Oracle 1z0-830 - Java SE 21 Developer Professional

Understanding Try-With-Resources (AutoCloseable)

BoomBoom implements AutoCloseable, meaning its close() method isautomatically calledat the end of the try block.

Step-by-Step Execution

Step 1: Enter Try Block

java

try (BoomBoom boomBoom = new BoomBoom()) {

System.out.print("bim ");

throw new Exception();

}

"bim " is printed.

Anexception (Exception) is thrown, butbefore it is handled, the close() method is executed.

Step 2: close() is Called

java

@Override

public void close() throws Exception {

System.out.print("bam ");

throw new RuntimeException();

}

"bam " is printed.

A new RuntimeException is thrown, but it doesnot override the existing Exception yet.

Step 3: Exception Handling

java

} catch (Exception e) {

System.out.print("boom ");

}

The catch (Exception e)catches the original Exception from the try block.

"boom " is printed.

Final Output

nginx

bim bam boom

Theoriginal Exception is caught, not the RuntimeException from close().

TheRuntimeException from close() is ignoredbecause thecatch block is already handling Exception.

Thus, the correct answer is:bim bam boom

References:

Java SE 21 - Try-With-Resources

Java SE 21 - AutoCloseable Interface

java.io.Console is used for interactive input from the console.

Existing Methods in java.io.Console

reader() → Returns a Reader object.

readLine() → Reads a line of text from the console.

readLine(String fmt, Object... args) → Reads a formatted line.

readPassword() → Reads a password, returning a char[].

readPassword(String fmt, Object... args) → Reads a formatted password.

read() Does Not Exist

Consoledoes not have a read() method.

If character-by-character reading is required, use:

java

Console console = System.console();

Reader reader = console.reader();

int c = reader.read(); // Reads one character

read() is available inReader, butnot in Console.

Thus, the correct answer is:read() does not exist.

References:

Java SE 21 - Console API

Java SE 21 - Reader API

The ServiceLoader class in Java is used to load service providers implementing a given service interface. The following methods are evaluated for their correctness in loading an implementation of MyService:

A. MyService service = ServiceLoader.load(MyService.class).iterator().next();

This method uses the ServiceLoader.load(MyService.class) to create a ServiceLoader instance for MyService. Calling iterator().next() retrieves the next available service provider. If no providers are available, a NoSuchElementException will be thrown. This approach is correct but requires handling the potential exception if no providers are found.

B. MyService service = ServiceLoader.load(MyService.class).findFirst().get();

This method utilizes the findFirst() method introduced in Java 9, which returns an Optional describing the first available service provider. Calling get() on the Optional retrieves the service provider if present; otherwise, a NoSuchElementException is thrown. This approach is correct and provides a more concise way to obtain the first service provider.

C. MyService service = ServiceLoader.getService(MyService.class);

The ServiceLoader class does not have a method named getService. Therefore, this method is incorrect and will result in a compilation error.

D. MyService service = ServiceLoader.services(MyService.class).getFirstInstance();

The ServiceLoader class does not have a method named services or getFirstInstance. Therefore, this method is incorrect and will result in a compilation error.

In summary, options A and B are correct methods to load an implementation of MyService using ServiceLoader.

In this code, a HashMap is instantiated using the var keyword:

java

var ceo = new HashMap<>();

The diamond operator <> is used without explicit type arguments. While the diamond operatorallows the compiler to infer types in many cases, when using var, the compiler requires explicit type information to infer the variable's type.

Therefore, the code will not compile because the compiler cannot infer the type of the HashMap when both var and the diamond operator are used without explicit type parameters.

To fix this issue, provide explicit type parameters when creating the HashMap:

java

var ceo = new HashMap();

Alternatively, you can specify the variable type explicitly:

java

Map

contentReference[oaicite:0]{index=0}

In Java, a lambda expression can be used where a target type is a functional interface. A functional interface is an interface that contains exactly one abstract method. This concept is also known as a Single Abstract Method (SAM) type.

Analyzing each interface:

Interface A: Contains a single default method ma(). Since default methods are not abstract, A has no abstract methods.

Interface B: Extends A and adds a static method mb(). Static methods are also not abstract, so B has no abstract methods.

Interface C: Extends B and declares two abstract methods: ma() (which overrides the default method from A) and mc(). Therefore, C has two abstract methods.

Interface D: Extends C and adds another abstract method md(). Thus, D has three abstract methods.

Interface E: Extends D and provides default implementations for ma(), mb(), and mc(). However, it does not provide an implementation for md(), leaving it as the only abstract method in E.

For an interface to be a functional interface, it must have exactly one abstract method. In this case, E has one abstract method (md()), so it qualifies as a functional interface. However, the question asks which interface can be the target of a lambda expression. Since E is a functional interface, it can be the target of a lambda expression.

Therefore, the correct answer is D (E).