Errors

In Java, situations where things might go wrong in the program are predominantly handled by the following subclasses of the java.lang.Throwable class: Error and Exception.

Errors

The Error class represents illegal operations that cause serious abnormalities in the program and are not recommended to catch. Some examples include the following:

• The ClassFormatError occurs when a class file cannot be read or interpreted.
• The IncompatibleClassChangeError occurs when a base class is altered after a child class has already been initialized.
• The NoClassDefFoundError occurs when the file with the class containing the main() method cannot be found.

Exceptions

The Exception class refers to abnormal and unexpected events that disrupt the flow of the program but can be reasonably handled by a catch-statement. Some examples include:

• The ArrayIndexOutOfBoundsException occurs when attempting to access an index that does not exist within a given array.
• The FileNotFoundException occurs when a file with the specified path cannot be found.
• The NumberFormatException occurs when an attempt is made to convert a string to a numeric type and the string contains non-numeric characters.
• The NullPointerException occurs when attempting to use a null value in place of where an object is required.

Example

A try...catch block is a means for a programmer to encapsulate a block of code and “catch” a potentially-thrown Exception (but never an Error) before it halts the execution of the program.

In the example below, the code used in the try block will run until the Exception is thrown by the assignment to the c variable because division by zero, 0, is not possible. In the catch block, an ArithmeticException is thrown and yields a printed message along with details about where the Exception can be traced.

class DivideByZero {
  public static void main(String[] args) {
    int a = 27, b = 0;
    try {
      System.out.println("I'm executed first!");
      int c = a / b; // This will throw an exception
      System.out.println("I'm never executed!");
    } catch (ArithmeticException e) {
      System.out.println("Exception Caught!");
      e.printStackTrace();
    }
      System.out.println("Done!");
  }
}

When a programmer needs to handle more than one type of error, there are two common ways to do so, as follows:

// Handle different errors separately
try {

} catch (Exception1 e) {

} catch (Exception2 e) {

}

// Handle different errors in the same way
try {

} catch (Exception1 | Exception2 e) {

}