The answer is line 8, where `int len = s.length();` is executed. A NullPointerException occurs whenever you call an instance method on a null object reference, and the stack trace explicitly points to that line as the source. In Java, local variables are not automatically initialized, so if `s` was declared but never assigned a non-null value before line 8, invoking `length()` on it triggers the exception. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this question tests your ability to read a stack trace and pinpoint the exact line where a null reference is dereferenced—a common trap is assuming the error is on the line where the variable was declared rather than where it was used. Remember the mnemonic: “Null pointer? Look for the dot.” The dot (method call) is always the line that throws, not the line that assigned null.
1Z0-829 Controlling Program Flow Practice Question
This 1Z0-829 practice question tests your understanding of controlling program flow. 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
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "s" is null
at com.example.Main.main(Main.java:8)
Refer to the exhibit. The exhibit shows a stack trace from a Java application. Which line in the code caused the NullPointerException?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Line 8: int len = s.length();
Option D is correct because the NullPointerException occurs when invoking the `length()` method on a null reference. In the stack trace, the exception is thrown at line 8, where `s.length()` is called, but `s` has not been assigned a non-null value (it is null by default if declared as a field, or uninitialized locally). The JVM throws NullPointerException when any instance method is called on a null object 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.
✗
Line 10: System.out.println(len);
Why it's wrong here
The stack trace does not include line 10.
✗
Line 12: String s = "hello";
Why it's wrong here
The stack trace points to line 8, not 12.
✗
Line 5: int x = 10;
Why it's wrong here
The stack trace shows line 8, not line 5.
✓
Line 8: int len = s.length();
Why this is correct
The stack trace explicitly shows line 8.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may confuse the line where the exception is thrown (line 8) with the line where the variable is declared (line 12), mistakenly thinking the declaration itself causes the error, but the exception only occurs when the null reference is actually used.
Trap categories for this question
Command / output trap
The stack trace shows line 8, not line 5.
Detailed technical explanation
How to think about this question
Under the hood, the Java Virtual Machine (JVM) checks for null references before dispatching any virtual method invocation. When `s.length()` is executed, the JVM attempts to dereference `s` to locate the object's method table; if `s` is null, the JVM throws a NullPointerException. In real-world scenarios, this often occurs when a method parameter or local variable is expected to be initialized but is not, such as when reading from a database or user input without proper null checks.
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.
Controlling Program Flow — This question tests Controlling Program Flow — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Line 8: int len = s.length(); — Option D is correct because the NullPointerException occurs when invoking the `length()` method on a null reference. In the stack trace, the exception is thrown at line 8, where `s.length()` is called, but `s` has not been assigned a non-null value (it is null by default if declared as a field, or uninitialized locally). The JVM throws NullPointerException when any instance method is called on a null object reference.
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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