Java Exceptions - Try...Catch

Java Exceptions

When executing Java code, different errors can occur: coding errors made by the programmer, errors due to wrong input, or other unforeseeable things.

When an error occurs, Java will normally stop and generate an error message. The technical term for this is: Java will throw an exception (throw an error).

Java try and catch

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The try and catch keywords come in pairs:

Create a HashSet object called cars that will store strings:

        
  try {
  //  Block of code to try
}
catch(Exception e ) {
  //  Block of code to handle errors
}
             

Finally

The finally statement lets you execute code, after try...catch, regardless of the result:

 
 public class Main { 
  public static void main (String[] args ) {
    try  {
      int[] myNumbers  = {1, 2, 3}; 
      System.out.println (myNumbers[10] );
    } catch  (Exception e ) {
      System.out.println("Something went wrong.");
    } finally  {
      System.out.println ("The 'try catch' is finished." );
    }
  }
}
                 

The throw keyword

The throw statement allows you to create a custom error.

The throw statement is used together with an exception type. There are many exception types available in Java: ArithmeticException, FileNotFoundException, ArrayIndexOutOfBoundsException, SecurityException , etc:

Throw an exception if age is below 18 (print "Access denied"). If age is 18 or older, print "Access granted":

              
            
    public class Main {
  static void checkAge(int age) {
    if (age < 18) {
      throw new ArithmeticException("Access denied - You must be at least 18 years old.");
    }
    else {
      System.out.println("Access granted - You are old enough!");
    }
  }

  public static void main(String[] args) {
    checkAge(15); // Set age to 15 (which is below 18...)
  }
}
 
The output will be:
Exception in thread "main" java.lang.ArithmeticException: Access denied - You must be at least 18 years old.
        at Main.checkAge(Main.java:4)
        at Main.main(Main.java:12)
                 

If age was 20, you would not get an exception:

Throw an exception if age is below 18 (print "Access denied"). If age is 18 or older, print "Access granted":

              
             
  checkAge(20);
  The output will be:
  Access granted - You are old enough!