Exception Hierarchy in Java

Every exception in Java is an object of a class, and those classes form a family tree. Knowing the tree tells you three useful things at a glance: whether the compiler will force you to handle a problem, whether you should try to recover from it, and which catch blocks will catch it. The whole tree grows from one root: Throwable.

The Family Tree

You can ask Java to print each class's ancestors:

public class Ancestors {
    static void show(Class<?> c) {
        StringBuilder line = new StringBuilder(c.getSimpleName());
        Class<?> p = c.getSuperclass();
        for (; p != Object.class; p = p.getSuperclass()) {
            line.append(" -> ").append(p.getSimpleName());
        }
        System.out.println(line);
    }
 
    public static void main(String[] args) {
        show(NumberFormatException.class);
        show(NullPointerException.class);
        show(java.io.FileNotFoundException.class);
        show(StackOverflowError.class);
    }
}

NumberFormatException -> IllegalArgumentException -> RuntimeException -> Exception -> Throwable
NullPointerException -> RuntimeException -> Exception -> Throwable
FileNotFoundException -> IOException -> Exception -> Throwable
StackOverflowError -> VirtualMachineError -> Error -> Throwable

How it works

getSuperclass() climbs one level at a time until it reaches Object. Every line ends at Throwable, the only class Java lets you throw and catch.

The Three Branches

The next lesson, "Checked vs Unchecked Exceptions", explains why the compiler treats these branches differently.

Why the Tree Matters for catch

A catch block catches its type and every subclass of it:

That's why specific catch blocks must come before general ones (see "Multiple Catch Blocks and Multi-Catch").

Common Mistakes

Catching Throwable or Error. You'd also catch OutOfMemoryError, which you can't fix by catching.

Thinking Error and Exception are the same thing. They're siblings with different purposes.

Catching Exception when you only expect one type. You'll also catch bugs you didn't intend to hide.

Interview Corner

Practice

1. Easy: Use the show method above to print the ancestors of ClassCastException, ArrayIndexOutOfBoundsException and java.sql.SQLException.
2. Harder: For each, say which branch it's in and whether the compiler forces you to handle it: IOException, IllegalStateException, OutOfMemoryError, InterruptedException.

Key Takeaways