THINKING IN JAVA 上的一道关于异常处理的题目

[aizhongwang83]

class AbortedConstruction extends Exception {
public AbortedConstruction() {
super("Construction aborted");
}
}

class WithCleanup {
private boolean constructed = false;
public WithCleanup(boolean abort)
throws AbortedConstruction {
// Perform construction that might be
// unsucessful (and throw an exception) here.
if(abort) throw new AbortedConstruction();
System.out.println("After exception");
constructed = true;
}
public void cleanup() {
System.out.println(
"constructed = " + constructed);
if(constructed == true)
System.out.println("Cleaning up");
else
System.out.println(
"Constructor didn't finish,"+
"not cleaning up");
}
}
public class E14_CleanupFlag {
public static void main(String args[]) {
WithCleanup wc = null;
try {
wc = new WithCleanup(false);
} catch(AbortedConstruction e) {
System.out.println("Caught " + e);
} finally {
System.out.println(
"In finally 1, preparing to clean up");
wc.cleanup();
}
wc = null; // Very important!
try {
try {
wc = new WithCleanup(true);
} catch(AbortedConstruction e) {
System.out.println("Caught " + e);
} finally {
System.out.println(
"In finally 2, preparing to clean up");
wc.cleanup();
}
} catch(Exception e) {
System.out.println("Caught exception "+ e);
}
}
} ///:~
结果：
After exception
In finally 1, preparing to clean up
constructed = true
Cleaning up
Caught AbortedConstruction: Construction aborted
In finally 2, preparing to clean up
Caught exception java.lang.NullPointerException

我想知道Caught exception java.lang.NullPointerException怎么产生的，大家帮帮忙

[li_d_s]

wc = new WithCleanup(true);
这里，在执行new WithCleanup的时候出现异常就跑到catch里面去了wc根本没有被初始化，还是null，然后到了finally,wc.cleanup()当然就抛出空指针异常。

[anqini]

wc.cleanup();最后一个这里出这个异常，因为wc = new WithCleanup(true);之后，构造方法里面出异常了，所以没有实例化其类，所以wc是个null,当你调用null.cleanup();方法的时候出这个异常

[zbo]

wc=null,
在wc=new WithCleanup(true)中，首先计算等式右边的值，再赋给wc,
在计算值的过程中抛出异常，程序跳转到finally段。
执行wc.cleanup()，因为wc是null,所以抛出空指针异常啊。

[ty8888]

帮上面的楼主顶上去~~~~~~~~！

[胡矣]

wc = new WithCleanup(false);
这局的时候出现AbortedConstruction 异常.
所以没有生成新对象.wc为null.
运行到下面wc.cleanup(); 的时候出现java.lang.NullPointerException