Java中try-catch-finally-return的执行顺序
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转载自:http://qing0991.blog.51cto.com/1640542/1387200
看到一篇关于try,catch,finally,return的面试题就顺便看了下。
1、try块中没有抛出异常,try、catch和finally块中都有return语句
public static int NoException(){ int i=10; try{ System.out.println("i in try block is:"+i); return --i; } catch(Exception e){ --i; System.out.println("i in catch - form try block is:"+i); return --i; } finally{ System.out.println("i in finally - from try or catch block is:"+i); return --i; } }
运行代码:
public static void main(String[] args) { System.out.println("=============NoException=================="); System.out.println(NoException()); System.out.println("==============================="); }
运行结果:
=============NoException==================i in try block is:10i in finally - from try or catch block is:98===============================
执行顺序: 执行try块,执行到return语句时,先执行return的语句,–i,但是不返回到main方法,执行finally块,遇到finally块中的return语句,执行–i,并将值返回到main方法,这里就不会再回去返回try块中计算得到的值。
结论: try-catch-finally都有return语句时,没有异常时,返回值是finally中的return返回的。
2.try块中没有抛出异常,仅try和catch中有return语句
public static int NoException1(){ int i=10; try{ System.out.println("i in try block is:"+i); return --i; } catch(Exception e){ --i; System.out.println("i in catch - form try block is:"+i); return --i; } finally{ System.out.println("i in finally - from try or catch block is:"+i); --i; System.out.println("i in finally block is:"+i); //return --i; }}
运行结果:
=============NoException1==================i in try block is:10i in finally - from try or catch block is:9i in finally block is:89===============================
执行顺序: try中执行完return的语句后,不返回,执行finally块,finally块执行结束后,返回到try块中,返回i在try块中最后的值。
结论: try-catch都有return语句时,没有异常时,返回值是try中的return返回的。
3.try块中抛出异常,try、catch和finally中都有return语句
public static int WithException(){ int i=10; try{ System.out.println("i in try block is:"+i); i = i/0; return --i; } catch(Exception e){ System.out.println("i in catch - form try block is:"+i); --i; System.out.println("i in catch block is:"+i); return --i; } finally{ System.out.println("i in finally - from try or catch block is--"+i); --i; System.out.println("i in finally block is--"+i); return --i; }}
执行结果:
=============WithException==================i in try block is:10i in catch - form try block is:10i in catch block is:9i in finally - from try or catch block is--8i in finally block is--76===============================
执行顺序: 抛出异常后,执行catch块,在catch块的return的–i执行完后,并不直接返回而是执行finally,因finally中有return语句,所以,执行,返回结果6。
结论: try块中抛出异常,try、catch和finally中都有return语句,返回值是finally中的return。
4.try块中抛出异常,try和catch中都有return语句
public static int WithException1(){ int i=10; try{ System.out.println("i in try block is:"+i); i=i/0; return --i; }catch(Exception e){ System.out.println("i in catch - form try block is:"+i); return --i; }finally{ System.out.println("i in finally - from try or catch block is:"+i); --i; System.out.println("i in finally block is:"+i); //return i; }}
执行结果:
=============WithException1==================i in try block is:10i in catch - form try block is:10i in finally - from try or catch block is:9i in finally block is:89===============================
执行顺序: 抛出异常后,执行catch块,执行完finally语句后,依旧返回catch中的执行return语句后的值,而不是finally中修改的值。
结论: 返回的catch中return值。
5.try、catch中都出现异常,在finally中有返回
public static int WithException2(){ int i=10; try{ System.out.println("i in try block is:"+i); i=i/0; return --i; } catch(Exception e){ System.out.println("i in catch - form try block is:"+i); int j = i/0; return --i; } finally{ System.out.println("i in finally - from try or catch block is:"+i); --i; --i; System.out.println("i in finally block is:"+i); return --i;}
执行结果:
=============WithException2==================i in try block is:10i in catch - form try block is:10i in finally - from try or catch block is:10i in finally block is:87===============================
执行顺序: try块中出现异常到catch,catch中出现异常到finally,finally中执行到return语句返回,不检查异常。
结论: 返回finally中return值。
6、只在函数最后出现return语句
public static int WithException3(){ int i=10; try{ System.out.println("i in try block is:"+i); i=i/0; //return --i; } catch(Exception e){ System.out.println("i in catch - form try block is:"+i); //int j = i/0; //return --i; } finally{ System.out.println("i in finally - from try or catch block is:"+i); --i; --i; System.out.println("i in finally block is:"+i); //return --i; } return --i;}
执行结果:
=============WithException3==================i in try block is:10i in catch - form try block is:10i in finally - from try or catch block is:10i in finally block is:87===============================
总体结论:
结论一:
return语句并不是函数的最终出口,如果有finally语句,这在return之后还会执行finally(return的值会暂存在栈里面,等待finally执行后再返回)
结论二:
finally里面不建议放return语句,根据需要,return语句可以放在try和catch里面和函数的最后。
可行的做法有四:
(1)return语句只在函数最后出现一次。
(2)return语句仅在try和catch里面都出现。
(3)return语句仅在try和函数的最后都出现。
(4)return语句仅在catch和函数的最后都出现。
注意,除此之外的其他做法都是不可行的,编译器会报错。
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