Reading a crash

Programs go wrong in two ways. You've met the first since module 1: javac finds a mistake, and the program doesn't run at all. The second kind happens while the program runs: it gets to a line it can't carry out, and it stops. This is a crash, and Java calls the problem an exception.

This program computes an average, but nothing has been added yet:

public class Main {
    public static void main(String[] args) {
        int total = 0;
        int count = 0;
        System.out.println("Scores added: " + count);
        int average = total / count;
        System.out.println("Average: " + average);
    }
}

Scores added: 0

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

A crash message has three parts worth reading:

• What happened: the exception's name, here ArithmeticException, a calculation that can't be done.
• Why: the message after the colon, / by zero. A whole number can't be divided by zero.
• Where: at Main.main(Main.java:6), the method main on line 6.

Everything printed before the crash still appears: Scores added: 0 came out, and the lines after line 6 never ran.

When the crash happens inside a method, Java lists every method that was running, the one that crashed first. This is the call stack from module 11:

public class Main {
    public static void main(String[] args) {
        printShare(12, 4);
        printShare(12, 0);
    }

    public static void printShare(int total, int people) {
        System.out.println("Each gets " + share(total, people));
    }

    public static int share(int total, int people) {
        return total / people;
    }
}

Each gets 3

Exception in thread "main" java.lang.ArithmeticException: / by zero
	at Main.share(Main.java:12)
	at Main.printShare(Main.java:8)
	at Main.main(Main.java:4)

Read it from the top: the division on line 12 in share failed. share was called by printShare on line 8, and that call came from line 4 of main, the second printShare. The first call worked; only the call with 0 people crashed.

Java Arena shows a crash the same way and explains it in plain words, with the line it happened on. The fix is often not on the crashing line itself: ask where the bad value came from.