异常处理 ：

  自己写一个异常类，为啥有时候IO中的方法总是提示添加try catch 或者 throws Exception

 因为本省io操作的就 throws 了IOexception 如下图他就给了一个nullpointException

 

 

运行结果：（无图无真相）

 

 1.创建一个Myexception

package test20140903.testexception;

public class MyException extends Exception {

	public MyException() {
		super();
		// TODO Auto-generated constructor stub
	}

	public MyException(String message, Throwable cause) {
		super(message, cause);
		// TODO Auto-generated constructor stub
	}

	public MyException(String message) {
		System.out.println("Exception : " + message);
		
	}

	public MyException(Throwable cause) {
		super(cause);
	}
	
 
}

 

 

 

 2.写一个 计算的类

package test20140903.testexception;

public class Calculation {
	
	//饿汉式单例类
	private static  Calculation calc =new Calculation();
	private Calculation(){}
	public static Calculation  getInstance(){
		return calc;
	}
	
	//懒汉式单例类
//	private static  Calculation calc = null ;
//	private Calculation(){}
//	public static Calculation  getInstance(){
//		if(calc == null){
//			calc =new Calculation();
//		}
//		return calc;
//	}

	public double divide(double a ,double b) throws MyException {
		if(b == 0){
				throw new MyException("dividend cannot be 0");
		}
		return 1.0 *a /b;
	}
	
	public void add(double a ,double b) throws MyException{}
	public void minus(double a ,double b) throws MyException{}
	public void multiply(double a ,double b) throws MyException{}
	
}

 

 

 3.测试类

package test20140903.testexception;

public class ExceptionTest {
	
	public static void main(String[] args) throws MyException {
		double a =  Calculation.getInstance().divide(1, 0);
		System.out.println(a);
	}
	
}