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Java – Exception Handling With Constructors in Inheritance

  • Difficulty Level : Hard
  • Last Updated : 24 Sep, 2021
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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



Java




// Java Program to Illustrate Exception handling with
// Constructors in inheritance where Parent class
// constructor does not throw any checked exception
  
// Class 1
// Parent class
class Parent {
  
    // Constructor of Parent class
    // Not throwing any checked exception
    Parent()
    {
  
        // Print statement whenever parent class
        // constructor is called
        System.out.println("parent class constructor");
    }
}
  
// Class 2
// Child class
public class Child extends Parent {
  
    // Constructor of child class
    Child()
    {
  
        // Print statement whenever child class
        // constructor is called
        System.out.println("child class constructor");
    }
  
    // main driver method
    public static void main(String[] args)
    {
  
        // Creating object of child class inside main()
        Child child = new Child();
    }
}
Output
parent class constructor
child class constructor

Example 2 

Java




// Java Program to Illustrate Exception handling with
// Constructors in inheritance where Parent class
// constructor does not throw any checked exception
  
// Class 1
// Parent class
class Parent {
  
    // Constructor of parent class
    // Not throwing any checked exception
    Parent()
    {
  
        // Print statement when constructor of
        // parent class is called
        System.out.println("parent class constructor");
    }
}
  
// Class 2
// Child class 
public class Child extends Parent {
    Child() throws Exception
    {
  
        // Print statement when constructor of
        // child class is called
        System.out.println(
            "child class constructor throwing Exception");
    }
  
    // Main driver method
    public static void main(String[] args) throws Exception
    {
  
        // Creating object of child class
        Child child = new Child();
    }
}
Output
parent class constructor
child class cosntructor 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 

Java




// Java Program to Illustrate Exception handling with
// Constructors in inheritance where Child class constructor
// Not throwing exception of same type or its parent classes
  
// Importing I/O classes
import java.io.*;
  
// Class 1
// Parent class
class Parent {
  
    // Constructor of parent class
    // Throwing checked exception
    Parent() throws FileNotFoundException
    {
  
        // Print statement when
        // parent class constructor is called
        System.out.println(
            "parent class constructor throwing exception");
    }
}
  
// Class 2
// Child class
class Child extends Parent {
  
    // Constructor of child class
    Child()
    {
        // Print statement when
        // child class constructor is called
        System.out.println("child class constructor");
    }
  
    // Main driver method
    public static void main(String[] args) throws Exception
    {
        // Creating object of child class inside main()
        Child child = new Child();
    }
}

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 



Java




// Java Program to Illustrate Exception handling with Constructors
// in Inheritance where we Resolve the Error we Need to
// Declare the Exceptions to be Thrown
  
// Importing I/O classes 
import java.io.*;
  
// Parent class 
class Parent {
    
    // throwing checked exception
    Parent() throws FileNotFoundException {
        
        System.out.println("parent class constructor throwing checked exception");
    }
}
  
public class Child extends Parent {
    
    Child() throws FileNotFoundException {
        
        System.out.println("child class constructor throwing same exception");
    }
  
    public static void main(String[] args) throws Exception {
        
        Child child = new Child();
    }
}

Output 

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

Example 2 

Java




// Java Program to Illustrate Exception handling with
// Constructors in Inheritance where we Resolve the Error we
// Need to Declare the Exceptions to be Thrown
  
// Importing I/O classes
  
// Importing package
package package1;
// Importing required I/O classes
import java.io.*;
  
// Class 1
// Parent class
class Parent {
  
    // throwing checked exception
    Parent() throws FileNotFoundException
    {
        System.out.println(
            "parent class constructor throwing checked exception");
    }
}
  
// Class 2
// Child class
public class Child extends Parent {
  
    // It can also throw same exception or its parent
    // classes exceptions
    Child() throws IOException
    {
  
        System.out.println(
            "child class constructor throwing super-class exception");
    }
  
    // Main driver method
    public static void main(String[] args) throws Exception
    {
  
        // Creating object of child class
        // inside main() method
        Child child = new Child();
    }
}

Output 

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

 

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