[JAVA100例] 009 - 異常捕獲與自訂異常類別

範例示範繼承 Exception 建立自訂異常類別 MyException，提供三種建構子（無參、訊息、訊息+代碼），並在 DemoException 中透過 try-catch 捕獲處理。

class MyException extends Exception {
    public MyException() {}
    public MyException(String msg) { super(msg); }
    public MyException(String msg, int x) { super(msg); i = x; }
    public int val() { return i; }
    private int i;
}

public class DemoException {
    public static void a() throws MyException {
        System.out.println("Throwing MyException from a()");
        throw new MyException();
    }
    public static void b() throws MyException {
        System.out.println("Throwing MyException from b()");
        throw new MyException("Originated in b()");
    }
    public static void c() throws MyException {
        System.out.println("Throwing MyException from c()");
        throw new MyException("Originated in c()", 47);
    }
    public static void main(String[] args) {
        try { a(); } catch (MyException e) { e.getMessage(); }
        try { b(); } catch (MyException e) { e.toString(); }
        try { c(); } catch (MyException e) {
            e.printStackTrace();
            System.out.println("error code: " + e.val());
        }
    }
}

執行結果展示三種建構子的異常拋出與捕獲處理方式：getMessage()、toString()、printStackTrace() 及自訂 val() 取得錯誤代碼。