Collections
Iterating an ArrayList
Predict exactly what the console prints after the loop runs.
Training
Practice Java by predicting the exact output.
Read the snippet, think through the mutations and branches, then write what Java would print line by line.
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Challenges
Choose an exercise to start practicing.
Beginner
Maps
Keeping insertion order
Fill in the two lines that this LinkedHashMap iteration will print.
Strings
Tweaking text with StringBuilder
Type the exact line printed, including spaces and capitalization.
Control Flow
Flow control with break/continue
What single number reaches System.out.println at the end?
Streams
Chaining Stream operations
Write the exact string printed when this stream pipeline executes.
Optional
Optional filtering
Predict the two lines printed when working with Optional.filter + orElseGet.
Collections
Deque pushes vs polls
Determine the three lines printed after mixing add, push, and poll operations.
Maps
Merging HashMap values
Observe how Map.merge mutates entries; type the two numbers printed.