初学Exceptions

异常的第一种用法是Java比C++增加的一种使用方法，但是其抛出异常的方法一般只有直接抛出到method体之外，不能用try catch（原因不明确，可能是因为try catch l里的variables都只能在curly brakets 里使用）；如果要将读入文件单独成为一个method的话，抛出的异常需要两次，先抛出到mathod之外，再抛出到main之外。
package myexception;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class Test2 {
public static void main(String[] args) throws FileNotFoundException {

    String fileName = new String("hello.txt");

    File textFile = new File(fileName);

    throwEx(textFile);

}

public static void throwEx(File file) throws FileNotFoundException  {
    Scanner in = new Scanner(file);
    int a = in.nextInt();
    System.out.println(a);

    while (in.hasNext()) {
        System.out.println(in.next());
    }

    in.close();

}

}

抛出异常的传统作用是继承C++的，是终止进程的方法，在不同的Java文件中的异常最终都要在main里再次抛出

import java.io.IOException;
import java.util.Scanner;

public class App {

public static void main(String[] args) {
    Test test = new Test();
    Scanner in = new Scanner(System.in);
    try {
        test.run(in.nextBoolean());
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}

}

/
import java.io.IOException;

public class Test {
public void run(boolean a) throws IOException {
/// a is false means this field is wrong , the method need to be
/// terminated by FIleException
if (a)
throw new FIleException(“a is true”);
/// a is false means this field is wrong , the method need to be
/// terminated by IOException
if (!a)
throw new IOException();
/// a is false means this field is wrong , the method need to be
/// terminated by FIleException
if (a)
throw new FIleException(“a is true”);
}

}
///
import java.io.IOException;

public class FIleException extends IOException {

public FIleException(String message) {
    super(message);

}

}

可以在一个method里抛出多个异常，则此时再main里抛出异常会有三种方法；如果要抛出子类和父类的异常则只需要抛出父类的异常即可，这是因为多态的性质（多态，其实是一种downcasting,而upcasting则是危险的）

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