Activity 4.9 — String Methods

Introduction

Activity 4-9

String Methods

Strings are objects

String is a class.
A String variable stores a String object.
Objects have methods you can call with dot notation.

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// Strings are objects
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String word = "hello";
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// Objects have methods
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word.length();
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word.toUpperCase();
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word.substring(1, 3);

One easy confusion

Arrays use length
Strings use length()

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// Arrays vs Strings
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arr.length      // array length field
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word.length()   // String length method

`substring`

substring(start, end) includes the start index.
It does not include the end index.
substring(start) goes from start to the end of the string.

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// substring(start, end)
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// start is included
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// end is NOT included
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String s = "computer";
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System.out.println(s.substring(0, 3)); // com

`indexOf`

Finds the first position of a character or substring.
If nothing is found, it returns -1.

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// indexOf returns the first match
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String s = "banana";
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System.out.println(s.indexOf("a"));  // 1
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System.out.println(s.indexOf("na")); // 2
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System.out.println(s.indexOf("z"));  // -1

Comparing Strings

Use equals to test if two strings have the same text.
Do not use == for String comparison on the AP exam.

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// equals vs ==
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String a = "cat";
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String b = "cat";
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// Use equals for String comparison
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a.equals(b);

`compareTo`

Used for alphabetical comparison.
Negative means “comes before.”
Positive means “comes after.”
Zero means the strings are equal.

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// compareTo
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"a".compareTo("b")   // negative
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"b".compareTo("a")   // positive
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"a".compareTo("a")   // 0

Strings are immutable

String methods do not change the original String.
They return a new String instead.
If you want the change, assign the result to a variable.

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// Strings are immutable
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String s = "hello";
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s.toUpperCase();      // does NOT change s
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s = s.toUpperCase();  // reassign if you want the change

Today's goals

Use common AP String methods correctly.
Extract pieces of text with substring.
Search for text with indexOf.
Compare Strings with equals and compareTo.

'F' → Fullscreen

Objectives

Use common String methods including length(), substring(), and indexOf().
Compare strings using equals() and compareTo().
Explain why Strings are immutable.
Solve small text-processing problems using String methods.

Activity Tasks

Create a new project named Activity4-9-StringMethods.
Complete Tasks 1-5.
Use the figures as references while you work.

Figures: Reference Examples

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// Figure 1 — Common String methods
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String word = "programming";
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        System.out.println(word.length());         // 11
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        System.out.println(word.substring(0, 4)); // prog
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        System.out.println(word.substring(3));    // gramming
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        System.out.println(word.indexOf("g"));    // 3
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        System.out.println(word.toUpperCase());   // PROGRAMMING
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    }
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}

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// Figure 2 — Comparing Strings correctly
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String a = "cat";
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        String b = "cat";
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        System.out.println(a.equals(b));     // true
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        System.out.println(a.compareTo(b));  // 0
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        System.out.println(a.compareTo("dog")); // negative
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    }
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}

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// Figure 3 — Strings are immutable
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String word = "hello";
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        word.toUpperCase();
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        System.out.println(word); // still hello
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        word = word.toUpperCase();
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        System.out.println(word); // now HELLO
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    }
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}

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// Figure 4 — A useful pattern with substring + indexOf
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String fullName = "Ada Lovelace";
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        int spaceIndex = fullName.indexOf(" ");
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        String firstName = fullName.substring(0, spaceIndex);
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        String lastName = fullName.substring(spaceIndex + 1);
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        System.out.println(firstName);
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        System.out.println(lastName);
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    }
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}

Task 1: String Analyzer

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// Task 1 — String Analyzer
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//
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// Create a String named word.
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// Then print:
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// 1) The word
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// 2) Its length
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// 3) The first character
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// 4) The last character
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// 5) The uppercase version
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//
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// Hint:
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// first character: substring(0, 1)
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// last character: substring(word.length() - 1)
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String word = "computer";
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        System.out.println("Word: " + word);
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        System.out.println("Length: " + word.length());
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        System.out.println("First character: " + word.substring(0, 1));
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        System.out.println("Last character: " + word.substring(word.length() - 1));
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        System.out.println("Uppercase: " + word.toUpperCase());
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    }
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}

Task 2: Initials Generator

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// Task 2 — Initials Generator
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//
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// Create a String fullName with a first and last name.
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// Example: "Grace Hopper"
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//
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// Use indexOf and substring to print the initials like:
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// G.H.
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//
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// Steps:
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// 1) Find the space index
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// 2) Get the first letter of the first name
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// 3) Get the first letter of the last name
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// 4) Print them with periods
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String fullName = "Grace Hopper";
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        int spaceIndex = fullName.indexOf(" ");
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        String firstInitial = fullName.substring(0, 1);
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        String lastInitial = fullName.substring(spaceIndex + 1, spaceIndex + 2);
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        System.out.println(firstInitial + "." + lastInitial + ".");
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    }
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}

Task 3: Username Generator

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// Task 3 — Username Generator
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//
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// Create a String fullName.
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// Generate a username using:
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// - the first letter of the first name
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// - the full last name
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// - all lowercase
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//
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// Example:
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// "Ada Lovelace" -> alovelace
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//
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// Use substring, indexOf, and toLowerCase.
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String fullName = "Ada Lovelace";
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        int spaceIndex = fullName.indexOf(" ");
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        String firstInitial = fullName.substring(0, 1);
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        String lastName = fullName.substring(spaceIndex + 1);
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        String username = (firstInitial + lastName).toLowerCase();
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        System.out.println("Username: " + username);
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    }
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}

Task 4: Word Finder

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// Task 4 — Word Finder
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//
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// Create a sentence String.
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// Use indexOf to search for a target word.
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//
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// Print:
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// 1) The sentence
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// 2) The index of the target word
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// 3) A message saying whether it was found
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//
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// Remember:
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// indexOf returns -1 if the word is not found.
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String sentence = "The quick brown fox jumps over the lazy dog";
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        String target = "fox";
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        int index = sentence.indexOf(target);
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        System.out.println("Sentence: " + sentence);
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        System.out.println("Index of target: " + index);
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        if (index == -1)
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        {
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            System.out.println("Target not found.");
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        }
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        else
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        {
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            System.out.println("Target found.");
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        }
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    }
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}

Task 5: String Comparison Mini-Challenge

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// Task 5 — String Method Practice Mini-Challenge
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//
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// Create two Strings and do all of the following:
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// 1) Check if they are equal using equals
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// 2) Compare them alphabetically using compareTo
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// 3) Print both lowercase
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// 4) Print both uppercase
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//
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// Suggested values:
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// "Apple"
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// "banana"
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public class Program
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{
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    public static void main(String[] args)
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    {
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        String a = "Apple";
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        String b = "banana";
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        System.out.println("Equals? " + a.equals(b));
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        System.out.println("compareTo: " + a.compareTo(b));
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        System.out.println("a lowercase: " + a.toLowerCase());
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        System.out.println("b lowercase: " + b.toLowerCase());
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        System.out.println("a uppercase: " + a.toUpperCase());
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        System.out.println("b uppercase: " + b.toUpperCase());
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    }
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}

Sample Output

Your program output should something similar to the sample output below.

Word: computer
Length: 8
First character: c
Last character: r
Uppercase: COMPUTER

G.H.

Username: alovelace

Sentence: The quick brown fox jumps over the lazy dog
Index of target: 16
Target found.

Equals? false
compareTo: -33
a lowercase: apple
b lowercase: banana
a uppercase: APPLE
b uppercase: BANANA

Reflection Questions

You may write your reflection answers as comments at the bottom of your code.

Why should you use equals() instead of == for Strings?
What does indexOf() return when the text is not found?
Why does substring(0, 3) return only the first three characters?
What does it mean that Strings are immutable?
How is compareTo() different from equals()?

Submission

Submit your activity and reflection answers to the appropriate dropbox.