千锋20200309

在千锋“逆战”学习第26天

      每日一句：其实你找不到错误不代表错误不存在，同样你看不到技术比你牛的人并不代表世界上没有技术比你牛的人。
      今天为Map集合收尾，主要讲异常的知识
      明天继续加油。

作业

1.填空

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2.查询API，填空

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3.下列代码补充完整

public class TestThrow {
	public static void main(String[] args) {
		throwException(10);
	}

	public static void throwException(int n) {
		if (n == 0) {
			// 抛出一个NullPointerException
			throw new NullPointerException();
		} else {
			// 抛出一个ClassCastException
			// 并设定详细信息为“类型转换出错”
			throw new ClassCastException("类型转换出错");
		}
	}
}

运行结果：

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4.有如下代码

public class TestException {
	public static void main(String[] args) {
		System.out.println("main 1");
		int n;
		// 读入n
		Scanner input = new Scanner(System.in);
		System.out.println("输入n的值：");
		n = input.nextInt();
		ma(n);
		System.out.println("main 2");
	}

	public static void ma(int n) {
		try {
			System.out.println("ma1");
			mb(n);
			System.out.println("ma2");
		} catch (EOFException e) {
			System.out.println("Catch EOFException");
		} catch (IOException e) {
			System.out.println("Catch IOException");
		} catch (SQLException e) {
			System.out.println("Catch SQLException");
		} catch (Exception e) {
			System.out.println("Catch Exception");
		} finally {
			System.out.println("in finally");
		}
	}

	public static void mb(int n) throws Exception {
		System.out.println("mb1");
		if (n == 1)
			throw new EOFException();
		if (n == 2)
			throw new IOException();
		if (n == 3)
			throw new SQLException();
		if (n == 4)
			throw new NullPointerException();
		System.out.println("mb2");
	}
}

当读入n分别为1、2、3、4、5时，输出结果是什么

main 1
输入n的值：
1
ma1
mb1
Catch EOFException
in finally
main 2

main 1
输入n的值：
2
ma1
mb1
Catch IOException
in finally
main 2

main 1
输入n的值：
3
ma1
mb1
Catch SQLException
in finally
main 2

main 1
输入n的值：
4
ma1
mb1
Catch Exception
in finally
main 2

main 1
输入n的值：
5
ma1
mb1
mb2
ma2
in finally
main 2

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7.代码改错

class MyException{}
public class TestException {
	public static void main(String[] args) {
		ma();
	}
	public static int ma() {
		try {
			m();
			return 100;
		} catch (Exception e) {
			System.out.println("Exception");
		}catch (ArithmeticException e) {
			System.out.println("ArithmeticException");
		}
	}
	public static void m() {
		throw new MyException();
	}
}

1.先catch ArithmeticException然后catch Exception
2.m方法只能抛出Throwable类下的异常

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9.有如下代码：

public class TestTryCatch {
	public static void main(String[] args) {
		System.out.println(ma());
	}
	public static int ma() {
		int n ;
		try {
			n=10/0;
		} catch (Exception e) {
		}
		return n;
	}
}

运行结果：

A.编译不通过。try执行不到的时候，n将未初始化，编译不通过

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10.有如下代码：

public class TestFinally {
	public static void main(String[] args) {
		System.out.println(ma());
	}

	public static int ma() {
		int b;
		Scanner sc = new Scanner(System.in);
		System.out.println("输入b");
		b = sc.nextInt();
		try {
			int n = 100;
			return n / b;
		} catch (Exception e) {
			return 10;
		} finally {
			return 100;
		}
	}
}

运行结果：

当读入b为100时，输出结果为：100
当读入b为0时，输出结果为：100

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11.有如下代码：

public class TestTryFinally {
	public static void main(String[] args) {
		try {
			ma();
		} catch (Exception ex1) {
		}
	}

	public static void ma() throws Exception {
		int n = 10;
		int b;
		Scanner sc = new Scanner(System.in);
		System.out.println("输入b");
		b = sc.nextInt();
		try {
			System.out.println("ma1");
			int result = n / b;
			System.out.println("ma2 " + result);
		} finally {
			System.out.println("In Finally");
		}
	}
}

运行结果：

当读入b为10时，输出结果为：
ma1
ma2 1
In Finally
当读入b为0时，输出结果为：
ma1
In Finally

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13.有如下代码：

public class TestException {
	public static void main(String[] args) {
		try {
			System.out.println("main1");
			ma();
			System.out.println("main2");
		} catch (Exception e) {
			System.out.println("In Catch");
		}
	}
	public static void ma() {
		System.out.println("ma1");
		throw new NullPointerException();
		System.out.println("ma2");
	}
}

运行结果:

编译出错，抛出异常后面的输出语句执行不到

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14.

public class TestException {
	public static void main(String[] args) {
		try {
			ma();
		} 
		/*1*/
		catch (Exception e) {
		}
	}
	public static void ma() throws IOException{}
}

下列哪些代码放在/1/处可以编译通过

catch(NullPointerException npe) {}//可以
catch(IOException ioe) {}//可以
catch(SQLException sqle) {}//不可以，无法访问SQLException的catch块。此异常从不从try语句体引发