Nested try catch: Java exception handling
10:41:00
The try catch blocks can be nested. One
try-catch block can be present in the another try’s body. This is called Nesting of try catchblocks. Each time a
try block does not have a catch handler for a particular exception, the stack is unwound
and the next try block’s catch (i.e., parent try block’s catch) handlers are
inspected for a match.
If no catch
block matches, then the java run-time system will handle the exception. Lets see the syntax
first then we will discuss this with an example.
Syntax of Nested try Catch
....
//Main try block
try
{
statement 1;
statement 2;
//try-catch block inside another try block
try
{
statement 3;
statement 4;
}
catch(Exception e1)
{
//Exception Message
}
//try-catch block inside another try block
try
{
statement 5;
statement 6;
}
catch(Exception e2)
{
//Exception Message
}
}
catch(Exception e3) //Catch of Main(parent) try block
{
//Exception Message
}
....
Nested try catch example – explanation
class Nest{
public static void main(String args[]){
//Parent try block
try{
//Child try block1
try{
System.out.println("Inside block1");
int b =45/0;
System.out.println(b);
}
catch(ArithmeticException e1){
System.out.println("Exception: e1");
}
//Child try block2
try{
System.out.println("Inside block2");
int b =45/0;
System.out.println(b);
}
catch(ArrayIndexOutOfBoundsException e2){
System.out.println("Exception: e2");
}
System.out.println("Just other statement");
}
catch(ArithmeticException e3){
System.out.println("Arithmetic Exception");
System.out.println("Inside parent try catch block");
}
catch(ArrayIndexOutOfBoundsException e4){
System.out.println("ArrayIndexOutOfBoundsException");
System.out.println("Inside parent try catch block");
}
catch(Exception e5){
System.out.println("Exception");
System.out.println("Inside parent try catch block");
}
System.out.println("Next statement..");
}
}
Output:
Inside block1
Exception: e1
Inside block2
Arithmetic Exception
Inside parent try catch block
Next statement..
The above
example shows Nested try catch use in Java. You can see that there are two
try-catch block inside main try block’s body. I’ve marked them as
Block1: I have divided an integer by zero and it caused an arithmetic exception however the catch of block1 is handling arithmetic exception so
block 1 and block 2 in
above example.Block1: I have divided an integer by zero and it caused an arithmetic exception however the catch of block1 is handling arithmetic exception so
"Exception: e1" got printed.
Block2: In block2 also,
ArithmeticException occurred but block 2 catch is only
handling ArrayIndexOutOfBoundsException so in this case control jump back
to Main try-catch(parent) body. Since catch of parent try block is handling
this exception that’s why “Inside parent try catch block” got printed as
output.
Parent try Catch block: Since all the exception handled properly
so program control didn’t get terminated at any point and at last “Next
statement..” came as output.
Note: The main point to note here is that
whenever the child try-catch blocks are not handling any exception, the control
comes back to the parent try-catch if the exception is not handled there also
then the program will terminate abruptly.
Consider this example:
Here we have deep (two level) nesting which means we have a try-catch block inside a child try block. To make you understand better I have given the names to each try block in comments like try-block2 etc.
Here we have deep (two level) nesting which means we have a try-catch block inside a child try block. To make you understand better I have given the names to each try block in comments like try-block2 etc.
This is how the
structure is: try-block3 is inside try-block2 and try-block2 is inside
main try-block, you can say that the main try-block is a grand parent of the
try-block3. Refer the explanation which is given at the end of this code.
class NestingDemo{
public static void main(String args[]){
//main try-block
try{
//try-block2
try{
//try-block3
try{
int arr[]= {1,2,3,4};
/* I'm trying to display the value of
* an element which doesn't exist. The
* code should throw an exception
*/
System.out.println(arr[10]);
}catch(ArithmeticException e){
System.out.print("Arithmetic Exception");
System.out.println(" handled in try-block3");
}
}
catch(ArithmeticException e){
System.out.print("Arithmetic Exception");
System.out.println(" handled in try-block2");
}
}
catch(ArithmeticException e3){
System.out.print("Arithmetic Exception");
System.out.println(" handled in main try-block");
}
catch(ArrayIndexOutOfBoundsException e4){
System.out.print("ArrayIndexOutOfBoundsException");
System.out.println(" handled in main try-block");
}
catch(Exception e5){
System.out.print("Exception");
System.out.println(" handled in main try-block");
}
}
}
Output:
ArrayIndexOutOfBoundsException handled in main try-block
As you can see
that the
ArrayIndexOutOfBoundsException has occurred in the grand child
try-block3. Since try-block3 is not handling this exception, the control then
gets transferred to the parent try-block2 and looked for the catch handlers in
try-block2. Since the try-block2 is also not handling that exception, the
control got transferred to the main grand parent try-block where it found the
appropriate catch block for exception. This is how the routing
of exception is done in nested structure.
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