Errors and Exceptions in Java in Java

public void spend(int amount) {
    if (amount > money) {
        throw new IllegalArgumentException("not enough money");
    }

    money -= amount;
}

try {
    wallet.spend(80);
} catch (IllegalArgumentException e) {
    System.out.println("error: " + e.getMessage());
}

Full lesson:

Some requests can't be done. A wallet with 50 can't spend 80.

The method could return a special value, like -1. But the caller may forget to check it. Then the program goes on with a wrong number.

An exception is better. It is an error that the program can't miss.

Throwing an exception

• throw new IllegalArgumentException("not enough money") stops the method at once.
• The lines after it don't run. So the wallet keeps its money.
• IllegalArgumentException is Java's built-in exception for a bad argument.
• The text in the parentheses is the exception's message.
• Some exceptions must be listed after throws in the method's header. This one doesn't need that.

Catching it

• Put the call inside a try block.
• When the call throws, the rest of the try block is skipped.
• The catch block for that exception runs instead. e is the exception.
• e.getMessage() returns its message.
• Then the program goes on below the catch block.

An exception that nobody catches stops the whole program. Java prints it and the line where it happened.

Your task

A Wallet starts with 50. Its spend method takes money out and returns what is left.

• In spend, throw an IllegalArgumentException when amount is more than money. Its message is "not enough money".
• In the loop, call spend inside a try block. Print what was spent and what is left.
• Catch the exception. Print error: and its message.

The loop tries to spend 20, then 40, then 30.

Expected output:

spent 20, 30 left
error: not enough money
spent 30, 0 left