📘 Java API: Packages and Exception Handling

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🔷 PART 1: PACKAGES IN JAVA

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🔹 1. Definition

A package in Java is a namespace (folder) that organizes related classes and interfaces into a hierarchical structure.

👉 It helps in:

• Avoiding name conflicts
• Making code reusable
• Improving organization

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🔹 2. Types of Packages

✅ Built-in Packages

Provided by Java

Examples:

• java.lang → basic classes (String, Math)
• java.util → utilities (Scanner, ArrayList)
• java.io → input/output

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✅ User-defined Packages

Created by programmer

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🔹 3. Creating a Package

✔ Syntax:

package mypackage;

✔ Example:

package mypackage;

public class Hello {
    public void show() {
        System.out.println("Hello from package");
    }
}

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🔹 4. Compiling and Running

Step-by-step:

1. Save file → Hello.java
2. Compile:

javac -d . Hello.java

3. Run:

java mypackage.Hello

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🔹 5. Accessing Packages

✔ Using import

import mypackage.Hello;

OR

import mypackage.*;

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✔ Example:

import mypackage.Hello;

class Test {
    public static void main(String[] args) {
        Hello h = new Hello();
        h.show();
    }
}

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🔹 6. Access Modifiers in Packages

Modifier	Same Class	Same Package	Different Package
public	✔	✔	✔
protected	✔	✔	✔ (via inheritance)
default	✔	✔	✘
private	✔	✘	✘

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🔹 7. Advantages of Packages

• Code reusability
• Better organization
• Access control
• Avoids naming conflicts

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🔹 8. Diagram Description

📊 Package Structure Diagram:

• Main folder → package name
• Inside → classes (files)
• Tree structure showing hierarchy

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🔷 PART 2: EXCEPTION HANDLING IN JAVA

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🔹 1. Definition

An exception is an error or abnormal condition that occurs during program execution and disrupts normal flow.

👉 Example:

• Division by zero
• File not found

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🔹 2. Types of Exceptions

✅ Checked Exceptions

• Checked at compile-time
• Must be handled

Examples:

• IOException
• SQLException

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✅ Unchecked Exceptions

• Occur at runtime

Examples:

• ArithmeticException
• NullPointerException

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🔹 3. Exception Handling Keywords

Keyword	Purpose
try	Code that may cause exception
catch	Handles exception
finally	Always executes
throw	Explicitly throw exception
throws	Declare exception

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🔹 4. Basic Syntax

try {
    // risky code
} catch (Exception e) {
    // handling code
} finally {
    // always runs
}

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🔹 5. Example

public class Test {
    public static void main(String[] args) {
        try {
            int a = 10 / 0;
        } catch (ArithmeticException e) {
            System.out.println("Cannot divide by zero");
        } finally {
            System.out.println("Done");
        }
    }
}

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✔ Explanation

1. 10 / 0 → causes exception
2. Control goes to catch
3. Error handled
4. finally always executes

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🔹 6. Using throw

throw new ArithmeticException("Error occurred");

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🔹 7. Using throws

void readFile() throws IOException {
    // code
}

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🔹 8. Multiple Catch Blocks

try {
    int a[] = new int[5];
    a[10] = 30;
} catch (ArithmeticException e) {
    System.out.println("Arithmetic Error");
} catch (ArrayIndexOutOfBoundsException e) {
    System.out.println("Index Error");
}

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🔹 9. Custom Exception

class MyException extends Exception {
    public MyException(String msg) {
        super(msg);
    }
}

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🔹 10. Important Rules

• Only one finally block
• Multiple catch allowed
• finally always executes (except system crash)
• Checked exceptions must be handled

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🔹 11. Diagram Description

📊 Exception Flow Diagram:

1. Try block executes
2. Exception occurs
3. Control jumps to catch
4. Finally block executes

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📝 Likely Exam Questions

1. Define package in Java.
2. What are advantages of packages?
3. Explain how to create a package.
4. What is exception handling?
5. Types of exceptions in Java.
6. Explain try-catch-finally with example.
7. Difference between throw and throws.
8. What are checked and unchecked exceptions?
9. Write a program demonstrating exception handling.
10. Explain access modifiers in packages.

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📌 Quick Revision Summary

🔹 Packages

• Organize classes
• Avoid conflicts
• Use package and import

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🔹 Exception Handling

• Handles runtime errors
• Keywords: try, catch, finally, throw, throws
• Types: Checked & Unchecked

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