GateShala
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Access Modifiers and Packages

Control who can see your classes and members, and organize code into packages.

Access modifiers decide which other code may use a class, field, method or constructor. They are the main tool for encapsulation: hiding the inner details of a class and exposing only a safe public surface.

The four levels

ModifierSame classSame packageSubclass (other package)Everywhere
privateyesnonono
(none), "package-private"yesyesnono
protectedyesyesyesno
publicyesyesyesyes

A good default: make fields private, and make methods public only when other code really needs them.

Encapsulation with private fields

Java
Open in playground
public class Main {
    public static void main(String[] args) {
        Student s = new Student("Kabir");
        s.setMarks(87);
        s.setMarks(140);                 // rejected by the setter
        System.out.println(s.getName() + ": " + s.getMarks() + " (" + s.getGrade() + ")");
        // s.marks = 140;                // compile error: marks has private access
    }
}

class Student {
    private final String name;
    private int marks;

    Student(String name) {
        this.name = name;
    }

    public String getName() { return name; }
    public int getMarks() { return marks; }

    public void setMarks(int marks) {
        if (marks < 0 || marks > 100) {
            System.out.println("Invalid marks: " + marks);
            return;
        }
        this.marks = marks;
    }

    public String getGrade() {
        return marks >= 75 ? "A" : marks >= 50 ? "B" : "C";
    }
}
Output
Invalid marks: 140
Kabir: 87 (A)

Methods like getMarks and setMarks are called getters and setters. The setter can validate input, which is impossible if the field is public.

protected

protected members are visible to subclasses, even in other packages, and to classes in the same package.

Java
Open in playground
public class Main {
    public static void main(String[] args) {
        SavingsAccount acc = new SavingsAccount(10000);
        acc.addInterest();
        System.out.println(acc.report());
    }
}

class Account {
    protected double balance;          // subclasses may use it
    Account(double balance) { this.balance = balance; }
}

class SavingsAccount extends Account {
    private static final double RATE = 0.04;
    SavingsAccount(double balance) { super(balance); }

    void addInterest() { balance += balance * RATE; }
    String report() { return "Balance: " + balance; }
}
Output
Balance: 10400.0

Packages

A package is a folder of related classes with a unique name, such as java.util or com.gateshala.bank. Packages prevent name clashes and work together with access modifiers. A project might look like this:

text
src/
  com/gateshala/bank/Account.java
  com/gateshala/bank/Bank.java
  com/gateshala/app/Main.java

The first line of each file declares its package, and other packages use import:

text
// File: com/gateshala/bank/Account.java
package com.gateshala.bank;

public class Account {
    private double balance;
    public void deposit(double amount) { balance += amount; }
    public double getBalance() { return balance; }
}

// File: com/gateshala/app/Main.java
package com.gateshala.app;

import com.gateshala.bank.Account;   // or: import com.gateshala.bank.*;

public class Main {
    public static void main(String[] args) {
        Account a = new Account();
        a.deposit(500);
        System.out.println(a.getBalance());
    }
}

Compile and run from the src folder with javac com/gateshala/*/*.java and then java com.gateshala.app.Main.