java中异常的分类

java中异常的分类

一个图，五个关键字

try、catch、finally、throw、throws

TestExceptions.java

//异常
public class TestExceptions {
	public static void main(String[] args) {
		TestExceptions te = new TestExceptions();
		te.m(0);
	}
	//throws抛出ArithmeticException
	void m(int i) throws ArithmeticException {
		if(i==0) { 
			//定义ArithmeticException的消息
			throw new ArithmeticException("被除数为0");
		}
	}
}

F:\java>javac TestExceptions.java

F:\java>java TestExceptions
Exception in thread "main" java.lang.ArithmeticException: 被除数为0
        at TestExceptions.m(TestExceptions.java:11)
        at TestExceptions.main(TestExceptions.java:5)

F:\java>

使用try...catch捕捉异常

TestExceptions.java

//异常
public class TestExceptions {
	public static void main(String[] args) {
		try {
			new TestExceptions().m(0);
		} catch (ArithmeticException ae) {
			ae.printStackTrace();
			System.out.println("出错了");
		}
	}
	//throws抛出ArithmeticException
	void m(int i) throws ArithmeticException {
		if(i==0) { 
			//定义ArithmeticException的消息
			throw new ArithmeticException("被除数为0");
		}
	}
}

F:\java>javac TestExceptions.java

F:\java>java TestExceptions
java.lang.ArithmeticException: 被除数为0
        at TestExceptions.m(TestExceptions.java:15)
        at TestExceptions.main(TestExceptions.java:5)
出错了

F:\java>

必须要捕捉的异常

TestExceptions.java

import java.io.*;

//异常
public class TestExceptions {
	public static void main(String[] args) {
		//io包下的一个类
		FileInputStream in = null;
		//一个try可以跟多个catch
		try {
			//必须要捕捉FileNotFoundException异常
			in = new FileInputStream("myfile.txt");
			int b;
			b = in.read();
			while (b != -1) {
				System.out.print((char) b);
				b = in.read();
			}
		} catch (FileNotFoundException e) {
			System.out.println(e.getMessage());

		} catch (IOException e) {
			e.printStackTrace(); 
		} finally {
			try {
				in.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

 myfile.txt存在时

F:\java>javac TestExceptions.java

F:\java>java TestExceptions
a
ab
abc
abcd
F:\java>

  myfile.txt不存在时
 

F:\java>javac TestExceptions.java

F:\java>java TestExceptions
myfile.txt (系统找不到指定的文件。)
Exception in thread "main" java.lang.NullPointerException
        at TestExceptions.main(TestExceptions.java:25)

F:\java>

myfile.txt

a
ab
abc
abcd