并不必然非要利用Java违例。这一点必需把握,因为常常都需要建设本身的违例,以便指出本身的库大概生成的一个非凡错误——但建设Java分级布局的时候,这个错误是无法预知的。
为建设本身的违例类,必需从一个现有的违例范例担任——最亏得寄义上与新违例近似。担任一个违例相当简朴:
//: Inheriting.java // Inheriting your own exceptions class MyException extends Exception { public MyException() {} public MyException(String msg) { super(msg); } } public class Inheriting { public static void f() throws MyException { System.out.println( "Throwing MyException from f()"); throw new MyException(); } public static void g() throws MyException { System.out.println( "Throwing MyException from g()"); throw new MyException("Originated in g()"); } public static void main(String[] args) { try { f(); } catch(MyException e) { e.printStackTrace(); } try { g(); } catch(MyException e) { e.printStackTrace(); } } } ///:~
担任在建设新类时产生:
class MyException extends Exception { public MyException() {} public MyException(String msg) { super(msg); } }
这里的要害是“extends Exception”,它的意思是:除包罗一个Exception的全部寄义以外,尚有更多的寄义。增加的代码数量很是少——实际只添加了两个构建器,对MyException的建设方法举办了界说。请记着,如果我们不明晰挪用一个基本类构建器,编译器会自动挪用基本类默认构建器。在第二个构建器中,通过利用super要害字,明晰挪用了带有一个String参数的基本类构建器。
该措施输出功效如下:
Throwing MyException from f() MyException at Inheriting.f(Inheriting.java:16) at Inheriting.main(Inheriting.java:24) Throwing MyException from g() MyException: Originated in g() at Inheriting.g(Inheriting.java:20) at Inheriting.main(Inheriting.java:29)
可以看到,在从f()“掷”出的MyException违例中,缺乏具体的动静。
建设本身的违例时,还可以采纳更多的操纵。我们可添加特另外构建器及成员:
//: Inheriting2.java // Inheriting your own exceptions class MyException2 extends Exception { public MyException2() {} public MyException2(String msg) { super(msg); } public MyException2(String msg, int x) { super(msg); i = x; } public int val() { return i; } private int i; } public class Inheriting2 { public static void f() throws MyException2 { System.out.println( "Throwing MyException2 from f()"); throw new MyException2(); } public static void g() throws MyException2 { System.out.println( "Throwing MyException2 from g()"); throw new MyException2("Originated in g()"); } public static void h() throws MyException2 { System.out.println( "Throwing MyException2 from h()"); throw new MyException2( "Originated in h()", 47); } public static void main(String[] args) { try { f(); } catch(MyException2 e) { e.printStackTrace(); } try { g(); } catch(MyException2 e) { e.printStackTrace(); } try { h(); } catch(MyException2 e) { e.printStackTrace(); System.out.println("e.val() = " + e.val()); } } } ///:~
此时添加了一个数据成员i;同时添加了一个非凡的要领,用它读取谁人值;也添加了一个特另外构建器,用它配置谁人值。输出功效如下:
Throwing MyException2 from f() MyException2 at Inheriting2.f(Inheriting2.java:22) at Inheriting2.main(Inheriting2.java:34) Throwing MyException2 from g() MyException2: Originated in g() at Inheriting2.g(Inheriting2.java:26) at Inheriting2.main(Inheriting2.java:39) Throwing MyException2 from h() MyException2: Originated in h() at Inheriting2.h(Inheriting2.java:30) at Inheriting2.main(Inheriting2.java:44) e.val() = 47
由于违例不外是另一种形式的工具,所以可以继承这个历程,进一步加强违例类的本领。但要留意,对利用本身这个包的客户措施员来说,他们大概错过所有这些加强。因为他们大概只是简朴地寻找筹备生成的违例,除此以外不做任何工作——这是大大都Java库违例的尺度用法。若呈现这种环境,有大概建设一个新违例范例,个中险些不包括任何代码:
//: SimpleException.java
class SimpleException extends Exception {
} ///:~
它要依赖编译器来建设默认构建器(会自动挪用基本类的默认构建器)。虽然,在这种环境下,我们不会获得一个SimpleException(String)构建器,但它实际上也不会常常用到。