Question 498 of 481
1Z0-811 Arrays and Methods Practice Question
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?
⚠ Common exam trap
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.
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
✓
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.
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 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.
- ✗
The outer loop uses data[i].length instead of data.length.
Why it's wrong here
Uses data.length correctly.
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Last reviewed: Jun 30, 2026
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