什么是Exceptions

首先我们要知道什么是Exceptions, 定义如下

Exceptions: An exception is an event that occurs during the execution of a program that disrupts the normal flow of instructions.

这是官方的定义，详细请点击

同时我们可以定义Exception为： An exception is an error condition that occurs during the execution of a Java Program.

这句话中两个要点：1，exception 是一个 error(想想另外一个我们在APCS中会碰到的error --  compile error)

                          2,  occurs during the exectution，也就是在运行过程中出现的错误。

APCS 会考到哪些不同种类的Exceptions

Exceptions有非常多的种类，每一个种类的Exception 都有自己的一个名字由“出错原因+Exception”组成。以下几类将会在APCS中cover到。

1，ArithmeticExceptions

什么情况会出现: Thrown when an exceptional arithmetic condition has occurred. For example, an integer "divide by zero"

1 public class Test2 {   3     public static void main(String args[]){4         int x = 0;5         int y = 6;6         System.out.println("y/x = " + y/x); //6/0 will cause an ArithmeticException here7     }8 }

运行结果为

java.lang.ArithmeticException: / by zero
    at Test.main(Test.java:6)

2，NullPointerException

什么情况会出现: Thrown when an application attempts to use null in a case where an object is required.

简单说来就是当你想调用一个object的public方法或者使用public instance variable的时候这个object并没有“存在”

 1 public class Test 2 {    3     private int x; 4     public int getX(){ 5         return x; 6     } 7      8     public static void main(String args[]){ 9         Test t = new Test();10         t = null;11         t.getX(); // may cause NullPointerException12     }13 }

运行结果为  

java.lang.NullPointerException
    at Test.main(Test.java:11)

3，ArrayIndexOutOfException

什么情况会出现: Thrown when an application attempts to use null in a case where an object is required.

About Exceptions in APCS JAVA