1z0-830 Exam Dumps - Java SE 21 Developer Professional

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

The DoubleSummaryStatistics class in Java is part of the java.util package and is used to collect and summarize statistics for a stream of double values. Let's analyze how the methods work:

Initialization and Data Insertion

stats1.accept(4.5); → Adds 4.5 to stats1.

stats1.accept(6.0); → Adds 6.0 to stats1.

stats2.accept(3.0); → Adds 3.0 to stats2.

stats2.accept(8.5); → Adds 8.5 to stats2.

Combining stats1 and stats2

stats1.combine(stats2); merges stats2 into stats1, resulting in one statistics summary containing all values {4.5, 6.0, 3.0, 8.5}.

Calculating Output Values

Sum= 4.5 + 6.0 + 3.0 + 8.5 = 22.0

Max= 8.5

Average= (22.0) / 4 = 5.5

Thus, the output is:

yaml

Sum: 22.0, Max: 8.5, Avg: 5.5

References:

Java SE 21 & JDK 21 - DoubleSummaryStatistics

Java SE 21 - Streams and Statistical Operations

Error in for Loop Initialization

The initialization part of a for loopcannot declare multiple variables with different types in a single statement.

Error:

java

for (int i = 0, int j = 3; i < j; i++) {

Fix:Declare variables separately:

java

for (int i = 0, j = 3; i < j; i++) {

lyrics.lines() in Java 21

The lines() method of String returns aStream<String>, splitting the string by line breaks.

Calling .toList() on a streamconverts it to a list.

Valid Code After Fixing the Loop:

java

var lyrics = """

Quand il me prend dans ses bras

Qu'il me parle tout bas

Je vois la vie en rose

""";

for (int i = 0, j = 3; i < j; i++) {

System.out.println(lyrics.lines()

toList()

get(i));

}

Expected Output After Fixing:

vbnet

Quand il me prend dans ses bras

Qu'il me parle tout bas

Je vois la vie en rose

Thus, the correct answer is:Compilation fails.

References:

Java SE 21 - String.lines()

Java SE 21 - for Statement Rules

In this code, an ExecutorService is created with a fixed thread pool of size 2 using Executors.newFixedThreadPool(2). A Runnable task is defined to print "Task is complete" to the console.

The sequence of operations is as follows:

service.submit(task);

This submits the task to the executor service for execution. Since the thread pool has a size of 2 and no other tasks are running, this task will be executed promptly, printing "Task is complete" to the console.

service.shutdown();

This initiates an orderly shutdown of the executor service. In this state, the service stops accepting new tasks

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.