Unit 6 · Heavy
A resizable list of objects. The unit adds one genuinely tricky behavior: removing during a traversal shifts every later element, so the loop and the list disagree about where things are.
What a strong answer looks like
A strong Unit 6 answer accounts for the shift after every insertion or removal, and states the size at the moment each index is used.
Know
An ArrayList grows and shrinks, reports size() as a method, and stores objects rather than primitives.
Apply
Use size() for bounds and get() for access, keeping in mind both change as the list is modified.
Watch out
Using length or length() on an ArrayList, or an index equal to size().
Study move
Write a loop over an ArrayList and state the valid index range at each step.
Know
Removing an element shifts every later element down by one and reduces size. Inserting shifts them up.
Apply
When removing inside a loop, either iterate backwards or avoid advancing the index after a removal.
Watch out
Removing while iterating forwards with a normal increment, which silently skips the element after each removal.
Study move
Remove every negative value from a list twice, once forwards and once backwards, and compare results.
Know
Traversal mirrors arrays but bounds come from size(), which is re-evaluated each pass and can change mid-loop.
Apply
Decide before writing the loop whether the list will change during it.
Watch out
Caching size() in a variable and then modifying the list, so the bound no longer matches.
Study move
Trace a loop that adds an element on a condition and explain the effect on the bound.
Know
ArrayList holds objects, so primitives are wrapped. Autoboxing hides the conversion, but equality comparison on wrappers is not the same as on primitives.
Apply
Compare wrapper values with equals rather than the equality operator.
Watch out
Comparing two wrapper objects with the equality operator, which compares references and can be false for equal values.
Study move
Compare two equal wrapper values both ways and explain the difference in result.
Connections and techniques that receive extra attention in this unit.
Related topics some schools attach to this unit and others leave out. Covered on request rather than assumed.
Modeling drill
Write code that removes every element below a threshold from an ArrayList, and state why your loop does not skip any.
