The answer is a ClassCastException caused by retrieving an Integer from a raw type Map and casting it to a String. This happens because raw types bypass compile-time generic type checking, so the compiler cannot verify that the value stored is actually an Integer, not a String. When the code later performs an explicit cast to String, the JVM detects the type mismatch at runtime and throws the ClassCastException. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this scenario tests your understanding of generics and type safety, often appearing in questions about legacy code or collections without type parameters. A common trap is assuming raw types behave like parameterized types, but they disable all generic checks, leaving you vulnerable to runtime errors. Remember the mnemonic: “Raw types are raw deals—no compile-time shields, only runtime spills.”
1Z0-829 Working with Arrays and Collections Practice Question
This 1Z0-829 practice question tests your understanding of working with arrays and collections. 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.
Exhibit
Refer to the exhibit.
Error log:
Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String
at com.example.Cache.get(Cache.java:12)
at com.example.Main.main(Main.java:10)
Source code:
public class Cache {
private Map<String, Object> store = new HashMap<>();
public <T> T get(String key, Class<T> type) {
Object value = store.get(key);
return type.cast(value);
}
}
public class Main {
public static void main(String[] args) {
Cache cache = new Cache();
cache.store.put("age", 30);
String age = cache.get("age", String.class);
System.out.println(age);
}
}
Refer to the exhibit.
Error log:
Exception in thread "main" java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String
at com.example.Cache.get(Cache.java:12)
at com.example.Main.main(Main.java:10)
Source code:
public class Cache {
private Map<String, Object> store = new HashMap<>();
public <T> T get(String key, Class<T> type) {
Object value = store.get(key);
return type.cast(value);
}
}
public class Main {
public static void main(String[] args) {
Cache cache = new Cache();
cache.store.put("age", 30);
String age = cache.get("age", String.class);
System.out.println(age);
}
}
A
The store map is not type-safe.
Why wrong: It's not the direct cause; the cast is.
B
The value stored is Integer but retrieved as String.
Correct cause.
C
The get method should use (T) cast instead of type.cast().
Why wrong: Still would throw ClassCastException.
D
The type parameter T is not used correctly.
Why wrong: It is used correctly; the mismatch is the issue.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The value stored is Integer but retrieved as String.
The ClassCastException occurs because the code retrieves a value from the store map and attempts to cast it to String, but the actual stored object is an Integer. Since the map is not using generics (raw type), the compiler does not enforce type safety, allowing the mismatch to be caught only at runtime.
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.
✗
The store map is not type-safe.
Why it's wrong here
It's not the direct cause; the cast is.
✓
The value stored is Integer but retrieved as String.
Why this is correct
Correct cause.
Related concept
Read the scenario before looking for a memorised answer.
✗
The get method should use (T) cast instead of type.cast().
Why it's wrong here
Still would throw ClassCastException.
✗
The type parameter T is not used correctly.
Why it's wrong here
It is used correctly; the mismatch is the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse the lack of type safety (Option A) with the direct cause of the exception, which is the actual type mismatch at retrieval time.
Detailed technical explanation
How to think about this question
Under the hood, the JVM performs a checkcast instruction when a reference is cast to a specific type. If the object's runtime type is not a subtype of the target type, a ClassCastException is thrown. In real-world scenarios, using raw types or unchecked casts in legacy code often leads to such exceptions, which is why generics were introduced in Java 5 to provide compile-time type safety.
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: The value stored is Integer but retrieved as String. — The ClassCastException occurs because the code retrieves a value from the store map and attempts to cast it to String, but the actual stored object is an Integer. Since the map is not using generics (raw type), the compiler does not enforce type safety, allowing the mismatch to be caught only at runtime.
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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