String Methods

String Methods: AP Computer Science A Study Guide

Welcome, code wizards! In this magical unit, we’re delving into the world of strings in Java. No, not the kind you use to tie your shoes, but the kind that makes your programs sing! Let's take a byte (pun intended) out of string manipulation and see what Java strings can do for us. 🧙‍♂️💻

The String class, located in the java.lang package (the all-inclusive resort of Java classes), is your go-to for managing sequences of characters. Think of it as the Swiss Army knife of text handling — you've got a tool for every string need, and best of all, you don’t even need to import it.

Imagine you have a magical scroll of text, and you want to read only specific passages. Java’s got you covered with its substring methods!

To find out how long your scroll (string) is, you use:

int length();

This method returns the number of characters in the string, spaces and special symbols included.

To extract a specific passage from your scroll, you use:

String substring(int beginIndex, int endIndex);

This method returns the characters starting from beginIndex up to, but not including, endIndex. Note: Java is a zero-indexed language, so the first character is at position 0. This is not Hogwarts, where "0" means "nothing."

Let's break down an example:

String str = "Java Wizard";

• str.substring(0, 4) returns "Java", which starts at index 0 and goes up to, but doesn’t include, index 4.
• str.substring(5) returns "Wizard", because it starts at index 5 and includes all characters till the end.

So, if you were to retrieve individual characters, remember the index dance, where index n says, "I'm not 1, but I won’t be outdone!"

Sometimes, strings need to be compared. Like, who gets the Dragon Egg between strings:

To check if two strings are identical, use:

boolean equals(String other);

String s1 = "magic";
String s2 = "Magic";
boolean result = s1.equals(s2); // returns false

Remember, equals() cares about case sensitivity, much like how "Java" is not a clone of "java."

For comparing strings lexicographically (fancy word for alphabetical order), use:

int compareTo(String other);

• A negative number means your string comes before 'other'.
• 0 means they are identical.
• A positive number means your string comes after 'other'.

Example:

String s3 = "Potion";
String s4 = "Magic";
int compareResult = s3.compareTo(s4); // returns a positive number

Worried about remembering all those magical string spells? Fear not! The AP CS A Java Quick Reference Sheet is your spellbook during the exam. Here’s a recap of methods you get to wield:

• int length() returns the number of characters in your string, including spaces and punctuation.

• String substring(int from, int to) returns a new string starting from from and ending before to.

• int indexOf(String str) searches for str in your main string and returns the starting index, or -1 if it's not found.

• int compareTo(String other) returns a value that tells you the relative order of strings.

• boolean equals(String other) checks if two strings are exactly the same.

Let’s flex those coding muscles with some practice!

indexOf Practice: What is the value of pos after executing the following code?

String s = "java coding";
int pos = s.indexOf("c");

• Answer: pos is 5 because "c" is the 6th character (remember, zero-indexed).

Length Practice: What is the value of len after executing this code?

String s = "spell";
int len = s.length();

• Answer: len is 5, because "spell" has 5 characters.

Substring Practice: What is the value of sub after the following code?

String mantra = "abracadabra";
String sub = mantra.substring(0, 4);

• Answer: sub is "abra".

compareTo Practice: What is the value of result after this comparison?

String one = "Aurora";
String two = "Belladonna";
int result = one.compareTo(two);

• Answer: result is negative because "Aurora" comes before "Belladonna" alphabetically.

• CompareTo(String other): Compares two strings lexicographically.
• External Libraries: Pre-written code you can import to add functionalities.
• IndexOf(String str): Finds the first occurrence of str in a string.
• Inheritance: Allows a class to inherit properties and methods from another class.
• Length(): Returns the number of characters in a string.
• Overloaded Methods: Same method name, but different parameters.
• Zero-indexed language: In Java, the first element is at index 0.

There you have it, fellow coders! You’re now equipped with the string spells to tackle any AP CS A challenge. The string class and its magic are now your allies. Remember, with great power (string manipulation) comes great coding responsibility. Now go forth and may your code be bug-free and your strings always well-trimmed. 🧙‍♀️✨