JAVA / CHAPTER 12

Exception handling

Advanced

Handle a specific failure without concealing unrelated errors.

By the end of this chapter

Trace try/catch/finally
Choose meaningful exception boundaries
Preserve useful failure information

The idea

An exception interrupts ordinary statement execution and searches for a matching handler. Code after the failing statement in the same try block is skipped. A catch clause handles compatible exception types; order more specific handlers before general ones. Use exceptions for failures that the caller must distinguish, rather than turning every possible problem into a plausible-looking default result.

A finally block normally runs as control leaves the try/catch structure, making it useful for cleanup. For resources such as files, prefer try-with-resources when the resource implements AutoCloseable. Checked exceptions must be caught or declared where required; unchecked exceptions still deserve a deliberate policy. Catch near a boundary that can recover or explain the failure meaningfully.

Java example

public class Main {
  public static void main(String[] args) {
    try {
      int count = Integer.parseInt("four");
      System.out.println(count);
    } catch (NumberFormatException ex) {
      System.out.println("Invalid count");
    } finally {
      System.out.println("Finished");
    }
  }
}

Result

Invalid count
Finished

Parsing fails before count can be printed. The matching handler explains the failure, and the finally block then runs.

Try it yourself

Parse a user-supplied integer and report a distinct validation error for malformed text.

Use your own editor for this exercise. Runnable challenges on CodeQuest are available for JavaScript, Python, and SQL.

CHECK YOUR UNDERSTANDING

Chapter checkpoint

Which output sequence occurs?

try {
  System.out.print("A");
  Integer.parseInt("x");
  System.out.print("B");
} catch (NumberFormatException e) { System.out.print("C"); }
finally { System.out.print("D"); }

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