Unit 4 · Moderate
Text as an immutable object. Two facts generate most of the unit: indexing starts at zero and stops one before the length, and no String method ever changes the String it was called on.
What a strong answer looks like
A strong Unit 4 answer marks the start and end index before evaluating, and states explicitly that the original String is unchanged.
Know
Indexes run from zero to length minus one. The length itself is never a valid index.
Apply
Write the index beneath each character before evaluating any call.
Watch out
Using length as an index, which is the standard out-of-bounds error.
Study move
Label every index of a short word, then predict the result of two charAt calls.
Know
substring includes its start index and excludes its end index. indexOf returns the first match, or negative one when there is none.
Apply
Mark both boundaries on paper before evaluating, and handle the not-found case explicitly.
Watch out
Treating the end index as inclusive, which produces a string one character too long.
Study move
Extract the portion of a string before a delimiter, then test it when the delimiter is absent.
Know
String methods return new Strings; they never modify the original. A result that is not assigned is discarded.
Apply
Assign the result of every String operation, or it has no effect.
Watch out
Calling a method and expecting the variable to change, which silently does nothing.
Study move
Write two lines that differ only by whether the result is assigned, and state both outputs.
Know
Traversing a String means looping over its indexes, most often to compare adjacent characters or build a new String.
Apply
Choose bounds that match the comparison: adjacent-pair loops must stop one index early.
Watch out
Running an adjacent-pair loop to the final index, which reads past the end.
Study move
Write a loop that counts adjacent equal characters and check it on a one-character string.
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 a method that returns the portion of an email-like string before the at sign, and state its behavior when there is no at sign.
