Java - Exception Handling With Constructors in Inheritance

Java provides a mechanism to handle exceptions. To learn about exception handling, you can refer to exceptions in java. In this article, we discuss exception handling with constructors when inheritance is involved. In Java, if the constructor of the parent class throws any checked exception, then the child class constructor can throw the same exception or its parent classes. There is no problem if the parent class or child class constructor throws any unchecked exceptions. The child class constructor can throw any unchecked exception without looking for a parent class constructor.

Understanding behavior of constructor calls

Whenever a method that throws some exception is called by another method, then the calling method is responsible for handling that exception (The calling method is the method that contains the actual call; the called method is the method being called). In case of constructors, the parent class constructor is called by the child class constructor. It means the child class constructor is responsible for handling the exception thrown by the parent class constructor. 

Now, for handling an exception there are two ways, one is to catch the exception and another is to throw it. But in the case of the constructor, we can't handle it using the try-catch mechanism. The reason is that we enclose our code which can raise an exception in the try block and then catch it. The exception is raised due to a call to parent class constructor, like super(). It means if we want to handle the exception using try-catch is depicted in the below illustration.

Illustration 1

Child() {

 // Try- catch block 
 try 
 {
 super();
 } 
 
 catch (FileNotFoundException exc) 
 {
 // Handling exception(code)
 }
}

Actually, it is not correct as a call to super must be first statement in the child class constructor (refer super in java  as it can be perceived from below illustration as follows:

Illustration 2

Child() {
 super(); // either called explicitly or added by the compiler in case of default constructor
 try {
 // your code 
 }
 catch(FileNotFoundException exc) {
 // handling code; 
 }
 }

and hence the exception can't be caught (as its not inside the try block) and we can't handle it using try-catch mechanism. That's why we need to throw the exception. The below code will compile fine which will appear as follows:

// parent class constructor throws FileNotFoundException 
Child() throws FileNotFoundException {
 super(); // either called explicitly or added by the compiler in case of default constructor
 try {
 // your code
 }
 catch(FileNotFoundException exc) {
 // handling code; 
 }
 }

Different Use-cases:

1. Parent class constructor does not throw any checked exception
2. Parent class constructor throws a checked exception

Now let us discuss each case in detail alongside justifying via clean java programs.

Case 1: Parent class constructor does not throw any checked exception

If the parent class constructor does not throw any exception then the child class can throw any exception or throw nothing.

Example 1

parent class constructor
child class constructor

Example 2 

parent class constructor
child class constructor throwing Exception

Case 2: Parent class constructor throws a checked exception

If the parent class constructor throws a checked exception, then the child class constructor can throw the same exception or its super-class exception. Now at this point, the child class constructors have to throw the exception.

Example 

Output 

error: unreported exception FileNotFoundException; must be caught or declared to be thrown
 Child() {
 ^

In order to resolve the error we need to declare the exceptions to be thrown. These exception can be of same or parent class.

Example 1 

Output 

parent class constructor throwing checked exception
child class constructor throwing same exception

Example 2 

Output 

parent class constructor throwing checked exception
child class constructor throwing super-class exception