Returning value from method having try-catch-finally blocks

In this article, we will discuss valid & invalid scenarios for returning a value when method is enclosed with try-catch-finally blocks

General rule:

Whenever return-type of method is defined in a method signature, then compulsorily that method has to return value of that type. Like,

• If return-type is defined as “int” data-type, then respective method has to return a value of “int” data-type
• If return-type is defined as object of some class is defined, then corresponding reference-type has to be returned from that method
• Otherwise, compile-time error will be thrown stating “This method must return a result of type <data-type>”

Solution:

• To fix this compile-time error –> provide return statement with some value

ReturnValueExample.java

package in.bench.resources.exception.handling;

public class ReturnValueExample {

	public static void main(String[] args) {
		calculate();
	}

	public static int calculate() {
		return 0;
	}
}

Q) Whatever we discussed till now is for simple case, but what if method is enclosed with try-catch-finally block ?

• In the above example, there is no possibility of raising any exception as it contains just single statement for returning value from method
• But if method contains a code which possibly might raise exception during execution, then we need to surround that code with try-catch block for exception handling
• Also, if that method returns a value, then we need to find a way to return value from that particular method involving try-catch block
• For example, consider below program

ReturnValueExample.java

package in.bench.resources.exception.handling;

public class ReturnValueExample {

	public static void main(String[] args) {

		// invoking static method
		returnValueFromMethod();
	}

	public static int returnValueFromMethod() {

		int result = 18/0;
		return result;
	}
}

Output:

Exception in thread "main" java.lang.ArithmeticException:/ by zero
	at in.bench.resources.exception.handling.ReturnValueExample
	.returnValueFromMethod(ReturnValueExample.java:13)
	at in.bench.resources.exception.handling.ReturnValueExample
	.main(ReturnValueExample.java:8)

Explanation:

• Above method throws exception during execution
• and terminates program abnormally
• because there is no try-catch block for exception handling
• Therefore, program execute unsuccessfully by throwing exception & terminates abnormally

 

try-catch block:

• Let us go for the improved version with try-catch-finally blocks,

Problem:

• But here the problem is, from where we need to return value
• In previous program, we had only one loop that is method loop
• i.e.; curly braces start right after method declaration and ends after some statements

Loops in above program:

Here, in the above example as shown in the screen-capture, we have 4 loops i.e.;

1. method loop
2. try-block loop
3. catch-block loop
4. finally-block loop

Q) Again, from where we need to return value for this method ?

• There are numerous cases to return value for valid scenarios, let’s go for each case with an example

 

Valid Cases:

Case 1: write return statement after completion of try-catch-finally blocks; that is just before end of method

Reason:

• This is valid case because after try-catch-finally block execution, method returns value
• Also, it can be seen as 2 independent entities with,
• 1^st being try-catch-finally block
• 2^nd is return statement after try-catch-finally block

 

Case 2: write return statement inside finally-block only; but there shouldn’t be any statement after finally-block

Reason:

• whether any exception is raised or NOT from try-block
• and its corresponding exception is being handled or NOT in the catch-block
• finally-block will always be executed irrespective of the program’s outcome
• except in one scenario when System.exit(0); is invoked explicitly
• Error-scenario : any statement after finally block will result in compile-time error stating “Unreachable code”

 

Case 3: write return statement inside both try-block & catch-block; there is no compulsion to write return inside finally-block and this completely optional (Case 6)

Reason:

• Whenever try-block executes successfully, then it can return value for this method
• Also, if any exception is raised from try-block then its corresponding exception will be caught in the catch-block
• And from catch-block also, it can return value for this method
• Error-scenario : any statement after finally block in this example, will result in compile-time error stating “Unreachable code”

 

Case 4.A: write return statement inside try-block & at the end of method; that is just before end of method

Reason:

• Whenever try-block executes successfully, then it can always return value for this method
• But if any exception is raised & it is handled in the corresponding catch-block –> return statement at the end of method will be executed and returns the value for this method after executing finally-block
• Error-scenario : any statement after return statement at the end of method, will result in compile-time error stating “Unreachable code”

 

Case 4.B: write return statement inside both try-block & finally-block; but no statement after finally block

Reason:

• Whenever try-block executes successfully, then it can always return value for this method
• But if any exception is raised & it is handled in the corresponding catch-block –> return statement inside finally-block will return value for this method (after executing any statement inside finally-block before encountering return statement)
• Error-scenario : any statement after return statement (i.e.; finally-block in this case) will result in compile-time error stating “Unreachable code”

 

Case 4.C: write return statement inside both try-block & finally-block; but no statement after finally block (no catch block for this case)

Reason:

• This case is very similar to Case 4.B but it has got no catch block in try-catch-finally blocks sequence
• So, whenever try-block executes successfully, then it can always return value for this method from try-block
• But if any exception is raised then it is NOT handled as there is no catch-block for this case
• So, whenever exception is raised then JVM checks for handler-code up in the runtime stack & finally-block gets executed to return value (after executing any statement inside finally-block before encountering return statement)
• Error-scenario : any statement after return statement (i.e.; finally-block in this case) will result in compile-time error stating “Unreachable code”

 

Case 5.A: write return statement inside catch-block & at the end of method; that is just before end of method

Reason:

• Whenever try-block executes successfully, then it can always return value from end of method
• If any exception is raised from try-block then it get caught in the corresponding catch-block and catch-block can also return value
• But if any exception is raised & it is handled in the corresponding catch-block –> return statement at the end of method will be executed and returns value for this method after executing finally-block
• Error-scenario : any statement after return statement at the end of method, will result in compile-time error stating “Unreachable code”

 

Case 5.B: write return statement inside both catch-block & finally-block; but no statement after finally-block

Reason:

• Whenever try-block executes successfully, then it can always return value from finally-block
• If any exception is raised from try-block then it is get caught in the corresponding catch-block and catch-block can also returns value
• But if any exception is raised & it is handled in the corresponding catch-block –> return statement inside finally-block will return value for this method (after executing any statement inside finally-block before encountering return statement)
• Error-scenario : any statement after return statement (i.e.; finally-block) will result in compile-time error stating “Unreachable code”

 

Case 6: write return statement inside try-block & catch-block & finally-block; but return value from try-block or catch-block will be overridden by return statement in the finally-block

Reason:

• Whenever try-block executes successfully, then it can return value for this method from try-block
• Similarly, if any exception is raised then exception gets caught in the catch-block & it can also return value (from catch-block)
• Since, we have finally-block returning value therefore returning value from try-block or catch-block will be overridden by return statement in the finally-block
• Because, on all cases finally-block gets executed irrespective of exception is raised or NOT from try-block and  it is handled or NOT inside catch-block.
• Therefore, overrides any return value from try-block or catch-block
• This is called overridden case
• Error-scenario : any statement after finally block in this example, will result in compile-time error stating “Unreachable code”

Conclusion for Valid case:

• Above 9 examples are valid cases to write return statement;
• except these 9 example mentioned in the above cases, all other cases results in compile-time error
• following examples depicts few of those cases

 

Let us see some compile-time error for invalid cases

Invalid Cases:

Case A: program returns value from try-block only

• Compile-time error: This method must return a result of type int

 

Case B: program returns value from catch-block only

• Compile-time error: This method must return a result of type int

 

Case C: program returns value from try-catch-finally blocks; but contains some statements after finally-block

• Compile-time error: Unreachable code

 

Case D: program returns value from finally-block only; but contains some statements after finally block

• Compile-time error: Unreachable code

Conclusion for Invalid cases:

• Any code present after finally-block will results compile-time error stating “Unreachable code“
• Similarly, any code after return statement will results compile-time error stating “Unreachable code“

 

Final Conclusion:

Out of all possible valid cases,

• If a method contains a finally-block
• then finally-block will always gets executed
• irrespective of any valid combination used in the program

In the next article, we will see detail example on return statement with finally block

 

References:

• https://docs.oracle.com/javase/tutorial/essential/exceptions/finally.html
• https://docs.oracle.com/javase/tutorial/essential/exceptions/try.html
• https://docs.oracle.com/javase/tutorial/essential/exceptions/catch.html
• https://docs.oracle.com/javase/tutorial/essential/exceptions/
• https://docs.oracle.com/javase/7/docs/api/java/lang/Exception.html
• https://docs.oracle.com/javase/7/docs/api/java/lang/Error.html
• https://docs.oracle.com/javase/specs/jls/se7/html/jls-11.html
• https://docs.oracle.com/javase/7/docs/api/java/lang/ArithmeticException.html
• https://docs.oracle.com/javase/7/docs/api/java/lang/NullPointerException.html
• http://docs.oracle.com/javase/7/docs/api/java/lang/ArrayIndexOutOfBoundsException.html
• http://www.oracle.com/technetwork/java/effective-exceptions-092345.html
• http://otfried.org/courses/cs206/slides/slides-stackframes.pdf

 

Read Also:

• Exception Handling in Java
• Exception Hierarchy in Java
• Checked Exception v/s Unchecked Exception
• try-catch block in Java
• finally block in Java
• try with multiple catch block in Java
• Various methods to print exception information in Java
• return statement with finally block
• final v/s finally v/s finalize

 

Happy Coding !!
Happy Learning !!