Question 365 of 509

Working with Streams and Lambda Expressions →hardMultiple SelectObjective-mapped

Quick Answer

The answer is that `this::toString`, `String::isEmpty`, and `SomeClass::new` are all valid method references in Java 17. These three examples cover the four fundamental types of method references: `this::toString` is a reference to an instance method of a particular object (the current instance), `String::isEmpty` is a reference to an instance method of an arbitrary object of a specific type (where the first parameter becomes the instance), and `SomeClass::new` is a constructor reference. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your ability to distinguish between static method references, instance method references on specific objects, instance method references on arbitrary objects, and constructor references—a common trap is confusing `String::isEmpty` with a static method call, when it actually requires an instance of String to be passed as the implicit first argument. A helpful memory tip is to think of the colon-colon operator as a "double colon double check": always verify whether the method is static, belongs to a specific instance, or is a constructor, and remember that `this` and `super` are valid as the qualifier for instance method references.

1Z0-829 Working with Streams and Lambda Expressions Practice Question

This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.

Which THREE are valid method references in Java 17? (Choose three.)

Question 1hardmulti select

Full question →

A
String::new
Why wrong: This is not a method reference; new is a constructor, but the syntax is String::new for constructor reference. Actually, String::new is a valid constructor reference. So D is correct? That makes four correct. Need to adjust. Let's make D invalid: e.g., 'System::out::println' is invalid. Or we can have one invalid among these. So change D to 'System::out::println' which is invalid because it's a two-level reference. Then we have A, B, C as correct, D invalid, E also correct? We need exactly three correct. So make E invalid too. Let's set: A correct, B correct, C correct, D invalid (e.g., 'String::valueOf' - that is valid too. Need an invalid. Use 'ArrayList::new' is valid. Use 'list::size' valid. Need an invalid: 'null::method' is invalid. So use 'null::equals' which is invalid. That works.
B
String::isEmpty
Valid: Reference to instance method isEmpty of an arbitrary String object.
C
null::equals
Why wrong: Invalid: Cannot reference a method on null.
D
Math::random
Valid: Reference to static method random of Math.
E
this::toString
Valid: Reference to instance method toString of the current object.

Full breakdown with real-world context →

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

String::isEmpty

All three are valid: static method reference (String::isEmpty), instance method of an arbitrary object (list::add, but that's for a specific instance? Actually, list::add is an instance method of a particular list object, that's valid. String::isEmpty is a static method reference? No, isEmpty is an instance method of String. But when used as String::isEmpty, it's a reference to an instance method of an arbitrary object of a particular type. It is valid. this::method is also valid. SomeClass::new is a constructor reference.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
String::new
Why it's wrong here
This is not a method reference; new is a constructor, but the syntax is String::new for constructor reference. Actually, String::new is a valid constructor reference. So D is correct? That makes four correct. Need to adjust. Let's make D invalid: e.g., 'System::out::println' is invalid. Or we can have one invalid among these. So change D to 'System::out::println' which is invalid because it's a two-level reference. Then we have A, B, C as correct, D invalid, E also correct? We need exactly three correct. So make E invalid too. Let's set: A correct, B correct, C correct, D invalid (e.g., 'String::valueOf' - that is valid too. Need an invalid. Use 'ArrayList::new' is valid. Use 'list::size' valid. Need an invalid: 'null::method' is invalid. So use 'null::equals' which is invalid. That works.
✓
String::isEmpty
Why this is correct
Valid: Reference to instance method isEmpty of an arbitrary String object.
Related concept
Read the scenario before looking for a memorised answer.
✗
null::equals
Why it's wrong here
Invalid: Cannot reference a method on null.
✓
Math::random
Why this is correct
Valid: Reference to static method random of Math.
Related concept
Read the scenario before looking for a memorised answer.
✓
this::toString
Why this is correct
Valid: Reference to instance method toString of the current object.
Related concept
Read the scenario before looking for a memorised answer.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A practitioner preparing for the 1Z0-829 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

What to study next

Got this wrong? Here's your next step.

Identify which 1Z0-829 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 1Z0-829 question test?

Working with Streams and Lambda Expressions — This question tests Working with Streams and Lambda Expressions — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: String::isEmpty — All three are valid: static method reference (String::isEmpty), instance method of an arbitrary object (list::add, but that's for a specific instance? Actually, list::add is an instance method of a particular list object, that's valid. String::isEmpty is a static method reference? No, isEmpty is an instance method of String. But when used as String::isEmpty, it's a reference to an instance method of an arbitrary object of a particular type. It is valid. this::method is also valid. SomeClass::new is a constructor reference.

What should I do if I get this 1Z0-829 question wrong?

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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