The answer is that `List<? extends Number> list = new ArrayList<Integer>();` is a valid wildcard generic declaration in Java 17. This works because the upper-bounded wildcard `? extends Number` allows the list reference to accept any concrete parameterized type that is a subtype of `Number`, such as `Integer`, making the assignment type-safe for reading. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your understanding of wildcard variance and assignment compatibility, often appearing as a multi-select trap where candidates confuse unbounded wildcards with upper-bounded ones or mistakenly think you can add elements to a wildcard list. A common pitfall is assuming `List<?>` can be assigned a raw type, but it actually requires a specific parameterized type like `ArrayList<String>`. Remember the memory tip: "Wildcards widen the type, but you can only read from them—never add, except null."
1Z0-829 Working with Arrays and Collections Practice Question
This 1Z0-829 practice question tests your understanding of working with arrays and collections. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 of the following variable declarations are valid in Java 17?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
List<?> list = new ArrayList<String>();
Option A is correct because `List<?>` is a wildcard type that can hold any type, and `new ArrayList<String>()` creates an `ArrayList` of a specific type (`String`). The wildcard `?` acts as a type-safe placeholder, allowing the assignment of a concrete parameterized type to an unbounded wildcard reference. This is valid because the wildcard represents an unknown type, and the list is read-only in terms of type safety (you cannot add elements except `null`).
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.
✓
List<?> list = new ArrayList<String>();
Why this is correct
Correct: wildcard captures any type.
Related concept
Read the scenario before looking for a memorised answer.
✗
List<Object> list = new ArrayList<String>();
Why it's wrong here
Invalid: generics are invariant; List<String> is not a subtype of List<Object>.
✓
List<? super Integer> list = new ArrayList<Number>();
Why this is correct
Correct: Number is a supertype of Integer, so this is valid.
Related concept
Read the scenario before looking for a memorised answer.
✓
List<? extends Number> list = new ArrayList<Integer>();
Why this is correct
Correct: Integer extends Number, so this assignment is valid.
Related concept
Read the scenario before looking for a memorised answer.
✗
List<?> list = new ArrayList<?>();
Why it's wrong here
Invalid: cannot create an ArrayList of wildcard type; the type argument must be a concrete type.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse generic invariance with array covariance, mistakenly thinking `List<Object>` can hold a `List<String>` (like `Object[]` can hold `String[]`), or they incorrectly assume wildcards can be used in instantiation expressions like `new ArrayList<?>()`.
Detailed technical explanation
How to think about this question
Under the hood, Java generics use type erasure, meaning the generic type information is removed at runtime, but the compiler enforces type safety at compile time. The wildcard `?` (unbounded) is essentially `? extends Object`, and it allows covariance in read-only contexts. For `List<? super Integer>`, the wildcard with a lower bound allows writing `Integer` values into the list (since `Integer` is a subtype of the lower bound), but reading yields `Object`. This is a common pattern in APIs like `Collections.copy()` or `Comparator` that need to accept a range of types.
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.
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
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 exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: List<?> list = new ArrayList<String>(); — Option A is correct because `List<?>` is a wildcard type that can hold any type, and `new ArrayList<String>()` creates an `ArrayList` of a specific type (`String`). The wildcard `?` acts as a type-safe placeholder, allowing the assignment of a concrete parameterized type to an unbounded wildcard reference. This is valid because the wildcard represents an unknown type, and the list is read-only in terms of type safety (you cannot add elements except `null`).
What should I do if I get this 1Z0-829 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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