Collections
Iterating an ArrayList
Predict exactly what the console prints after the loop runs.
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Training
Practice Java syntax by predicting console output.
Choose an exercise, read the snippet, and write the exact output before you ever run the code.
- Trace collections, strings, and loops without a compiler
- Instant feedback that nudges you toward the right answer
- Hints appear after the second attempt if you get stuck
Challenges
Pick a focused exercise to start practicing.
Each card opens a dedicated route where you can work through the snippet and get tailored feedback.
Beginner
Maps
Keeping insertion order
Fill in the two lines that this LinkedHashMap iteration will print.
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Strings
Tweaking text with StringBuilder
Type the exact line printed, including spaces and capitalization.
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Control Flow
Flow control with break/continue
What single number reaches System.out.println at the end?
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Streams
Chaining Stream operations
Write the exact string printed when this stream pipeline executes.
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Optional
Optional filtering
Predict the two lines printed when working with Optional.filter + orElseGet.
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Collections
Deque pushes vs polls
Determine the three lines printed after mixing add, push, and poll operations.
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Maps
Merging HashMap values
Observe how Map.merge mutates entries; type the two numbers printed.
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