Quick Answer
The answer is that an empty array will cause an ArrayIndexOutOfBoundsException. This happens because when the array has a length of zero, `arr.length - 1` evaluates to -1, and Java arrays do not allow negative indices—accessing `arr[-1]` immediately throws the exception. On the Oracle Java Foundations 1Z0-811 exam, this tests your understanding of array bounds and the importance of defensive programming; a common trap is assuming the array always has at least one element. The exam often presents such a method without a guard clause, expecting you to recognize that an empty array violates the index range of 0 to length-1. A helpful memory tip: always check for an empty array before accessing index zero, because zero length means zero valid indices.
This 1Z0-811 practice question tests your understanding of arrays and methods. 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.
A new developer writes a method that accepts an array and intends to swap the first and last elements. The code is: public static void swapEnds(int[] arr) { int temp = arr[0]; arr[0] = arr[arr.length-1]; arr[arr.length-1] = temp; } What is a potential issue with this method if the array is empty?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "first"
Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
It will throw a NullPointerException.
Why wrong: NullPointerException occurs if arr is null, not for empty array.
It will throw an ArrayIndexOutOfBoundsException.
Accessing index 0 and -1 on an empty array throws this exception.
It will produce incorrect result because arr.length-1 is negative.
Why wrong: It throws an exception, not an incorrect result.
It will compile and run correctly.
Why wrong: It will throw an exception at runtime.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
It will throw an ArrayIndexOutOfBoundsException.
When the array is empty, `arr.length` is 0, so `arr.length - 1` evaluates to -1. Accessing `arr[-1]` throws an `ArrayIndexOutOfBoundsException` because array indices must be non-negative and less than the array length. The method does not check for an empty array before attempting the swap.
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.
It will throw a NullPointerException.
Why it's wrong here
NullPointerException occurs if arr is null, not for empty array.
It will throw an ArrayIndexOutOfBoundsException.
Why this is correct
Accessing index 0 and -1 on an empty array throws this exception.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
It will produce incorrect result because arr.length-1 is negative.
Why it's wrong here
It throws an exception, not an incorrect result.
It will compile and run correctly.
Why it's wrong here
It will throw an exception at runtime.
Common exam traps
Oracle often tests the distinction between a `null` reference and an empty array, leading candidates to mistakenly think an empty array causes a `NullPointerException` instead of an `ArrayIndexOutOfBoundsException`.
Detailed technical explanation
In Java, array indices are zero-based and bounds-checked at runtime. Accessing an index of -1 triggers an `ArrayIndexOutOfBoundsException` because the JVM verifies the index is within [0, length-1]. A defensive programming practice would be to add a guard clause like `if (arr == null || arr.length < 2) return;` to handle edge cases gracefully.
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: It will throw an ArrayIndexOutOfBoundsException. — When the array is empty, `arr.length` is 0, so `arr.length - 1` evaluates to -1. Accessing `arr[-1]` throws an `ArrayIndexOutOfBoundsException` because array indices must be non-negative and less than the array length. The method does not check for an empty array before attempting the swap.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "first". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 30, 2026
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