Try with Resources

Video Materials

Lastly, Java includes a special type of Try-Catch statement, known as a Try with Resources statement, that can perform some of the work typically handled by the finally block.

Try with Resources

Let’s look at an example of a Try with Resources statement:

import java.util.Scanner;
import java.io.File;
import java.lang.NumberFormatException;
import java.io.FileNotFoundException;

public class Resources{
  
  public static void main(String[] args){
    
    try(
      Scanner reader = new Scanner(new File("input.txt"))
    ){
      
      int x = Integer.parseInt(reader.nextLine().trim();
      System.out.println(x + 5);
      
    }catch(NumberFormatException e){
      System.out.println("Error: Invalid Number Format!");
    }catch(FileNotFoundException e){
      System.out.println("Error: File Not Found!");
    }
    
  }
}

In this example, there is a new statement after the try keyword, surrounded by parentheses. Inside of that statement, we are declaring and initializing a new Scanner object to read data from a file. That Scanner object is the resource that we are using in our Try with Resources statement. We can add multiple resources to that section, separated by semicolons ;.

When the code in the try statement throws an exception, Java will automatically try to close the resources declared in the Try with Resources statement. So, when we are reading input from a file, it would close that file and make sure that it isn’t damaged or left open when our program crashes.

In addition, any additional exceptions thrown when trying to close the file are suppressed by the system, so we only see the exception that caused the initial error. This is much better than using a finally statement to close the file, since we’d have to run the risk of throwing a second exception inside of the finally statement.

Any Java class that implements the AutoCloseable interface can be used as a resource in this way. The Java 8 API documentation for AutoCloseable lists all known classes that implement that interface. We’ll discuss interfaces more later in this course.

Specifically, this type of statement is great when writing programs that will handle large amounts of input, either by connecting to a database, reading from a file, or using the Internet to communicate with a server. A Try with Resources statement is a great way to make sure those programs are able to handle exceptions without leaving the system in an unstable state.

// attach scanner to System stdin
Scanner reader = new Scanner( System.in);  
// use as part of try with resources
try (Scanner scan = reader){...

} catch (Exception e){ 

}
// now System.in is closed and unavailable to rest of program