Exception Hierarchy & Types

Java's exception system is a class hierarchy rooted at Throwable. Understanding the distinction between checked exceptions (must be declared or caught) and unchecked exceptions (runtime bugs) is fundamental to writing robust Java code.

12 min•By Priygop Team•Updated 2026

The Exception Hierarchy

Throwable - root of all errors and exceptions
Error - JVM-level problems. Do NOT catch: OutOfMemoryError, StackOverflowError, VirtualMachineError
Exception - application-level problems. Can catch and handle.
Checked exceptions extend Exception directly (not RuntimeException). Must be caught or declared with throws.
Unchecked exceptions extend RuntimeException. Not required to catch - indicates programming bugs.
Common checked: IOException, SQLException, FileNotFoundException, ParseException
Common unchecked: NullPointerException, IllegalArgumentException, IndexOutOfBoundsException, ClassCastException, ArithmeticException

Checked vs Unchecked in Code

import java.io.*;
import java.util.*;

public class ExceptionTypes {

    // Checked exception - caller MUST handle or declare
    public static String readFile(String path) throws IOException {
        // If you don't declare 'throws IOException', compile error!
        BufferedReader reader = new BufferedReader(new FileReader(path));
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) sb.append(line).append("\n");
        reader.close();
        return sb.toString();
    }

    // Unchecked - no 'throws' needed (but should still be documented)
    public static int divide(int a, int b) {
        if (b == 0) throw new ArithmeticException("Cannot divide by zero: a=" + a);
        return a / b;
    }

    public static void lookupById(Map<String, String> map, String key) {
        Objects.requireNonNull(key, "key must not be null");  // throws NPE with message
        String value = map.get(key);
        if (value == null) throw new NoSuchElementException("No entry for key: " + key);
    }

    public static void main(String[] args) {
        // Checked: must try/catch or propagate
        try {
            String content = readFile("catalogue.txt");
        } catch (IOException e) {
            System.err.println("Failed to read file: " + e.getMessage());
        }

        // Unchecked: optional to catch (but good to handle at service boundary)
        System.out.println(divide(10, 2));    // 5
        // divide(10, 0) → ArithmeticException at runtime
    }
}

Quick Quiz

Tip

Practice Exception Hierarchy Types in small, isolated examples before integrating into larger projects. Breaking concepts into small experiments builds genuine understanding faster than reading alone.

Diagram

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Visual architecture and concepts for Exception Hierarchy & Types

Practice Task

Note

Practice Task - (1) Write a working example of Exception Hierarchy Types from scratch without looking at notes. (2) Modify it to handle an edge case (empty input, null value, or error state). (3) Share your solution in the Priygop community for feedback.

Common Mistake

Warning

A common mistake with Exception Hierarchy Types is skipping edge case testing - empty inputs, null values, and unexpected data types. Always validate boundary conditions to write robust, production-ready java code.

Key Takeaways

Java's exception system is a class hierarchy rooted at Throwable.
Throwable - root of all errors and exceptions
Error - JVM-level problems. Do NOT catch: OutOfMemoryError, StackOverflowError, VirtualMachineError
Exception - application-level problems. Can catch and handle.

Topics in This Module

The Exception Hierarchy

Throwable - root of all errors and exceptions

Error - JVM-level problems. Do NOT catch: OutOfMemoryError, StackOverflowError, VirtualMachineError

Exception - application-level problems. Can catch and handle.

Checked exceptions extend Exception directly (not RuntimeException). Must be caught or declared with throws.

Unchecked exceptions extend RuntimeException. Not required to catch - indicates programming bugs.

Common checked: IOException, SQLException, FileNotFoundException, ParseException

Common unchecked: NullPointerException, IllegalArgumentException, IndexOutOfBoundsException, ClassCastException, ArithmeticException

Checked vs Unchecked in Code

import java.io.*;
import java.util.*;

public class ExceptionTypes {

    // Checked exception - caller MUST handle or declare
    public static String readFile(String path) throws IOException {
        // If you don't declare 'throws IOException', compile error!
        BufferedReader reader = new BufferedReader(new FileReader(path));
        StringBuilder sb = new StringBuilder();
        String line;
        while ((line = reader.readLine()) != null) sb.append(line).append("\n");
        reader.close();
        return sb.toString();
    }

    // Unchecked - no 'throws' needed (but should still be documented)
    public static int divide(int a, int b) {
        if (b == 0) throw new ArithmeticException("Cannot divide by zero: a=" + a);
        return a / b;
    }

    public static void lookupById(Map<String, String> map, String key) {
        Objects.requireNonNull(key, "key must not be null");  // throws NPE with message
        String value = map.get(key);
        if (value == null) throw new NoSuchElementException("No entry for key: " + key);
    }

    public static void main(String[] args) {
        // Checked: must try/catch or propagate
        try {
            String content = readFile("catalogue.txt");
        } catch (IOException e) {
            System.err.println("Failed to read file: " + e.getMessage());
        }

        // Unchecked: optional to catch (but good to handle at service boundary)
        System.out.println(divide(10, 2));    // 5
        // divide(10, 0) → ArithmeticException at runtime
    }
}

Tip

Diagram

Loading diagram…

Visual architecture and concepts for Exception Hierarchy & Types

Topics in This Module