Pass the Oracle Java SE 1z0-830 Questions and answers with CertsForce

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 the verifyNotNull method, the following operations are performed:

Assertion to Enable Assertions:

java

boolean enabled = false;

assert enabled = true;

assert enabled;

The variable enabled is initially set to false.

The first assertion assert enabled = true; assigns true to enabled if assertions are enabled. If assertions are disabled, this assignment does not occur.

The second assertion assert enabled; checks if enabled is true. If assertions are enabled and the previous assignment occurred, this assertion passes. If assertions are disabled, this assertion is ignored.

Dereferencing the input Object:

java

System.out.println(input.toString());

This line attempts to call the toString() method on the input object. If input is null, this will throw a NullPointerException.

Assertion to Check input for null:

java

assert input != null;

This assertion checks that input is not null. If input is null and assertions are enabled, this assertion will fail, throwing an AssertionError. If assertions are disabled, this assertion is ignored.

Analysis:

If Assertions Are Enabled:

The enabled variable is set to true by the first assertion, and the second assertion passes.

If input is null, calling input.toString() will throw a NullPointerException before the final assertion is reached.

If input is not null, input.toString() executes without issue, and the final assertion assert input != null; passes.

If Assertions Are Disabled:

The enabled variable remains false, but the assertions are ignored, so this has no effect.

If input is null, calling input.toString() will throw a NullPointerException.

If input is not null, input.toString() executes without issue.

Conclusion:

A NullPointerException is thrown if input is null, regardless of whether assertions are enabled or disabled. Therefore, the correct answer is:

C. Only if assertions are disabled and the input argument is null

Understanding Variable Assignments

java

Integer frenchRevolution = 1789;

Object o1 = new String("1789");

Object o2 = frenchRevolution;

frenchRevolution = null;

frenchRevolution is an Integer with value1789.

o1 is aString with value "1789".

o2 storesa 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 theString 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 areequal strings, the output is:

arduino

true

Thus, the correct answer is:true

References:

Java SE 21 - Integer.toString()

Java SE 21 - String.equals()

In Java generics, wildcards are used to relax the type constraints of generic types. The extends wildcard () denotes an upper bounded wildcard, allowing any type that is a subclass of Type. Conversely, the super wildcard () denotes a lower bounded wildcard, allowing any type that is a superclass of Type.

Option A:

java

public List getListSuper() {

return new ArrayList();

}

Here, List represents a list that can hold IOException objects and objects of its supertypes. Since Exception is a superclass of IOException, ArrayList is compatible with List. Therefore, this method compiles successfully.

Option B:

java

public List getListExtends() {

return new ArrayList();

}

In this case, List represents a list that can hold objects of IOException and its subclasses. Since FileNotFoundException is a subclass of IOException, ArrayList is compatible with List. Thus, this method compiles successfully.

Option C:

java

public List getListExtends() {

return new ArrayList();

}

Here, List expects a list of IOException or its subclasses. However, Exception is a superclass of IOException, not a subclass. Therefore, ArrayList is not compatible with List, and this method will not compile.

Option D:

java

public List getListSuper() {

return new ArrayList();

}

In this scenario, List expects a list that can hold IOException objects and objects of its supertypes. Since FileNotFoundException is a subclass of IOException, ArrayList is not compatible with List, and this method will not compile.

Therefore, the methods in options A and B compile successfully, while those in options C and D do not.

Understanding Constructors vs. Methods in Java

In Java, aconstructormustnot have a return type.

The followingis NOT a constructorbut aregular method:

java

void Versailles() { // This is NOT a constructor!

Correct way to define a constructor:

java

public Versailles() { // Constructor must not have a return type

Since there isno constructor explicitly defined,Java provides a default no-argument constructor, which does nothing.

Why Does Compilation Fail?

void Versailles() is interpreted as amethod,not a constructor.

This means the default constructor (which does nothing) is called.

Since the method Versailles() is never called, the object fields remain uninitialized.

If the constructor were correctly defined, the output would be:

nginx

Hall of Mirrors has 17 mirrors.

The gardens cover 800 hectares.

How to Fix It

java

public Versailles() { // Corrected constructor

this.mirrorsCount = 17;

this.gardensHectares = 800;

System.out.println("Hall of Mirrors has " + mirrorsCount + " mirrors.");

System.out.println("The gardens cover " + gardensHectares + " hectares.");

}

Thus, the correct answer is:Compilation fails at line n1.

References:

Java SE 21 - Constructors

Java SE 21 - Methods vs. Constructors