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1. Introduction

This tutorial focuses on some common Java exceptions.

We’ll start by discussing what an exception basically is. Later, we’ll discuss different types of checked and unchecked exceptions in detail.

2. Exceptions

An exception is an abnormal condition that occurs in a code sequence during the execution of a program. This abnormal condition arises when a program violates certain constraints at runtime.

All exception types are subclasses of the class Exception. This class is then subclassed into checked exceptions and unchecked exceptions. We’ll consider them in detail in the subsequent sections.

3. Checked Exceptions

Checked exceptions are mandatory to handle. They are direct subclasses of the class Exception.

There’s a debate on their importance that’s worth taking a look.

Let’s define some checked exceptions in detail.

3.1. IOException

A method throws an IOException or a direct subclass of it when any Input/Output operation fails. 

Typical uses of these I/O operations include:

  • Working with the file system or data streams using java.io package
  • Creating network applications using java.net package

FileNotFoundException

FileNotFoundException is a common type of IOException while working with the file system:

try {
    new FileReader(new File("/invalid/file/location"));
} catch (FileNotFoundException e) {
    LOGGER.info("FileNotFoundException caught!");
}

MalformedURLException

When working with URLs, we might encounter with MalformedURLException – if our URLs are invalid.

try {
    new URL("malformedurl");
} catch (MalformedURLException e) {
    LOGGER.error("MalformedURLException caught!");
}

3.2. ParseException

Java uses text parsing to create an object based on a given String. If parsing causes an error, it throws a ParseException.

For instance, we could represent Date in different ways e.g. dd/mm/yyyy or dd,mm,yyyy, but try to parse a string with a different format:

try {
    new SimpleDateFormat("MM, dd, yyyy").parse("invalid-date");
} catch (ParseException e) {
    LOGGER.error("ParseException caught!");
}

Here, the String is malformed and causes a ParseException.

3.3. InterruptedException

Whenever a Java thread calls join(), sleep() or wait() it goes into either the WAITING state or the TIMED_WAITING state.

In addition, a thread can interrupt another thread by calling another thread’s interrupt() method.

Consequently, the thread throws an InterruptedException if another thread interrupts it while it is in the WAITING or in the TIMED_WAITING state.

Consider the following example with two threads:

  • The main thread starts the child thread and interrupts it
  • The child thread starts and calls sleep()

This scenario results in an InterruptedException:

class ChildThread extends Thread {

    public void run() {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            LOGGER.error("InterruptedException caught!");
        }
    }
}

public class MainThread {

    public static void main(String[] args) 
      throws InterruptedException {
        ChildThread childThread = new ChildThread();
        childThread.start();
        childThread.interrupt();
    }
}

4. Unchecked Exceptions

For Unchecked Exceptions, the compiler doesn’t check during the compilation process. Hence, it isn’t mandatory for the method to handle these exceptions.

All unchecked exceptions extend the class RuntimeException.

Let’s discuss some unchecked exceptions in detail.

4.1. NullPointerException

If an application attempts to use null where it actually requires an object instance, the method will throw a NullPointerException

There are different scenarios where illegal uses of null causes NullPointerException. Let’s consider some of them.

Calling a method of the class that has no object instance:

String strObj = null;
strObj.equals("Hello World"); // throws NullPointerException.

Also, if an application tries to access or modify an instance variable with a null reference, we get a NullPointerException:

Person personObj = null;
String name = personObj.personName; // Accessing the field of a null object
personObj.personName = "Jon Doe"; // Modifying the field of a null object

4.2. ArrayIndexOutOfBoundsException

An array stores its elements in contiguous fashion. Thus, we can access its elements via indices.

However, if a piece of code tries to access an illegal index of an array, the respective method throws an ArrayIndexOutOfBoundException.

Let’s see a few examples that throw ArrayIndexOutOfBoundException:

int[] nums = new int[] {1, 2, 3};
int numFromNegativeIndex = nums[-1]; // Trying to access at negative index
int numFromGreaterIndex = nums[4];   // Trying to access at greater index
int numFromLengthIndex = nums[3];    // Trying to access at index equal to size of the array

4.3. StringIndexOutOfBoundsException

The String class in Java provides the methods to access a particular character of the string or to slice out a character array out of the String. When we use these methods, internally it converts the String into a character array.

Again, there could be an illegal use of indexes on this array. In such cases, these methods of the String class throws the StringIndexOutOfBoundsException.

This exception indicates that the index is either greater than or equal to the size of the String. StringIndexOutOfBoundsException extends IndexOutOfBoundsException.

The method charAt(index) of the class String throws this exception when we try to access a character at the index equal to the String’s length or some other illegal index:

String str = "Hello World";
char charAtNegativeIndex = str.charAt(-1); // Trying to access at negative index
char charAtLengthIndex = str.charAt(11);   // Trying to access at index equal to size of the string		

4.4. NumberFormatException

Quite often an application ends up with numeric data in a String. In order to interpret this data as numeric, Java allows the conversion of String to numeric types. The wrapper classes such as Integer, Float, etc. contains utility methods for this purpose.

However, if the String doesn’t have an appropriate format during the conversion, the method throws a NumberFormatException.

Let’s consider the following snippet.

Here, we declare a String with an alphanumeric data. Further, we try to use the methods of the Integer wrapper class to interpret this data as numeric.

Consequently, this results in NumberFormatException:

String str = "100ABCD";
int x = Integer.parseInt(str); // Throws NumberFormatException
int y = Integer.valueOf(str); //Throws NumberFormatException

4.5. ArithmeticException

When a program evaluates an arithmetic operation and it results in some exceptional condition, it throws ArithmeticException. In addition, ArithmeticException applies to only int and long data types.

For instance, if we try to divide an integer by zero, we get an ArithmeticException:

int illegalOperation = 30/0; // Throws ArithmeticException

4.6. ClassCastException

Java allows typecasting between the objects in order to support inheritance and polymorphism. We can either upcast an object or downcast it.

In upcasting, we cast an object to its supertype. And in downcasting, we cast an object to one of its subtypes.

However, at runtime, if the code attempts to downcast an object to a subtype of which it isn’t an instance, the method throws a ClassCastException.

The runtime instance is what actually matters in typecasting. Consider the following inheritance between AnimalDog, and Lion:

class Animal {}

class Dog extends Animal {}

class Lion extends Animal {}

Further, in the driver class, we cast the Animal reference containing an instance of Lion into a Dog.

However, at the runtime, the JVM notices that instance Lion isn’t compatible with the subtype of the class Dog.

This results in ClassCastException:

Animal animal = new Lion(); // At runtime the instance is Lion
Dog tommy = (Dog) animal; // Throws ClassCastException

4.7. IllegalArgumentException

A method throws an IllegalArgumentException if we call it with some illegal or inappropriate arguments.

For instance, the sleep() method of the Thread class expects positive time and we pass a negative time interval as an argument. This results in IllegalArgumentException:

Thread.currentThread().sleep(-10000); // Throws IllegalArgumentException

4.8. IllegalStateException

IllegalStateException signals that a method’s been invoked at an illegal or inappropriate time.

Every Java object has a state (instance variables) and some behavior (methods). Thus, IllegalStateException means it’s illegal to invoke the behavior of this object with the current state variables.

However, with some different state variables, it might be legal.

For example, we use an iterator to iterate a list. Whenever we initialize one, it internally sets its state variable lastRet to -1.

With this context, the program tries to call the remove method on the list:

//Initialized with index at -1
Iterator<Integer> intListIterator = new ArrayList<>().iterator(); 

intListIterator.remove(); // IllegalStateException

Internally, the remove method checks the state variable lastRet and if it is less than 0, it throws IllegalStateException. Here, the variable is still pointing to the value -1.

As a result, we get an IllegalStateException.

5. Conclusion

In this article, we first discussed what are exceptions. An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program’s instructions.

Then, we categorized the exceptions into the Checked Exceptions and the Unchecked Exceptions.

Next, we discussed different types of exceptions that can come up during the compile time or at the runtime.

We can find the code for this article over on GitHub.

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