The correct answer is to replace Files.readAllLines() with Files.lines() and process the stream line by line. This is because Files.readAllLines() loads the entire 500 MB CSV file into a List<String> in heap memory, which, combined with other application objects, exceeds the 2 GB heap limit and triggers the OutOfMemoryError. In contrast, Files.lines() returns a Stream<String> that reads lines lazily from the file, processing each line on demand without holding the entire file in memory—this is the essence of memory efficient file reading in Java NIO.2. On the Oracle Certified Professional Java SE 17 Developer 1Z0-829 exam, this scenario tests your understanding of the NIO.2 file API and the critical difference between eager and lazy file I/O; a common trap is assuming readAllLines() is safe for large files. Remember the memory tip: “Lines lazy, heap happy; readAll eager, heap meager.”
1Z0-829 Java I/O API and Securing Applications Practice Question
This 1Z0-829 practice question tests your understanding of java i/o api and securing applications. 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 financial services company runs a Java 17 application on a server with 8 GB RAM. The application reads daily transaction files in CSV format (each file is about 500 MB). It processes each line, validates it against a SQL database, and writes results to an output file. Recently, after processing about 60% of a file, the application crashes with an OutOfMemoryError: Java heap space. The heap size is set to 2 GB. The code uses Files.readAllLines() to load the entire file into a List<String>, then iterates. The team is evaluating solutions to avoid memory issues. Which approach is the best course of action?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Replace Files.readAllLines() with Files.lines() and process the stream line by line.
Option D is correct because Files.readAllLines() loads the entire 500 MB CSV file into memory as a List<String>, which causes the OutOfMemoryError when combined with other heap usage. Replacing it with Files.lines() returns a Stream<String> that reads lines lazily, processing each line one at a time without holding the entire file in memory. This directly solves the heap space issue without requiring additional hardware or manual file splitting.
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.
✗
Increase the JVM heap size to 4 GB.
Why it's wrong here
Does not solve the root cause; may still OOM with larger files.
✗
Enable class data sharing to reduce memory footprint.
Why it's wrong here
CDS reduces startup memory, not heap usage for data.
✗
Split the file into 100 MB chunks and process each chunk sequentially.
Why it's wrong here
Still uses readAllLines on chunks; OOM possible for 500 MB if not streamed.
✓
Replace Files.readAllLines() with Files.lines() and process the stream line by line.
Why this is correct
Streams lines lazily, fits in memory regardless of file size.
Clue confirmation
The clue word "best" in the question point toward this answer.
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 often choose to increase heap size (Option A) as a quick fix, missing the fundamental design flaw of loading the entire file into memory, which is a classic Java anti-pattern tested in the 1Z0-829 exam.
Detailed technical explanation
How to think about this question
Files.lines() internally uses a BufferedReader that reads from the file in chunks (typically 8 KB) and decodes lines on demand, leveraging lazy evaluation. The stream must be closed properly (e.g., via try-with-resources) to release the underlying file handle; failing to do so can cause resource leaks. In real-world scenarios, this pattern is essential for processing large log files or data streams where memory is constrained.
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.
Java I/O API and Securing Applications — This question tests Java I/O API and Securing Applications — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Replace Files.readAllLines() with Files.lines() and process the stream line by line. — Option D is correct because Files.readAllLines() loads the entire 500 MB CSV file into memory as a List<String>, which causes the OutOfMemoryError when combined with other heap usage. Replacing it with Files.lines() returns a Stream<String> that reads lines lazily, processing each line one at a time without holding the entire file in memory. This directly solves the heap space issue without requiring additional hardware or manual file splitting.
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.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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