Exception Handling: A Deeper Look — Java Cheat Sheets

1 BASIC try / catch

try { int result = 10 / divisor; } catch (ArithmeticException e) { System.out.println("Can't divide by zero: " + e.getMessage()); }

Why exception handling?

Without a try/catch, an error like division-by-zero or bad input (InputMismatchException) crashes the whole program. Handling it lets the program recover or fail gracefully.

2 THE JAVA EXCEPTION HIERARCHY

Throwable

Exception

Error

RuntimeException
(unchecked)

IOException etc.
(checked)

Everything throwable extends Throwable → splits into Exception (recoverable problems) and Error (serious JVM-level problems, not meant to be caught).

3 CHECKED vs. UNCHECKED EXCEPTIONS

Checked

Subclasses of Exception (excluding RuntimeException)
Compiler forces you to catch or declare (throws) them
e.g. IOException, SQLException

Unchecked

Subclasses of RuntimeException
Compiler does not require handling — usually programmer errors
e.g. NullPointerException, ArithmeticException

4 finally BLOCK & STACK UNWINDING

try { riskyOperation(); } catch (Exception e) { handle(e); } finally { System.out.println("Always runs — cleanup here"); }

Stack unwinding

When an exception isn't caught in the current method, Java "unwinds" back through the call stack, exiting each calling method in turn, until a matching catch is found (or the program terminates).

5 CHAINED & CUSTOM EXCEPTIONS

public class InsufficientFundsException extends Exception { public InsufficientFundsException(String msg) { super(msg); } } // chained: wrap a lower-level cause throw new RuntimeException("Save failed", originalCause);

Why write custom exceptions?

Gives your program's own, meaningful exception types (e.g. InsufficientFundsException) instead of forcing every error into a generic one — makes calling code's catch clauses more precise.

6 PRECONDITIONS, POSTCONDITIONS & ASSERTIONS

Precondition

A condition that must be true before a method runs (e.g. "amount must be positive").

Postcondition

A condition guaranteed true after a method completes successfully.

assert

assert age >= 0 : "age can't be negative";

7 try-WITH-RESOURCES

try (BufferedReader br = new BufferedReader( new FileReader("data.txt"))) { System.out.println(br.readLine()); } // br.close() is called automatically

Why use it?

Any resource implementing AutoCloseable (files, streams, DB connections) is automatically closed when the block ends — even if an exception occurs — eliminating manual finally { close(); } boilerplate.

8 METHOD REFERENCE TABLE — class Throwable / Exception

Method	Purpose
`getMessage()`	returns the exception's detail message
`getCause()`	returns the underlying exception that caused this one (chaining)
`printStackTrace()`	prints the full call-stack trace to System.err
`getStackTrace()`	returns the stack trace as a StackTraceElement[]
`toString()`	class name + message as text
`addSuppressed(e)`	records extra exceptions suppressed during try-with-resources cleanup

JAVA — CHAPTER 10