AP Computer Science A – 1.15 String Manipulation

What Is a String?

In Java, a String is an object used to store and work with text.

String name = "Jordan";
String school = "Klein Collins";
String message = "Hello, world!";

The String class contains many built-in instance methods that allow you to:

• Analyze text
• Modify text
• Compare text
• Extract parts of text

These skills are used constantly in real programs, such as:

• Usernames and passwords
• Searching text
• Checking input
• Sorting names
• Formatting output

1. substring() – Extract Part of a String

The substring() method is used to cut out a portion of a string.

Format:

string.substring(startIndex, endIndex)

• startIndex is included
• endIndex is not included

Example 1:

String word = "Computer";
System.out.println(word.substring(0, 3));

Output:

Com

Example 2:

System.out.println(word.substring(4));

Output:

uter

Common uses:

• Getting initials
• Removing prefixes
• Extracting usernames

2. indexOf() – Find the Position of a Character or Word

The indexOf() method finds the location of a character or substring in a string.

Format:

string.indexOf("text")

Example:

String word = "Programming";
System.out.println(word.indexOf("g"));

Possible Output:

3

If the value is not found, the result is:

-1

Common uses:

• Finding spaces
• Finding specific characters
• Validating input

3. equals() – Compare Two Strings for Equality

The equals() method checks if two Strings have the exact same value.

Important: Never use == to compare Strings in Java.

Correct Way:

String a = "Java";
String b = "Java";

System.out.println(a.equals(b));

Output:

true

If they are different, the result is:

false

4. compareTo() – Alphabetical Comparison

The compareTo() method compares two Strings alphabetically.

Format:

string1.compareTo(string2)

Results:

Example:

System.out.println("Apple".compareTo("Banana"));

Output: a negative number (because "Apple" comes before "Banana")

Common uses:

• Sorting lists of names
• Ranking words alphabetically

Common String Methods to Know

Why String Manipulation Matters in APCS A

In AP Computer Science A, you will use String manipulation to:

• Read and process user input
• Validate passwords and other data
• Process text from files
• Search inside larger pieces of data
• Format output for users
• Compare values correctly
• Build real programs that work with text

AP Exam Tips

• Know how substring() works (start included, end excluded).
• Remember that == does not compare String contents.
• Use equals() to compare the value of Strings.
• Understand what the results of compareTo() mean (0, negative, positive).
• Remember that Strings are objects, not primitive types.