- A
String::new
Why wrong: String::new is a constructor reference, not a method reference, so it is invalid for a question asking specifically about method references.
- B
String::isEmpty
String::isEmpty is a reference to an instance method of an arbitrary object of type String, equivalent to (String s) -> s.isEmpty().
- C
null::equals
Why wrong: null::equals is invalid because null cannot be used as a qualifier in a method reference; method references require a class name or an expression that produces a non-null reference.
- D
Math::random
Math::random is a reference to a static method with no parameters, returning a double, which matches a Supplier functional interface.
- E
this::toString
this::toString is a reference to an instance method of a specific object (this), usable where a zero-argument function is needed.
1Z0-829 Method Reference Practice Question
This 1Z0-829 practice question tests your understanding of working with streams and lambda expressions. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: method Reference. 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.)
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
Option A is a constructor reference, which is a valid type of method reference in Java. Option B is a reference to an instance method of an arbitrary object of type String, equivalent to `(String s) -> s.isEmpty()`. Option D is a reference to a static method, matching a Supplier functional interface. Option C is invalid because `null` cannot be used as a qualifier. Option E is invalid because `this` is not allowed as a qualifier in a method reference; the qualifier must be a class name or an expression that yields a non-null object reference, and `this` does not satisfy this requirement in all contexts.
Key principle: Method Reference
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
String::new is a constructor reference, not a method reference, so it is invalid for a question asking specifically about method references.
- ✓
String::isEmpty
Why this is correct
String::isEmpty is a reference to an instance method of an arbitrary object of type String, equivalent to (String s) -> s.isEmpty().
Related concept
Method Reference
- ✗
null::equals
Why it's wrong here
null::equals is invalid because null cannot be used as a qualifier in a method reference; method references require a class name or an expression that produces a non-null reference.
- ✓
Math::random
Why this is correct
Math::random is a reference to a static method with no parameters, returning a double, which matches a Supplier functional interface.
Related concept
Method Reference
- ✓
this::toString
Why this is correct
this::toString is a reference to an instance method of a specific object (this), usable where a zero-argument function is needed.
Related concept
Method Reference
Common exam traps
Common exam trap: answer the scenario, not the keyword
A common trap is incorrectly excluding constructor references like String::new, assuming they are not method references. Also, candidates may think null::equals is valid, but null cannot be a qualifier. Another trap is including this::toString, which is invalid because this is not an appropriate qualifier.
Detailed technical explanation
How to think about this question
Method references in Java are syntactic sugar for lambda expressions and are compiled into invokedynamic instructions. The four kinds are: static method reference (e.g., `Math::random`), instance method of a particular object (e.g., `this::toString`), instance method of an arbitrary object of a particular type (e.g., `String::isEmpty`), and constructor reference (e.g., `String::new`). The compiler resolves the method reference against the functional interface's single abstract method, ensuring parameter and return type compatibility at compile time.
KKey Concepts to Remember
- Method Reference
- Constructor Reference
- Static Method Reference
- Instance Method Reference
TExam Day Tips
- 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
Method Reference
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. Method Reference 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.
Review method Reference, then practise related 1Z0-829 questions on the same topic to reinforce the concept.
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Working with Streams and Lambda Expressions — study guide chapter
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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 — Method Reference.
What is the correct answer to this question?
The correct answer is: String::isEmpty — Option A is a constructor reference, which is a valid type of method reference in Java. Option B is a reference to an instance method of an arbitrary object of type String, equivalent to `(String s) -> s.isEmpty()`. Option D is a reference to a static method, matching a Supplier functional interface. Option C is invalid because `null` cannot be used as a qualifier. Option E is invalid because `this` is not allowed as a qualifier in a method reference; the qualifier must be a class name or an expression that yields a non-null object reference, and `this` does not satisfy this requirement in all contexts.
What should I do if I get this 1Z0-829 question wrong?
Review method Reference, then practise related 1Z0-829 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Method Reference
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Last reviewed: Jul 4, 2026
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