Quick Answer
The answer is that one or more rows in the 2D array are null, causing the NullPointerException. This occurs because Java treats a 2D array as an array of arrays, meaning each row reference can independently be null. When the code calls `data[i].length` or accesses `data[i][j]` on a null row, the JVM throws the exception—the loop assumes every row is a valid array object, but no null check is performed before iterating over columns. On the Oracle Java Foundations 1Z0-811 exam, this scenario tests your understanding of jagged or ragged arrays and the fact that null rows are a common pitfall in sensor data processing. A frequent trap is assuming a 2D array is always rectangular and fully initialized; the exam deliberately includes null rows to catch this oversight. Memory tip: "Null row, no go—check before you .length."
This 1Z0-811 practice question tests your understanding of arrays and methods. 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.
A developer is working on a Java program that processes sensor data. The data is stored in a 2D array 'double[][] readings', where each row represents a sensor and each column a time interval. The method 'public static double[] averagePerSensor(double[][] data)' should compute the average reading for each sensor (row) and return a 1D array of averages. The developer writes the following implementation: 'double[] result = new double[data.length]; for (int i = 0; i < data.length; i++) { double sum = 0; for (int j = 0; j < data[i].length; j++) { sum += data[i][j]; } result[i] = sum / data[i].length; } return result;'. However, the program sometimes throws a NullPointerException. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The method returns a double[] but should return double[][].
Why wrong: Return type is correct.
The rows have different numbers of columns.
Why wrong: That is handled correctly by the inner loop.
One or more rows in the 2D array are null.
Accessing length on null row causes NPE.
The outer loop uses data[i].length instead of data.length.
Why wrong: Uses data.length correctly.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
One or more rows in the 2D array are null.
The NullPointerException occurs because the code attempts to access `data[i].length` when `data[i]` is null. In Java, a 2D array is an array of arrays, and any row can be null. The code does not check for null rows before iterating over the columns, so if a row is null, accessing `data[i][j]` or `data[i].length` throws a NullPointerException. Option C correctly identifies this as the most likely cause.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The method returns a double[] but should return double[][].
Why it's wrong here
Return type is correct.
The rows have different numbers of columns.
Why it's wrong here
That is handled correctly by the inner loop.
One or more rows in the 2D array are null.
Why this is correct
Accessing length on null row causes NPE.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
The outer loop uses data[i].length instead of data.length.
Why it's wrong here
Uses data.length correctly.
Common exam traps
Oracle often tests the distinction between a null row and a row with zero columns, where candidates mistakenly think a zero-length row causes the exception, but only a null reference triggers a NullPointerException when accessing array length or elements.
Detailed technical explanation
In Java, a 2D array is stored as an array of references to 1D arrays. Each row reference can be null, meaning the row has not been initialized. The code must explicitly check `if (data[i] != null)` before accessing elements or length. This is a common pitfall when arrays are partially populated or when data comes from external sources like file parsing or database queries, where null rows may represent missing sensor data.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A practitioner preparing for the 1Z0-811 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
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FAQ
Arrays and Methods — This question tests Arrays and Methods — Read the scenario before looking for a memorised answer..
The correct answer is: One or more rows in the 2D array are null. — The NullPointerException occurs because the code attempts to access `data[i].length` when `data[i]` is null. In Java, a 2D array is an array of arrays, and any row can be null. The code does not check for null rows before iterating over the columns, so if a row is null, accessing `data[i][j]` or `data[i].length` throws a NullPointerException. Option C correctly identifies this as the most likely cause.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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