☕ Java📅 Day 18

Exception Handling in Java

Stop programs from crashing on bad input: try, catch and finally, checked vs unchecked exceptions, throw, throws and your own exception classes.

🎯

Learning objectives

→Explain what an exception is and what happens when nobody handles it
→Handle exceptions with try, catch and finally
→Tell checked exceptions apart from unchecked ones
→Use throw and throws, and write a custom exception class

💡 Key points

An exception is a problem that happens while the program runs, such as dividing by zero. If nobody catches it, the program stops and prints a stack trace.
Put risky code inside try. A catch (Type e) block handles a matching exception, and the program then carries on after the try-catch.
finally runs whether or not an exception happened. Use it for clean-up such as closing a file.
Catch blocks are checked from top to bottom, so put specific exceptions (ArithmeticException) before general ones (Exception).
Checked exceptions (IOException, FileNotFoundException) must be caught or declared with throws, and the compiler enforces this. Unchecked exceptions (subclasses of RuntimeException) do not have to be.
throw raises an exception right now. throws, in a method header, warns callers that the method may pass one on to them.
A custom exception is a class that extends Exception (checked) or RuntimeException (unchecked).

💻 Code examples(6)

#1An unhandled exception

java

public class Crash {
    public static void main(String[] args) {
        int total = 500;
        int students = 0;
        System.out.println("Calculating average...");
        int avg = total / students;
        System.out.println("Average: " + avg);
    }
}

Output

Calculating average...
Exception in thread "main" java.lang.ArithmeticException: / by zero
	at Crash.main(Crash.java:6)

Dividing an int by zero throws an ArithmeticException. Nothing catches it, so Java prints the exception's name, its message and the line number (6), and the last println never runs.

#2try and catch

java

public class SafeDivide {
    public static void main(String[] args) {
        int total = 500;
        int students = 0;
        try {
            int avg = total / students;
            System.out.println("Average: " + avg);
        } catch (ArithmeticException e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("Program continues");
    }
}

Output

Error: / by zero
Program continues

When the division fails, Java skips the rest of the try block and jumps to the matching catch. e.getMessage() gives a short description. After the catch, the program runs normally.

#3Several catch blocks and finally

java

public class ParseMarks {
    public static void main(String[] args) {
        String[] inputs = {"85", "abc", "92"};
        int[] marks = new int[2];

        for (int i = 0; i < inputs.length; i++) {
            try {
                marks[i] = Integer.parseInt(inputs[i]);
                System.out.println("Saved " + marks[i]);
            } catch (NumberFormatException e) {
                System.out.println("Not a number: "
                    + inputs[i]);
            } catch (ArrayIndexOutOfBoundsException e) {
                System.out.println("No space for "
                    + inputs[i]);
            } finally {
                System.out.println("-- done with " + i);
            }
        }
    }
}

Output

Saved 85
-- done with 0
Not a number: abc
-- done with 1
No space for 92
-- done with 2

"abc" cannot be parsed, and the array has no index 2, so each problem goes to the catch that matches its type. finally runs after every attempt, success or failure.

#4A checked exception: catch it or declare it

java

import java.io.FileReader;
import java.io.IOException;

public class ReadFile {
    public static void main(String[] args) {
        try {
            FileReader fr = new FileReader("marks.txt");
            System.out.println("File opened");
            fr.close();
        } catch (IOException e) {
            System.out.println("Could not open marks.txt");
        }
    }
}

Output

Could not open marks.txt

new FileReader(...) can throw FileNotFoundException, a kind of IOException. These are checked: remove the try-catch and javac refuses to compile. Here there is no marks.txt, so the catch runs.

#5throw: raise an exception yourself

java

public class Ticket {
    static void book(int age) {
        if (age < 18) {
            throw new IllegalArgumentException(
                "Age " + age + " is below 18");
        }
        System.out.println("Booked for age " + age);
    }

    public static void main(String[] args) {
        try {
            book(21);
            book(15);
            book(30);
        } catch (IllegalArgumentException e) {
            System.out.println("Refused: "
                + e.getMessage());
        }
    }
}

Output

Booked for age 21
Refused: Age 15 is below 18

throw new ... creates an exception object and raises it immediately. book(15) throws, so control jumps to the catch, and book(30) never runs.

#6A custom checked exception with throws

java

class LowBalanceException extends Exception {
    LowBalanceException(String msg) {
        super(msg);
    }
}

class Wallet {
    private double balance = 1000;

    void pay(double amt) throws LowBalanceException {
        if (amt > balance) {
            throw new LowBalanceException("Need Rs."
                + amt + ", have Rs." + balance);
        }
        balance -= amt;
        System.out.println("Paid Rs." + amt
            + ", left Rs." + balance);
    }
}

public class WalletDemo {
    public static void main(String[] args) {
        Wallet w = new Wallet();
        try {
            w.pay(400);
            w.pay(800);
        } catch (LowBalanceException e) {
            System.out.println("Failed: "
                + e.getMessage());
        }
    }
}

Output

Paid Rs.400.0, left Rs.600.0
Failed: Need Rs.800.0, have Rs.600.0

LowBalanceException extends Exception, so it is checked: pay() must declare throws LowBalanceException, and every caller must handle it. super(msg) stores the message that getMessage() returns.

🎯 Practice

Q1. What does finally do, and when does it run?+

It holds code that runs after the try (and any catch), whether or not an exception happened, for example closing a file. It is skipped only if the JVM itself stops, such as with System.exit().

Q2. Is NumberFormatException checked or unchecked? How can you tell?+

Unchecked. It extends IllegalArgumentException, which extends RuntimeException, so the compiler does not force you to catch it.

Q3. What is the difference between throw and throws?+

throw is a statement that raises one exception object: throw new ArithmeticException("..."); throws goes in a method header and lists the exceptions the method may pass to its caller: void read() throws IOException

Q4. Why is this a compile error? catch (Exception e) { } catch (ArithmeticException e) { }+

Exception already catches every ArithmeticException, so the second block could never run. javac reports 'exception ArithmeticException has already been caught'. Put the specific catch first.

Q5. Write a custom unchecked exception called InvalidMarksException.+

class InvalidMarksException extends RuntimeException { InvalidMarksException(String msg) { super(msg); } } // use: throw new InvalidMarksException("Marks must be 0-100");

📝 Notes

The exception family tree

Throwable is the root of everything that can be thrown.
- Error covers serious problems inside the JVM, such as StackOverflowError and OutOfMemoryError. Don't catch these.
- Exception covers problems a program can recover from. It and its subclasses, such as IOException, are checked, except for one branch:
  - RuntimeException and its subclasses are unchecked: ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException, NumberFormatException.

Unchecked exceptions usually mean a bug you can fix with a check, such as if (students != 0). Checked exceptions are problems outside your control, such as a missing file, so Java makes you plan for them. Forget one and you see unreported exception FileNotFoundException; must be caught or declared to be thrown.

try-with-resources (Java 7+)

Anything that must be closed, such as a file or a Scanner, can be opened in the brackets after try. Java closes it for you, even if an exception happens:

try (Scanner sc = new Scanner(System.in)) {
    int n = sc.nextInt();
    System.out.println("Read " + n);
}   // sc.close() happens automatically

Common mistakes

An empty catch block. catch (Exception e) { } hides the problem completely. At least print e.getMessage().
Catching Exception everywhere. Catch the specific type you expect, so real bugs still show up.
Using exceptions instead of a simple check. Test if (index < arr.length) rather than waiting for ArrayIndexOutOfBoundsException.
Putting the general catch first. It makes the later, specific catches unreachable, which is a compile error.

Next: Day 19, Recursion. You will write methods that call themselves, and learn why every one needs a base case.