第八周动手动脑

Posted 2021-01-19 yishaui

1.请阅读并运行AboutException.java示例，然后通过后面的几页PPT了解Java中实现异常处理的基础知识。

import javax.swing.*;

class AboutException {
public static void main(String[] a)
{
int i=1, j=0, k;
k=i/j;

try
{

k = i/j; // Causes division-by-zero exception
//throw new Exception("Hello.Exception!");
}

catch ( ArithmeticException e)
{
System.out.println("被0除. "+ e.getMessage());
}

catch (Exception e)
{
if (e instanceof ArithmeticException)
System.out.println("被0除");
else
{
System.out.println(e.getMessage());

}
}

finally
{
JOptionPane.showConfirmDialog(null,"OK");
}

}
}

输出结果：Exception in thread "main" java.lang.ArithmeticException: / by zero
at ketangceshi.AboutException.main(AboutException.java:9)

结论：异常 （Exception）：发生于程序执行期间，表明出现了一个非法的运行状况。许多JDK中的方法在检测到非法情况时，都会抛出一个异常对象。

2.多层的异常捕获-1

 

public class CatchWho {
public static void main(String[] args) {
try {
try {
throw new ArrayIndexOutOfBoundsException();
}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println( "ArrayIndexOutOfBoundsException" + "/内层try-catch");
}

throw new ArithmeticException();
}
catch(ArithmeticException e) {
System.out.println("发生ArithmeticException");
}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println( "ArrayIndexOutOfBoundsException" + "/外层try-catch");
}
}
}

运行结果：

ArrayIndexOutOfBoundsException/内层try-catch

发生ArithmeticException

 

3.多层的异常捕获=2

public class CatchWho2 {
public static void main(String[] args) {
try {
try {
throw new ArrayIndexOutOfBoundsException();
}
catch(ArithmeticException e) {
System.out.println( "ArrayIndexOutOfBoundsException" + "/内层try-catch");
}
throw new ArithmeticException();
}
catch(ArithmeticException e) {
System.out.println("发生ArithmeticException");
}
catch(ArrayIndexOutOfBoundsException e) {
System.out.println( "ArrayIndexOutOfBoundsException" + "/外层try-catch");
}
}
}

运行结果：ArrayIndexOutOfBoundsException/外层try-catch

4.多个嵌套try,catcj,finally,finally的时机需要注意

public class EmbededFinally {

public static void main(String args[]) {

int result;

try {

System.out.println("in Level 1");

try {

System.out.println("in Level 2");
// result=100/0; //Level 2

try {

System.out.println("in Level 3");

result=100/0; //Level 3

}

catch (Exception e) {

System.out.println("Level 3:" + e.getClass().toString());

}


finally {

System.out.println("In Level 3 finally");

}


// result=100/0; //Level 2

}

catch (Exception e) {

System.out.println("Level 2:" + e.getClass().toString());

}
finally {

System.out.println("In Level 2 finally");

}

// result = 100 / 0; //level 1

}

catch (Exception e) {

System.out.println("Level 1:" + e.getClass().toString());

}

finally {

. System.out.println("In Level 1 finally");
}
}

}

运行结果：

in Level 1

in Level 2

in Level 3

Level 3:class java.lang.ArithmeticException

In Level 3 finally

In Level 2 finally

In Level 1 finally    

结论：当有多层嵌套的finally时，异常在不同的层次抛出，在不同的位置抛出，可能会导致不同的finally语句块执行顺序。

5.finally语句块一定会执行吗?

public class SystemExitAndFinally {

public static void main(String[] args)
{
try{

System.out.println("in main");
throw new Exception("Exception is thrown in main");

//System.exit(0);

}
catch(Exception e)

{
System.out.println(e.getMessage());
System.exit(0);
}
finally
{
System.out.println("in finally");
}
}

}

运行结果：

in main

Exception is thrown in main

结论：finally不一定会执行。

6.如何跟踪异常的传播路径？

// UsingExceptions.java
// Demonstrating the getMessage and printStackTrace
// methods inherited into all exception classes.

public class PrintExceptionStack {
public static void main( String args[] )
{
try {
method1();
}
catch ( Exception e ) {
System.err.println( e.getMessage() + " " );
e.printStackTrace();
}
}

public static void method1() throws Exception
{
method2();
}

public static void method2() throws Exception
{
method3();
}

public static void method3() throws Exception
{
throw new Exception( "Exception thrown in method3" );
}
}

1.当程序中出现异常时，JVM会依据方法调用顺序依次查找有关的错误处理程序。

2.可使用printStackTrace 和 getMessage方法了解异常发生的情况：

3.printStackTrace：打印方法调用堆栈。

4.每个Throwable类的对象都有一个getMessage方法，它返回一个字串，这个字串是在Exception构造函数中传入的，通常让这一字串包含特定异常的相关信息。