01
Overview

Prefix detection plus case conversion.

Three methods that work together: startsWith for prefix check, substring for extraction, and case conversion for normalization.

startsWith(prefix) is a boolean test: does this String begin with the given prefix? substring(n) drops the first n characters. toUpperCase() and toLowerCase() create a new String in the requested case.

Together they handle prefix stripping, case-insensitive prefix detection, and normalization before comparison.

On the AP exam, these appear in FRQs about command parsing, tag detection, and word-prefix problems.

02
Core Concept

Detect prefix; strip prefix; convert case.

Three independent operations; combine as needed.

Prefix detection

word.startsWith("pre")    // true if word begins with "pre"

Prefix stripping

public String stripPrefix(String word, String prefix) {
    if (word.startsWith(prefix)) {
        return word.substring(prefix.length());
    }
    return word;
}

If the prefix is present, return the rest; otherwise return the original.

Case-insensitive prefix detection

public boolean startsWithIgnoreCase(String word, String prefix) {
    return word.toLowerCase().startsWith(prefix.toLowerCase());
}

Normalize both to lowercase before comparing.

Case conversion is non-destructive

toUpperCase and toLowerCase return new Strings — they don't modify the original.

String s = "Hello";
s.toUpperCase();         // "HELLO" returned but not stored
System.out.println(s);   // still "Hello"

s = s.toUpperCase();     // reassign to capture
System.out.println(s);   // "HELLO"
03
Key Vocabulary

Prefix + case vocabulary.

Used in command and tag detection.

Vocabulary
  • Prefix — the beginning portion of a String.
  • startsWith — boolean prefix test.
  • endsWith — boolean suffix test.
  • toUpperCase / toLowerCase — non-destructive case conversion.
  • Normalization — converting to a uniform case for comparison.
  • Strip — remove a portion (prefix or suffix).
04
AP Exam Focus

How these methods are tested.

Reassignment after case conversion is the common trap.

Exam Focus
  • Case methods return new Strings. Must reassign to keep the change.
  • startsWith is case-sensitive. Normalize first for case-insensitive checks.
  • substring(n) skips the first n chars.
  • Strings are immutable. No method modifies the original.
05
Worked Example

Strip "Mr. " or "Mrs. " from a name.

Prefix detection + substring + case normalization.

Worked Example AP-style reasoning

Problem. Remove the title from "Mr. Smith" or "Mrs. Jones" or "DR. Lee".

public String stripTitle(String name) {
    String lower = name.toLowerCase();
    if (lower.startsWith("mrs. ")) return name.substring(5);
    if (lower.startsWith("mr. ")) return name.substring(4);
    if (lower.startsWith("dr. ")) return name.substring(4);
    return name;
}

Trace.

  • "Mr. Smith" → lower starts with "mr. " → return name.substring(4) = "Smith".
  • "Mrs. Jones" → lower starts with "mrs. " → return name.substring(5) = "Jones".
  • "DR. Lee" → lower is "dr. lee" → starts with "dr. " → return name.substring(4) = "Lee".
  • "Smith" → no title detected → return "Smith".

Note. The "mrs. " check must come before "mr. " — otherwise "Mrs. Jones" would match "mr. " first and strip too little.

06
Common Mistakes

Prefix + case pitfalls.

Most miss reassignment.

Watch Out
  • Calling toLowerCase without assigning. Result is discarded.
  • Wrong substring index. Must equal the prefix's length.
  • Order of prefix checks. Longer prefixes must come first.
  • Case-sensitive startsWith on user input. Misses uppercase variations.
07
Reference Table

Prefix and case methods.

Each returns; none mutates.

Prefix & Case Reference

AP Quick Reference
MethodReturnsNote
startsWith(p)booleanCase-sensitive.
substring(n)StringFrom index n onward.
toUpperCase()StringNew copy.
toLowerCase()StringNew copy.
08
Practice Tip

How to master prefix + case.

Reassign after conversion; order checks longest-first.

Practice Strategy
  • MCQ Practice — for each option, verify case-conversion results are assigned back.
  • Java Lab — write stripTitle, isHttpsUrl. Order prefix checks longest first.
  • FRQ Practice — normalize case before any case-insensitive comparison.
09
Section · 09

Practice — attempt these now.

AP-style assessments aligned to this lesson. Time them.

All practice
FRQ Practice 30 min 16 marks Pending

AP CSA Unit 12 Topic 07 substring, startsWith, and Case Conversion FRQ Practice

AP-style topic practice assessment

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