Mutators

public class Player {
    private String name;
    private int health;
    private int score;
    
    public Player(String name) {
        this.name = name;
        this.health = 100;
        this.score = 0;
    }
    
    // Mutator — decreases health
    public void takeDamage(int damage) {
        health -= damage;
        if (health < 0) {
            health = 0;   // can't go below 0
        }
    }
    
    // Mutator — increases score
    public void addPoints(int points) {
        if (points > 0) {
            score += points;
        }
    }
    
    // Mutator — restores health
    public void heal(int amount) {
        health += amount;
        if (health > 100) {
            health = 100;   // cap at 100
        }
    }
    
    // Accessor
    public boolean isAlive() {
        return health > 0;
    }
    
    public String toString() {
        return name + " [HP: " + health + ", Score: " + score + "]";
    }
}

Player p = new Player("Hero");
System.out.println(p);          // Hero [HP: 100, Score: 0]
p.takeDamage(30);
System.out.println(p);          // Hero [HP: 70, Score: 0]
p.addPoints(50);
p.heal(10);
System.out.println(p);          // Hero [HP: 80, Score: 50]
p.takeDamage(200);
System.out.println(p.isAlive()); // false (health clamped to 0)

public class BankAccount {
    private double balance;
    
    public BankAccount(double balance) {
        this.balance = balance;
    }
    
    // Returns true if withdrawal succeeded
    public boolean withdraw(double amount) {
        if (amount > 0 && amount <= balance) {
            balance -= amount;
            return true;   // success
        }
        return false;      // failed
    }
    
    // Returns the old balance after setting new one
    public double setBalance(double newBalance) {
        double old = balance;
        if (newBalance >= 0) {
            balance = newBalance;
        }
        return old;   // returns previous balance
    }
}

public class BankAccount {
    private String owner;
    private double balance;
    
    public BankAccount(String owner, double balance) {
        this.owner = owner;
        this.balance = balance;
    }
    
    // Transfer money to another account
    public boolean transfer(BankAccount other, double amount) {
        if (amount > 0 && amount <= this.balance) {
            this.balance -= amount;   // deduct from this account
            other.balance += amount;  // add to other account
            return true;
        }
        return false;
    }
    
    public double getBalance() { return balance; }
}

BankAccount alice = new BankAccount("Alice", 1000);
BankAccount bob = new BankAccount("Bob", 500);
alice.transfer(bob, 300);
System.out.println(alice.getBalance());  // 700
System.out.println(bob.getBalance());    // 800

public class GradeBook {
    private int[] scores;
    private int count;
    
    // Private helper — only used by other methods in this class
    private double calculateAverage() {
        int sum = 0;
        for (int i = 0; i < count; i++) {
            sum += scores[i];
        }
        return (double) sum / count;
    }
    
    // Public method uses the private helper
    public String getLetterGrade() {
        double avg = calculateAverage();  // calls private helper
        if (avg >= 90) return "A";
        if (avg >= 80) return "B";
        if (avg >= 70) return "C";
        if (avg >= 60) return "D";
        return "F";
    }
}

public class WordGame {
    private String secretWord;
    private int guessesLeft;
    private String revealed;
    
    public WordGame(String word, int maxGuesses) {
        this.secretWord = word.toLowerCase();
        this.guessesLeft = maxGuesses;
        this.revealed = "";
        for (int i = 0; i < word.length(); i++) {
            revealed += "_";
        }
    }
    
    // Check if a letter is in the word and reveal it
    public boolean guess(String letter) {
        letter = letter.toLowerCase();
        boolean found = false;
        String newRevealed = "";
        
        for (int i = 0; i < secretWord.length(); i++) {
            if (secretWord.substring(i, i + 1).equals(letter)) {
                newRevealed += letter;
                found = true;
            } else {
                newRevealed += revealed.substring(i, i + 1);
            }
        }
        
        revealed = newRevealed;
        if (!found) {
            guessesLeft--;
        }
        return found;
    }
    
    public boolean isWon() {
        return revealed.equals(secretWord);
    }
    
    public boolean isLost() {
        return guessesLeft <= 0 && !isWon();
    }
    
    public String getRevealed() { return revealed; }
    public int getGuessesLeft() { return guessesLeft; }
}