Java异常处理深入理解

public class ExceptionTest{
    //NullPointerException
    @Test
    public void test1(){
        int[] arr = null;
        System.out.println(arr[3]);
        str = null;
        System.out.println(str.charAt(0));
    }
    //IndexOutOfBoundsException
    @Test
    public void test2(){
        //ArrayIndexOutOfBoundsException
        int[] arr = new int[10];
        System.out.println(arr[10]);
        //StringIndexOutOfBoundsException
        str = "abc";
        System.out.println(str.charAt(3));
    }
    //ClassCastException：类型不匹配
    @Test
    public void test3(){
        Object obj = new Date();
        String str = (String)obj;
    }
    //NumberFormatException:数据格式异常
    @Test
    public void test4(){
        str = "abc";
        int num = Integer.parseInt(str);
    }
    //InputMismatchException:输入不匹配异常
    @Test
    public void test5(){
        Scanner scanner = new Scanner(System.in);
        int score = scanner.nextInt();
        System.out.println(score);
    }
    //ArithmeticException:算数异常
    @Test
    public void test6(){
        int a = 10;
        int b= 0;
        System.out.println(a\b);
    }
}

try{
    //可能出现异常的代码
}catch(异常类型1 变量名1){//一段代码可能有多个异常类型
    //处理异常的方式1
}catch(异常类型2 变量名2){
    //处理异常的方式2
}catch(异常类型3 变量名3){
    //处理异常的方式3
}
.....
finally{
    //一定会执行的代码
}

    @Test
	public void test1() {
		str = "abc";
		int num = 0;// 声明放在外面，出了try,还可以使用num;
        //但是此时要注意：在try-catch结构中，num不一定被赋值，所以要手动的给赋默认初始化值
		try {
			num = Integer.parseInt(str);
		} catch (NullPointerException e) {
			System.out.println(e.getMessage());
		} catch (NumberFormatException e) {
			e.printStackTrace();
		}
		System.out.println(num);
	}

@Test//catch中又出现异常了
public void test1(){
    try{
        int a = 10;
        int b = 0;
        System.out.println(a / b);
    }catch(ArithmeticException e){
        int[] arr = new int[10];
        System.out.println(arr[10]);
    }
        //System.out.println("catch中又出现异常了，但我一定不会输出！");
    finally{
        System.out.println("catch中又出现异常了，但我一定会输出！");
    }
}

@Test//try中有return语句、catch中有return语句
public void testMethod(){
    int num = method();
}
public int method(){
    try{
        int[] arr = new int[10];
        System.out.println("arr[10]");
        return 1;
    }catch(ArrayIndexOutOfBoundException e){
        e.printStockTrace();
        return 2;
    }finally{
        System.out.println("我一定会被执行！");
        return 3;
    }
}
//finally中一定会执行的结构在执行顺序上优于try、catch中的return语句
//由于方法只能有一个返回值，所以最后返回的是return 3;

public class ExceptionTest2 {
	public static void main(String[] args) {
		try {
			method2();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		method3();// 在method2()里已经解决了异常，所以可以调用
	}
	public static void method3() {
		try {
			method2();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	public static void method2() throws FileNotFoundException, IOException {
		method1();
	}
	public static void method1() throws FileNotFoundException, IOException {//异常代码
		File file = new File("hello.txt");
		FileInputStream fis = new FileInputStream(file);
		int data = fis.read();
		while (data != -1) {
			System.out.print((char) data);
			data = fis.read();
		}
		fis.close();
		System.out.println("hanhan");// 不会被执行！
	}
}

public class StudentTest {
	public static void main(String[] args) {
		Student s = new Student();
		s.regist(-1001);
		System.out.println(s);
	}
}
class Student {
	private int id;
	public void regist(int id) {
		if (id > 0) {
			this.id = id;
		} else {//方式一：抛出运行时异常，运行时会报错
			// 手动抛出异常对象
			throw new RuntimeException("您输入的数据非法！");
		}
	}
}

public class StudentTest {
	public static void main(String[] args) {
		try {
			Student s = new Student();
			s.regist(-1001);
			System.out.println(s);
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
	}
}
class Student {
	private int id;
	public void regist(int id) throws Exception {//throws:体现异常的处理
		if (id > 0) {
			this.id = id;
		} else {//抛出编译时异常：必须显式的处理
			throw new Exception("您输入的数据非法！");//throw:体现生成一个异常对象
		}
	}
}

public class StudentTest {
	public static void main(String[] args) {
		try {
			Student s = new Student();
			s.regist(-1001);
			System.out.println(s);
		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
	}
}
class Student {
	private int id;
	public void regist(int id) throws Exception {
		if (id > 0) {
			this.id = id;
		} else {
			throw new MyException("不能输入负数");//自定义异常类的使用
		}
	}
}
public class MyException extends RuntimeException {//自定义异常类
	static final long serialVersionUID = -7034897190745766939L;
	public MyException() {
	}
	public MyException(String msg) {
		super(msg);
	}
}