關於try-catch-finally-return語句的執行順序
直接貼程式碼
main函式
public static void main(String[] args) {
System.out.println("=============NoException==================");
System.out.println(noException());
System.out.println("===============================");
System.out.println("=============NoException1==================" );
System.out.println(noException1());
System.out.println("===============================");
System.out.println("=============WithException==================");
System.out.println(WithException());
System.out.println("===============================");
System.out .println("=============WithException1==================");
System.out.println(WithException1());
System.out.println("===============================");
System.out.println("=============WithException2==================");
System.out.println(WithException2());
System.out .println("===============================");
System.out.println("=============WithException3==================");
System.out.println(WithException3());
System.out.println("===============================");
}
1.try中沒有異常,try,catch,finally中都有return語句
測試程式碼:
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 block is :" + i);
return --i;
} finally {
System.out.println("i in finally - from try or catch block is:" + i);
return --i;
}
}
執行結果:
=============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,這裡就不會再回去返回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中沒有異常丟擲,僅try和catch中有return
測試程式碼:
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;
}
}
執行結果:
=============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塊中丟擲異常,try、catch和finally中都有return語句
測試程式碼:
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:
丟擲異常後,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塊中丟擲異常,try、catch和finally中都有return語句,返回值是finally中的return。
4.try塊中丟擲異常,try和catch中都有return語句
測試程式碼:
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:
丟擲異常後,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
測試程式碼:
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、只在函式最後出現return語句
測試程式碼:
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
===============================
總結:
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。
結論一:
return語句並不是函式的最終出口,如果有finally語句,這在return之後還會執行finally
(return的值會暫存在棧裡面,等待finally執行後再返回)
結論二:
finally裡面不建議放return語句,根據需要,return語句可以放在try和catch裡面和函式的最後。可行的做法有四:
(1)return語句只在函式最後出現一次。
(2)return語句僅在try和catch裡面都出現。
(3)return語句僅在try和函式的最後都出現。
(4)return語句僅在catch和函式的最後都出現。
注意,除此之外的其他做法都是不可行的,編譯器會報錯。
2017.2.21
原來以為碰到這種型別的題目,再也不會錯,但是碰到了一下的“套中套” 。。 。 防不勝防
public class ZeroTest {
public static void main(String[] args) {
try{
int i = 100 / 0;
System.out.print(i);
}catch(Exception e){
System.out.print(1);
throw new RuntimeException();
}finally{
System.out.print(2);
}
System.out.print(3);
}
}
這段程式的輸出為12
‘3’是不會輸出的,因為在try
中捕捉到異常後,程式跳進catch
,會執行throw newRuntimeException()
,這個時候,程式出了會執行finally
中的程式碼,其他位置處的程式碼都不會執行。