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:
- Parent class constructor does not throw any checked exception
- 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 classclass 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 classpublic 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(); }} |
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 classclass 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 classpublic 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(); }} |
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
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 classesimport java.io.*;// Class 1// Parent classclass 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 classclass 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 classesimport java.io.*;// Parent classclass 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 packagepackage package1;// Importing required I/O classesimport java.io.*;// Class 1// Parent classclass Parent { // throwing checked exception Parent() throws FileNotFoundException { System.out.println( "parent class constructor throwing checked exception"); }}// Class 2// Child classpublic 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
