Packages and Access Modifiers

Declaring and Using Packages

A package declaration, if present, must be the very first non-comment line in a source file: package com.webnest.util;. Classes in other packages access it with an import statement, such as import com.webnest.util.Helper;, or by using the fully qualified name inline.

The Four Access Modifiers

Each modifier controls visibility at a different scope, from the most restrictive to the most open.

• private — accessible only within the same class.
• default (package-private) — no keyword; accessible within the same package only.
• protected — accessible within the same package, plus subclasses in other packages (through inheritance).
• public — accessible from anywhere, in any package.

Visibility Table

The relationship between modifier and accessible scope is fixed and worth memorizing exactly.

• private: same class only.
• default: same class, same package.
• protected: same class, same package, subclass (even in a different package).
• public: same class, same package, subclass, and any other package.

Example

package com.webnest.util;

class InternalHelper { // package-private (default) class, not visible outside this package
    static int square(int n) {
        return n * n;
    }
}

public class MathTool {
    public static int squareOfSum(int a, int b) {
        return InternalHelper.square(a + b);
    }

    public static void main(String[] args) {
        System.out.println(MathTool.squareOfSum(2, 3));
    }
}

25

Exercise

Everything in Wallet is public, so any code could set the balance directly. Tighten the access: make the field and the helper method private and keep only deposit and getBalance available to other classes. The output must not change.

Expected output: 500

class Wallet {
    public int balance;

    public boolean isValid(int amount) {
        return amount > 0;
    }

    public void deposit(int amount) {
        if (isValid(amount)) {
            balance += amount;
        }
    }

    public int getBalance() {
        return balance;
    }
}

public class WalletDemo {
    public static void main(String[] args) {
        Wallet wallet = new Wallet();
        wallet.deposit(500);
        wallet.deposit(-200);
        System.out.println(wallet.getBalance());
    }
}

class Wallet {
    private int balance;

    private boolean isValid(int amount) {
        return amount > 0;
    }

    public void deposit(int amount) {
        if (isValid(amount)) {
            balance += amount;
        }
    }

    public int getBalance() {
        return balance;
    }
}

public class WalletDemo {
    public static void main(String[] args) {
        Wallet wallet = new Wallet();
        wallet.deposit(500);
        wallet.deposit(-200);
        System.out.println(wallet.getBalance()); // 500
    }
}

Quick quiz

Interview questions