Topic 1.15 — String Manipulation

Goal: use common String methods to examine and build new strings (length, substring, index, concatenation), and trace code that combines these operations.

The big idea

Strings are objects. You don’t “change” a string using methods—instead, methods usually return a new string (or a value like an int).

This matters for tracing: if you don’t assign the result, the original string stays the same.

Two super-common patterns

Get info (returns numbers/characters)

int n = s.length();
char c = s.charAt(0);

Make a new string (returns a String)

String t = s.substring(1, 4);
String u = s + "!!";

Key String methods (you’ll see these a LOT)

Method / Operation	What it does	Returns
`s.length()`	Number of characters in `s`	`int`
`s.substring(a, b)`	Characters from index `a` up to (not including) `b`	`String`
`s.substring(a)`	Characters from index `a` to the end	`String`
`s.indexOf(x)`	Index of the first occurrence of `x` (often a String)	`int`
`s.charAt(i)`	Character at index `i`	`char`
`+` (concatenation)	Joins strings together	`String`

You’ll practice these a lot because AP loves asking “what is the output?” or “what value is stored?”

Indexes: remember they start at 0

String s = "APCSA";
// indexes:   01234

s.charAt(0) is 'A'
s.charAt(4) is 'A'
s.length() is 5

Substring rule (most-tested detail)

substring(a, b) includes a but does not include b.

String s = "computer";
String sub = s.substring(1, 4); // "omp"

Concatenation tricks

If either side of + is a String, Java turns the other side into a String and concatenates.

String s = "Score: " + 10;   // "Score: 10"
String t = "A" + 1 + 2;      // "A12"
String u = 1 + 2 + "A";      // "3A"

Common errors (what would break)

charAt(i) with an invalid index (like -1 or i >= length)
substring(a, b) with invalid bounds (like a > b or out of range)

On the exam, they usually avoid truly “crashy” cases unless the question is about spotting the error.

Exam mindset

Indexing starts at 0.
substring(a, b) excludes b.
Many String methods return a new String—store the result if you need it.
Trace concatenation left-to-right.

Quick self-check

If s = "hello", what is s.length()?
If s = "hello", what is s.substring(1, 4)?
What is "A" + 1 + 2?
What is 1 + 2 + "A"?
Why does substring “stop before” the end index?

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