Access Modifiers

1// private - Only accessible within the same class
2public class BankAccount {
3    private double balance;          // Only this class can access
4    private String accountNumber;
5    
6    public BankAccount(String accountNumber) {
7        this.accountNumber = accountNumber;
8        this.balance = 0;
9    }
10    
11    // Private helper method
12    private boolean isValidAmount(double amount) {
13        return amount > 0;
14    }
15    
16    public void deposit(double amount) {
17        if (isValidAmount(amount)) {  // Can call private method
18            balance += amount;         // Can access private field
19        }
20    }
21    
22    public double getBalance() {
23        return balance;  // Controlled access via getter
24    }
25}
26
27// Outside the class:
28BankAccount acc = new BankAccount("123");
29// acc.balance = 1000000;        // ERROR! private
30// acc.isValidAmount(100);       // ERROR! private
31acc.deposit(100);                // OK - public method
32acc.getBalance();                // OK - public method

1// default (package-private) - No modifier keyword
2// File: com/myapp/User.java
3package com.myapp;
4
5class User {              // default class - package access only
6    String name;          // default field
7    int age;
8    
9    void display() {      // default method
10        System.out.println(name + ", " + age);
11    }
12}
13
14// File: com/myapp/UserManager.java
15package com.myapp;
16
17public class UserManager {
18    public void createUser() {
19        User user = new User();   // OK - same package
20        user.name = "Alice";      // OK - same package
21        user.display();           // OK - same package
22    }
23}
24
25// File: com/other/External.java
26package com.other;
27import com.myapp.*;
28
29public class External {
30    public void test() {
31        // User user = new User();   // ERROR! User class not visible
32        UserManager um = new UserManager(); // OK - public class
33    }
34}

1// protected - Package + Subclass access
2package com.animals;
3
4public class Animal {
5    protected String name;        // Accessible in subclasses
6    protected int age;
7    
8    protected void makeSound() {  // Accessible in subclasses
9        System.out.println("Some sound");
10    }
11}
12
13// Same package - full access
14package com.animals;
15
16public class Zoo {
17    public void test() {
18        Animal a = new Animal();
19        a.name = "Leo";         // OK - same package
20        a.makeSound();          // OK - same package
21    }
22}
23
24// Different package, but subclass - limited access
25package com.pets;
26import com.animals.Animal;
27
28public class Dog extends Animal {
29    public void bark() {
30        name = "Buddy";         // OK - inherited protected
31        makeSound();            // OK - inherited protected
32        
33        // But NOT through another instance!
34        Animal other = new Animal();
35        // other.name = "X";    // ERROR! Not via different instance
36    }
37}

1// public - Accessible from everywhere
2public class PublicAPI {
3    public static final String VERSION = "1.0";
4    
5    public String publicField;
6    
7    public PublicAPI() { }
8    
9    public void publicMethod() {
10        System.out.println("Anyone can call this");
11    }
12}
13
14// Complete Example - Encapsulation Best Practices
15public class Employee {
16    // Private fields - hide internal state
17    private int id;
18    private String name;
19    private double salary;
20    
21    // Public constructor - allow creation
22    public Employee(int id, String name) {
23        this.id = id;
24        this.name = name;
25        this.salary = 0;
26    }
27    
28    // Public getters - read access
29    public int getId() { return id; }
30    public String getName() { return name; }
31    public double getSalary() { return salary; }
32    
33    // Public setter with validation
34    public void setSalary(double salary) {
35        if (salary >= 0) {
36            this.salary = salary;
37        }
38    }
39    
40    // Private helper - internal use only
41    private void log(String message) {
42        System.out.println("[Employee " + id + "] " + message);
43    }
44    
45    // Protected method - for subclasses
46    protected void updateRecord() {
47        log("Record updated");
48    }
49}