Create 18a 3 Types Exceptions, TRY-CATCH, throws FileNotFoundException

Week 3: Objects, files and exceptions/18a 3 Types Exceptions, TRY-CATCH, throws FileNotFoundException

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+try-catch block
+try block is used to perform an operation that may produce a run-time error, i.e. an exception. 
+The exception is caught in the catch block following the try block. 
+
+In addition, when the operation that might cause the error is written in parentheses after the try statement (as in the previous examples), 
+Java AUTOMATICALLY takes care of CLOSing the RESOURCE. 
+This prevents the file from being accidentally left open.
+
+
+
+In addition to reading files, the mechanism is useful in other ways. 
+For example, if you try to convert an input that does not contain just numbers into an integer, the execution will give an error:
+
+import java.util.Scanner;
+
+public class Example {
+    public static void main(String[] args){
+        Scanner reader = new Scanner(System.in);
+        System.out.print("Give a number: ");
+        int num = Integer.valueOf(reader.nextLine()); // this expects an Integer
+
+        System.out.println("The number doubled is " + num * 2);
+    }
+}
+
+Example output:
+Give a number: eight
+Exception in thread "main" java.lang.NumberFormatException: For input string: "eight"
+        at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:68)
+        at java.base/java.lang.Integer.parseInt(Integer.java:658)
+        at java.base/java.lang.Integer.valueOf(Integer.java:989)
+        at Example.main(Example.java:10)
+
+
+VS
+
+
+
+preparing for an error situation - TRY CATCH
+
+import java.util.Scanner;
+
+public class Example {
+    public static void main(String[] args){
+        Scanner reader = new Scanner(System.in);
+        try {
+            System.out.print("Give a number: ");
+            int num = Integer.valueOf(reader.nextLine());
+
+            System.out.println("The number doubled is " + num * 2);
+        } 
+        catch (NumberFormatException e) {
+            System.out.println("You didn't give a number!");
+        }
+    }
+}
+
+Example output:
+Give a number: eight
+You didn't give a number!
+
+
+
+
+==================================================================
+
+
+
+3 types of errors in Java:
+
+1
+TRANSLATION errors - these must be fixed before the program can run. 
+For example, a MISSPELLED variable name or a MISSING RETURN statement.
+
+2
+RUNTIME errors that MUST be HANDLED - for example, a FileNotFoundException that may be thrown by a FILE operation
+
+3
+RUNTIME errors that DO NOT NEED to be HANDLED - for example, NullPointerException. 
+These errors are usually not caught, but are allowed to abort the execution of the program. 
+Errors are also more typically caused by programmer error.
+
+
+
+
+
+Errors in category '2' must be handled by a try-catch block or passed up to the caller.
+
+
+
+
+
+
+For example, a method that reads a file may pass the error to the caller instead of handling the error itself (in the program). 
+The forwarding is done in the method signature with the 'throws' keyword.
+
+import java.util.Scanner;
+
+public class Example {
+    public static void main(String[] args){
+        try {
+            printFile("names.txt");
+        } 
+        catch (FileNotFoundException e) {
+            System.out.println("Error: " + e);
+        }
+    }
+
+    // HERE - TOPUP method with 'throws'
+    public static void printFile(String name) throws FileNotFoundException{
+        File file = new File(name);             //name = filename and extension
+        Scanner reader = new Scanner(file);
+        
+        while (reader.hasNextLine()) {
+            System.out.println(reader.nextLine());
+        }
+
+        // now reader must be closed by the programmer
+        reader.close();
+    }
+}
+
+
+
+
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