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

Understanding java.util.function.Predicate<T>

The Predicate interface represents a function thattakes an input and returns a boolean(true or false).

It is often used for filtering operations in functional programming and streams.

Analyzing the Methods:

and(Predicate<? super T> other)→Default method

Combines two predicates usinglogical AND(&&).

java

Predicate startsWithA = s -> s.startsWith("A");

Predicate hasLength3 = s -> s.length() == 3;

Predicate combined = startsWithA.and(hasLength3);

❌isEqual(Object targetRef)→Static method

Not a default method, because it doesnot operate on an instance.

java

Predicate isEqualToHello = Predicate.isEqual("Hello");

negate()→Default method

Negates a predicate (! operator).

java

Predicate notEmpty = s -> !s.isEmpty();

Predicate isEmpty = notEmpty.negate();

❌not(Predicate<? super T> target)→Static method (introduced in Java 11)

Not a default method, since it is static.

or(Predicate<? super T> other)→Default method

Combines two predicates usinglogical OR(||).

❌test(T t)→Abstract method

Not a default method, because every predicatemust implement this method.

Thus, the correct answers are:and(Predicate<? super T> other), negate(), or(Predicate<? super T> other)

References:

Java SE 21 - Predicate Interface

Java SE 21 - Functional Interfaces

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.

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