Oracle 1z0-830 - Java SE 21 Developer Professional

In this scenario, we have a Person class that extends ListResourceBundle and a properties file named Person.properties. Both define a resource with the key "name" but with different values:

Person class (ListResourceBundle):Defines the key "name" with the value "Jeanne".

Person.properties file:Defines the key "name" with the value "James".

When the ResourceBundle.getBundle("Person") method is called, the Java runtime searches for a resource bundle with the base name "Person". The search order is as follows:

Class-Based Resource Bundle:The runtime first looks for a class named Person (i.e., Person.class).

Properties File Resource Bundle:If the class is not found, it then looks for a properties file named Person.properties.

In this case, since the Person class is present and accessible, the runtime will load the Person class as the resource bundle. Therefore, the getBundle method returns an instance of the Person class.

Subsequently, when bundle.getString("name") is called, it retrieves the value associated with the key "name" from the Person class, which is "Jeanne".

Thus, the output of the program is:

nginx

Jeanne

Understanding post++ (Post-increment)

post++uses the value first, then increments it.

postResult = post++ + 10;

post starts as 5.

post++ returns 5, then post is incremented to 6.

postResult = 5 + 10 = 15.

Final value of post after this line is 6.

Understanding ++pre (Pre-increment)

++preincrements the value first, then uses it.

preResult = ++pre + 10;

pre starts as 5.

++pre increments pre to 6, then returns 6.

preResult = 6 + 10 = 16.

Final value of pre after this line is 6.

Thus, the final output is:

yaml

postResult: 15, preResult: 16, Final value of post: 6, Final value of pre: 6

References:

Java SE 21 - Operators and Expressions

Java SE 21 - Arithmetic Operators

In this code, two StringBuffer objects, us and uk, are created with the values "US" and "UK", respectively. A stream is then created from these objects using Stream.of(us, uk).

The collect method is used with Collectors.joining("-", "=", ""). The joining collector concatenates the elements of the stream into a single String with the following parameters:

Delimiter ("-"):Inserted between each element.

Prefix ("="):Inserted at the beginning of the result.

Suffix (""):Inserted at the end of the result.

Therefore, the elements "US" and "UK" are concatenated with "-" between them, resulting in "US-UK". The prefix "=" is added at the beginning, resulting in the final output =US-UK.

In this code, the try block throws an IOException. The catch block catches this exception and throws a new RuntimeException. Regardless of exceptions thrown in the try or catch blocks, the finally block is always executed. In this case, the finally block throws an ArithmeticException.

When an exception is thrown in a finally block, it overrides any previous exceptions that were thrown in the try or catch blocks. Therefore, the ArithmeticException thrown in the finally block is the exception that propagates out of the method. As a result, the program terminates with an ArithmeticException.

In Java, the ConcurrentHashMap class provides several constructors that allow for the customization of its initial capacity, load factor, and concurrency level. To configure a ConcurrentHashMap with an initial capacity of 42 and a concurrency level of 10, you can use the following constructor:

java

public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)

Parameters:

initialCapacity: The initial capacity of the hash table. This is the number of buckets that the hash table will have when it is created. In this case, it is set to 42.

loadFactor: A measure of how full the hash table is allowed to get before it is resized. The default value is 0.75, but in this case, it is set to 0.88.

concurrencyLevel: The estimated number of concurrently updating threads. This is used as a hint for internal sizing. In this case, it is set to 10.

Therefore, to create a ConcurrentHashMap with an initial capacity of 42, a load factor of 0.88, and a concurrency level of 10, you can use the following code:

java

var concurrentHashMap = new ConcurrentHashMap<>(42, 0.88f, 10);

Option Evaluations:

A. var concurrentHashMap = new ConcurrentHashMap(42);: This constructor sets the initial capacity to 42 but uses the default load factor (0.75) and concurrency level (16). It does not meet the requirement of setting the concurrency level to 10.

B. None of the suggestions.: This is incorrect because option E provides the correct configuration.

C. var concurrentHashMap = new ConcurrentHashMap();: This uses the default constructor, which sets the initial capacity to 16, the load factor to 0.75, and the concurrency level to 16. It does not meet the specified requirements.

D. var concurrentHashMap = new ConcurrentHashMap(42, 10);: This constructor sets the initial capacity to 42 and the load factor to 10, which is incorrect because the load factor should be a float value between 0 and 1.

E. var concurrentHashMap = new ConcurrentHashMap(42, 0.88f, 10);: This correctly sets the initial capacity to 42, the load factor to 0.88, and the concurrency level to 10, meeting all the specified requirements.

Therefore, the correct answer is option E.

1. Valid Use Cases of var

var is alocal variable type inferencefeature.

The compilerinfers the type from the assigned value.

Example of valid use:

java

var a = 10; // Type inferred as int

var str = "Hello"; // Type inferred as String

2. Analyzing the Given Statements

Statement

Valid/Invalid

Reason

var a = 1;

Valid

Type inferred as int.

var b = 2, c = 3.0;

❌Invalid

var doesnot allow multiple declarationsin one statement.

var d[] = new int[4];

❌Invalid

Array brackets []are not allowedwith var.

var e;

❌Invalid

varrequires an initializer(cannot be declared without assignment).

var f = { 6 };

❌Invalid

{ 6 } is anarray initializer, which must have an explicit type.

var h = (g = 7);

Valid

g is assigned 7, and h gets its value.

Thus, the correct answers are:B, C, D, E

References:

Java SE 21 - Local Variable Type Inference (var)

Java SE 21 - var Restrictions

In this code, three static methods from the Boolean class are used: logicalAnd, logicalOr, and logicalXor. Each method takes two boolean arguments and returns a boolean result based on the respective logical operation.

Evaluation of Each Statement:

Boolean.logicalAnd(1 == 1, 2 < 1)

Operands:

1 == 1 evaluates to true.

2 < 1 evaluates to false.

Operation:

Boolean.logicalAnd(true, false) performs a logical AND operation.

The result is false because both operands must be true for the AND operation to return true.

Output:

System.out.print(false); prints false.

Boolean.logicalOr(1 == 1, 2 < 1)

Operands:

1 == 1 evaluates to true.

2 < 1 evaluates to false.

Operation:

Boolean.logicalOr(true, false) performs a logical OR operation.

The result is true because at least one operand is true.

Output:

System.out.print(true); prints true.

Boolean.logicalXor(1 == 1, 2 < 1)

Operands:

1 == 1 evaluates to true.

2 < 1 evaluates to false.

Operation:

Boolean.logicalXor(true, false) performs a logical XOR (exclusive OR) operation.

The result is true because exactly one operand is true.

Output:

System.out.print(true); prints true.

Combined Output:

Combining the outputs from each statement, the final printed result is:

nginx

falsetruetrue

Understanding Optional.ofNullable(null)

Optional.ofNullable(null); creates an empty Optional (i.e., it contains no value).

Optional.of(null); would throw a NullPointerException, but ofNullable(null); safely creates an empty Optional.

Execution of orElse, orElseGet, and orElseThrow

orElse("Baguette")

Since optionalName is empty, "Baguette" is returned.

bread = "Baguette"

Output:"bread:Baguette"

orElseGet(() -> "Frog legs")

Since optionalName is empty, "Frog legs" is returned from the lambda expression.

dish = "Frog legs"

Output:", dish:Frog legs"

orElseThrow(() -> new Exception())

Since optionalName is empty, an exception is thrown.

The catch block catches this exception and prints ", no cheese.".

Thus, the final output is:

makefile

bread:Baguette, dish:Frog legs, no cheese.

References:

Java SE 21 & JDK 21 - Optional

Java SE 21 - Functional Interfaces

Why does the compilation fail?

TheJava switch statement does not support primitive type pattern matchingin switch expressions as of Java 21.

The case pattern case int i -> "integer"; isinvalidbecausepattern matching with primitive types (like int or long) is not yet supported in switch statements.

The error occurs at case int i -> "integer";, leading to acompilation failure.

Correcting the Code

Since myVar is of type Object,autoboxing converts 0 into an Integer.

To make the code compile, we should use Integer instead of int:

java

Object myVar = 0;

String print = switch (myVar) {

case Integer i -> "integer";

case Long l -> "long";

case String s -> "string";

default -> "";

};

System.out.println(print);

Output:

bash

integer

Thus, the correct answer is:Compilation fails.

References:

Java SE 21 - Pattern Matching for switch

Java SE 21 - switch Expressions