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学习java的经典书籍

</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
like this only if you didn&#8217;t know when the terminating condition was
going to occur. The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement causes execution to go back to the top of the iteration loop (thus
incrementing 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">)
whenever 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is not evenly divisible by 9. When it is, the value is printed.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
second portion shows an &#8220;infinite loop&#8221; that would, in theory,
continue forever. However, inside the loop there is a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
statement that will break out of the loop. In addition, you&#8217;ll see that
the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
moves back to the top of the loop without completing the remainder. (Thus
printing happens only when the value of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>i</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is divisible by 9.) The output is:
</FONT><P></DIV>

<font color="#990000"><PRE>0
9
18
27
36
45
54
63
72
10
20
30
40</PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
value 0 is printed because 0 % 9 produces 0.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
second form of the infinite loop is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>for(;;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
The compiler treats both 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>while(true)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>for(;;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in the same way so whichever one you use is a matter of programming taste.
</FONT><P></DIV>
<A NAME="Heading132"></A><H4 ALIGN=LEFT>
The
infamous &#8220;goto&#8221;
</H4>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
<A NAME="Index252"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">keyword
has been present in programming languages from the beginning. Indeed, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
was the genesis of program control in assembly language: &#8220;if condition A,
then jump here, otherwise jump there.&#8221; If you read the assembly code that
is ultimately generated by virtually any compiler, you&#8217;ll see that
program control contains many jumps. However, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">jumps
at the source-code level, and that&#8217;s what brought it into disrepute. If a
program will always jump from one point to another, isn&#8217;t there some way
to reorganize the code so the flow of control is not so jumpy? 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">fell
into true disfavor with the publication of the famous &#8220;Goto considered
harmful&#8221; paper by Edsger Dijkstra, and since then goto-bashing has been a
popular sport, with advocates of the cast-out keyword scurrying for cover.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
is typical in situations like this, the middle ground is the most fruitful. The
problem is not the use of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
but the overuse of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and in rare situations 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is the best way to structure control flow.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Although
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">is
a reserved word in Java, it is not used in the language; Java has no 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
However, it does have something that looks a bit like a jump tied in with the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keywords. It&#8217;s not a jump but rather a way to break out of an iteration
statement. 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
reason it&#8217;s often thrown in with discussions of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>goto</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is because it uses the same mechanism: a label.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">A
<A NAME="Index253"></A>label
is an identifier followed by a colon, like this:
</FONT><P></DIV><DIV ALIGN=LEFT><TT><FONT FACE="Courier New" SIZE=3 COLOR="Black">label1:</FONT></TT><P></DIV><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>only</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
place a label is useful in Java is right before an iteration statement. And
that means 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>right</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
before &#8211; it does no good to put any other statement between the label and
the iteration. And the sole reason to put a label before an iteration is if
you&#8217;re going to nest another iteration or a switch inside it.
That&#8217;s because the <A NAME="Index254"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index255"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
keywords will normally interrupt only the current loop, but when used with a
label they&#8217;ll interrupt the loops up to where the label exists:
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>label1:
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>outer-iteration</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>{</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
 
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>inner-iteration</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>{</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   //...
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   break; // 1
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   //...
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   continue;  // 2
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   //...
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   continue label1; // 3
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   //...
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
   break label1;  // 4
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>
 }
</B></FONT><P><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>}</B></FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">In
case 1, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
breaks out of the inner iteration and you end up in the outer iteration. In
case 2, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
moves back to the beginning of the inner iteration. But in case 3, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>continue
label1
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
breaks out of the inner iteration 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>and</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
the outer iteration, all the way back to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>label1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Then it does in fact continue the iteration, but starting at the outer
iteration. In case 4, the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>break
label1
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
also breaks all the way out to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>label1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
but it does not re-enter the iteration. It actually does break out of both
iterations.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Here
is an example using 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>for</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
loops:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: LabeledFor.java</font>
<font color="#009900">// Java&#8217;s "labeled for loop"</font>

<font color="#0000ff">public</font> <font color="#0000ff">class</font> LabeledFor {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">int</font> i = 0;
    outer: <font color="#009900">// Can't have statements here</font>
    <font color="#0000ff">for</font>(; <font color="#0000ff">true</font> ;) { <font color="#009900">// infinite loop</font>
      inner: <font color="#009900">// Can't have statements here</font>
      <font color="#0000ff">for</font>(; i &lt; 10; i++) {
        prt("i = " + i);
        <font color="#0000ff">if</font>(i == 2) {
          prt("<font color="#0000ff">continue</font>");
          <font color="#0000ff">continue</font>;
        }
        <font color="#0000ff">if</font>(i == 3) {
          prt("<font color="#0000ff">break</font>");
          i++; <font color="#009900">// Otherwise i never</font>
               <font color="#009900">// gets incremented.</font>
          <font color="#0000ff">break</font>;
        }
        <font color="#0000ff">if</font>(i == 7) {
          prt("<font color="#0000ff">continue</font> outer");
          i++; <font color="#009900">// Otherwise i never</font>
               <font color="#009900">// gets incremented.</font>
          <font color="#0000ff">continue</font> outer;
        }
        <font color="#0000ff">if</font>(i == 8) {
          prt("<font color="#0000ff">break</font> outer");
          <font color="#0000ff">break</font> outer;
        }
        <font color="#0000ff">for</font>(<font color="#0000ff">int</font> k = 0; k &lt; 5; k++) {
          <font color="#0000ff">if</font>(k == 3) {
            prt("<font color="#0000ff">continue</font> inner");
            <font color="#0000ff">continue</font> inner;
          }
        }
      }
    }
    <font color="#009900">// Can't break or continue</font>