# Correct Uses Files.write() to write bytes to a file.
Efficient and concise method for writing small text files.

Option C (FileOutputStream)# Correct
Uses a FileOutputStream to write raw bytes to a file.
Works for both text and binary data.

Option D (PrintWriter)# Correct
Uses PrintWriter for formatted text output.

Option F (FileWriter)# Correct
Uses FileWriter to write text data.

Option E (None of the suggestions)# Incorrect because option B is incorrect.

Understanding Variable Assignments

java

Integer frenchRevolution = 1789;

Object o1 = new String("1789");

Object o2 = frenchRevolution;

frenchRevolution = null;

frenchRevolution is an Integer with value 1789 .

o1 is a String with value "1789" .

o2 stores a 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 the String 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 are equal strings , the output is:

arduino

true

Thus, the correct answer is:true

References:
Java SE 21 - Integer.toString()
Java SE 21 - String.equals()

Analyzing the thrower() Method Execution java

int i = 0;

return i / i;
i / i evaluates to 0 / 0, which throws ArithmeticException (/ by zero) .
Since catch (NumberFormatException e) does not match ArithmeticException, it is skipped.
The finally block always executes, printing:

nginx

Beaujolais Nouveau, The exception propagates back to main().
Handling the Exception in main() java

try {

thrower();

}
catch (Exception e) {

System.out.print("Chablis, ");

}
finally {

}
Since thrower() throws ArithmeticException, it is caught by catch (Exception e).
"Chablis, "is printed.
Final Output

nginx

References:
Java SE 21 - Exception Handling
Java SE 21 - finally Block Execution

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

List<String> 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.

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, 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].

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 this code, now is assigned the current date using LocalDate.now(). The goal is to format this date to the ISO week date format, which represents dates in the YYYY-'W'WW-E pattern, where:

YYYY: Week-based year

'W': Literal 'W' character

WW: Week number

E: Day of the week

Given that the present day is December 31, 2024, this date falls in the first week of the week-based year 2025. Therefore, the ISO week date representation would be 2025-W01-2, where '2' denotes Tuesday.

Among the provided formatters:

format1: This line attempts to create a DateTimeFormatter using a constructor, which is incorrect because DateTimeFormatter does not have a public constructor that accepts a pattern directly. This would result in a compilation error.

format2: This is correctly assigned the predefined DateTimeFormatter.ISO_WEEK_DATE, which formats dates in the ISO week date format.

format3: This line attempts to create a DateFormat instance using a field, which is incorrect because DateFormat does not have such a constructor. This would result in a compilation error.

format4: This line attempts to get a DateFormat instance using an integer field, which is incorrect because DateFormat.getDateInstance() does not accept such parameters. This would result in a compilation error.

Therefore, the only correct and applicable formatter is format 2. Using format2 in the now.format() method will produce the desired output: 2025-W01-2.