Exception Handling

Before we start

Why this matters

Outline · Lectures 15–18

Topics in this chapter

Select a topic to jump to the slide that explains it.

1. What is an exception
2. Exception types: checked, unchecked, errors
3. try, catch and finally
4. Multiple catch clauses
5. throw and throws
6. Custom exceptions

Concept 1 of 6

What is an exception?

public class Crash {
    static int avg(int[] a) { return sum(a) / a.length; }
    static int sum(int[] a) { int s = 0; for (int x : a) s += x; return s; }
    public static void main(String[] args) {
        System.out.println(avg(new int[]{4, 6}));
        System.out.println(avg(new int[]{}));
        System.out.println("never printed");
    }
}

Concept 1 of 6

What is an exception? (cont.)

5
Exception in thread "main" java.lang.ArithmeticException: / by zero
	at Crash.avg(Crash.java:2)
	at Crash.main(Crash.java:6)

• An exception is an object that describes an error that happened while the program was running.
• When it is thrown, normal execution stops and Java looks back through the call stack for a matching handler.
• If no method handles it, the thread ends and the stack trace shows where it happened — read it from the top.

Concept 2 of 6

Types of exceptions

Types of exceptions

Concept 3 of 6

try, catch and finally

try, catch and finally

try {
    int n = Integer.parseInt(text);   // may throw
    System.out.println(100 / n);      // may throw
} catch (NumberFormatException e) {
    System.out.println("Not a number");
} catch (ArithmeticException e) {
    System.out.println("Cannot divide by zero");
} finally {
    System.out.println("done");       // always runs
}

• try wraps code that may throw. The rest of the try block is skipped after an exception.
• catch handles one type (and its subclasses). The first matching catch runs.
• finally runs whether or not an exception occurred — even after return. Use it for clean-up.

Step through the code

Trace: three inputs, three paths

Choose an input above the code, then step through. Compare which lines run for "10", "0" and "abc".

Step through the code

Trace: three inputs, three paths (cont.)

Interactive

Catch-clause matcher

Concept 4 of 6

Multiple catch clauses and multi-catch

try {
    ...
} catch (FileNotFoundException e) {    // most specific first
    System.out.println("Missing file");
} catch (IOException e) {              // then its superclass
    System.out.println("I/O problem");
} catch (NumberFormatException | ArithmeticException e) {
    System.out.println("Bad data: " + e.getMessage());
} catch (Exception e) {                // catch-all last
    e.printStackTrace();
}

Concept 5 of 6

throw and throws

static void setAge(int age) {
    if (age < 0)
        throw new IllegalArgumentException("age < 0: " + age);
    this.age = age;
}

static String firstLine(String path) throws IOException {
    try (BufferedReader br = new BufferedReader(new FileReader(path))) {
        return br.readLine();
    }
}

Step through the code

Trace: an exception travels up the stack

level3 throws; level2 does not catch it, but its finally still runs; main catches it. Watch the frames disappear.

Step through the code

Trace: an exception travels up the stack (cont.)

Code example

Creating your own exceptions

Withdrew 200.0, left 300.0
Insufficient funds: short by 150.0

• Extend Exception for a checked exception, RuntimeException for an unchecked one.
• Pass a message to super(msg); add fields for extra details.

Step through the code

Trace: throwing a custom exception

A custom exception is an ordinary object. Watch it get created, thrown, and caught two frames up.

Step through the code

Trace: throwing a custom exception (cont.)

Code example

try-with-resources

try (BufferedReader br = new BufferedReader(new FileReader("marks.txt"))) {
    String line;
    int total = 0;
    while ((line = br.readLine()) != null)
        total += Integer.parseInt(line.trim());
    System.out.println("Total = " + total);
} catch (FileNotFoundException e) {
    System.out.println("File missing: " + e.getMessage());
} catch (IOException | NumberFormatException e) {
    System.out.println("Bad data: " + e);
}

Total = 233

Code example

try-with-resources (cont.)

• Resources declared in try ( … ) are closed automatically at the end, in reverse order, even if an exception occurs.
• Works with any class that implements AutoCloseable: files, sockets, database connections, Scanner.

Concept 6 of 6

Good exception habits

Worked example 1 of 3

Worked example 1

Worked example 2 of 3

Worked example 2

Worked example 3 of 3

Worked example 3

static int readScore(Scanner in) {
    while (true) {
        try {
            return Integer.parseInt(in.nextLine().trim());
        } catch (NumberFormatException e) {
            System.out.println("Please type a whole number.");
        }
    }
}

Practice

Practice (1 of 2)

Try each one on paper first, then check your answer.

static double sqrt(double x) {
    if (x < 0) throw new IllegalArgumentException("negative: " + x);
    return Math.sqrt(x);
}

Practice

Practice (2 of 2)

Try each one on paper first, then check your answer.

class InvalidMarksException extends RuntimeException {
    InvalidMarksException(String msg) { super(msg); }
}

Check yourself

Quiz

Summary

Summary

• An exception is an object describing a run-time error; uncaught ones end the thread with a stack trace.
• Checked exceptions must be caught or declared; unchecked (RuntimeException) and Errors need not be.
• try runs risky code, the first matching catch handles it, finally always runs.
• throw raises an exception object; throws declares what a method may pass on.
• Custom exceptions extend Exception or RuntimeException; try-with-resources closes resources automatically.