轉載自:http://qing0991.blog.51cto.com/1640542/1387200
看到一篇關於try,catch,finally,return的面試題就順便看了下。
1、try塊中沒有拋出異常,try、catch和finally塊中都有return語句
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;
}
}
運行代碼:
public static void main(String[] args) {
System.out.println("=============NoException==================");
System.out.println(NoException());
System.out.println("===============================");
}
運行結果:
=============NoException==================
i in try block is:10
i in finally - from try or catch block is:9
8
===============================
執行順序: 執行try塊,執行到return語句時,先執行return的語句,–i,但是不返回到main方法,執行finally塊,遇到finally塊中的return語句,執行–i,並將值返回到main方法,這裏就不會再回去返回try塊中計算得到的值。
結論: try-catch-finally都有return語句時,沒有異常時,返回值是finally中的return返回的。
2.try塊中沒有拋出異常,僅try和catch中有return語句
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;
}
}
運行結果:
=============NoException1==================
i in try block is:10
i in finally - from try or catch block is:9
i in finally block is:8
9
===============================
執行順序: try中執行完return的語句後,不返回,執行finally塊,finally塊執行結束後,返回到try塊中,返回i在try塊中最後的值。
結論: try-catch都有return語句時,沒有異常時,返回值是try中的return返回的。
3.try塊中拋出異常,try、catch和finally中都有return語句
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;
}
}
執行結果:
=============WithException==================
i in try block is:10
i in catch - form try block is:10
i in catch block is:9
i in finally - from try or catch block is--8
i in finally block is--7
6
===============================
執行順序: 拋出異常後,執行catch塊,在catch塊的return的–i執行完後,並不直接返回而是執行finally,因finally中有return語句,所以,執行,返回結果6。
結論: try塊中拋出異常,try、catch和finally中都有return語句,返回值是finally中的return。
4.try塊中拋出異常,try和catch中都有return語句
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;
}
}
執行結果:
=============WithException1==================
i in try block is:10
i in catch - form try block is:10
i in finally - from try or catch block is:9
i in finally block is:8
9
===============================
執行順序: 拋出異常後,執行catch塊,執行完finally語句後,依舊返回catch中的執行return語句後的值,而不是finally中修改的值。
結論: 返回的catch中return值。
5.try、catch中都出現異常,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;
}
執行結果:
=============WithException2==================
i in try block is:10
i in catch - form try block is:10
i in finally - from try or catch block is:10
i in finally block is:8
7
===============================
執行順序: try塊中出現異常到catch,catch中出現異常到finally,finally中執行到return語句返回,不檢查異常。
結論: 返回finally中return值。
6、只在函數最後出現return語句
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;
}
執行結果:
=============WithException3==================
i in try block is:10
i in catch - form try block is:10
i in finally - from try or catch block is:10
i in finally block is:8
7
===============================
總體結論:
結論一:
return語句並不是函數的最終出口,如果有finally語句,這在return之後還會執行finally(return的值會暫存在棧裏面,等待finally執行後再返回)
結論二:
finally裏面不建議放return語句,根據需要,return語句可以放在try和catch裏面和函數的最後。
可行的做法有四:
(1)return語句只在函數最後出現一次。
(2)return語句僅在try和catch裏面都出現。
(3)return語句僅在try和函數的最後都出現。
(4)return語句僅在catch和函數的最後都出現。
注意,除此之外的其他做法都是不可行的,編譯器會報錯。