Java

 // This program demonstrates string functions.
 
 class Main {
     public static void main(String[] args) {
         String str = "Hello";
 
         System.out.println("string: " + str);
         System.out.println("string.toLowerCase(): " + str.toLowerCase());
         System.out.println("string.toUpperCase(): " + str.toUpperCase());
         System.out.println("string.indexOf('e'): " + str.indexOf('e'));
         System.out.println("string.length(): " + str.length());
         System.out.println("string.replace('H', 'j'): " + str.replace('H', 'j'));
         System.out.println("string(substring(2,4): " + str.substring(2, 4));
         System.out.println("string.trim(): " + str.trim());
 
         String name = "Bob";
         double value = 123.456;
         System.out.println(String.format("%s earned $%.2f", name, value));
     }
 }

 // This program creates a file, adds data to the file, displays the file,
 // appends more data to the file, displays the file, and then deletes the file.
 // It will not run if the file already exists.
 //
 // References:
 //     https://www.mathsisfun.com/temperature-conversion.html
 //     https://en.wikibooks.org/wiki/Java_Programming
 
 import java.util.*;
 
 class Main {
     public static void main(String[] args) {
         String FILENAME = "~file.txt";
     
         if(fileExists(FILENAME)) {
             System.out.println("File already exists.\n");
         } else {
             createFile(FILENAME);
             readFile(FILENAME);
             appendFile(FILENAME);
             readFile(FILENAME);
             deleteFile(FILENAME);
         }
     }
     
     private static double calculateFahrenheit(double celsius) {
         double fahrenheit;
 
         fahrenheit = celsius * 9 / 5 + 32;
         return fahrenheit;
     }
 
     private static void createFile(String filename) {
         try {
             java.io.File file = new java.io.File(filename);
             java.io.BufferedWriter writer = 
                 new java.io.BufferedWriter(new java.io.FileWriter(file));
             double celsius;
             double fahrenheit;
             
             writer.write("Celsius,Fahrenheit\n");
             for(celsius = 0; celsius <= 50; celsius++) {
                 fahrenheit = calculateFahrenheit(celsius);
                 writer.write(celsius + "," + fahrenheit + "\n");
             }
             writer.close();
         } catch(Exception exception) {
             System.out.println("Error creating " + filename);
             exception.printStackTrace();
         }
     }
     
     private static void readFile(String filename) {
         try {
             java.io.File file = new java.io.File(filename);
             java.io.BufferedReader reader = 
                 new java.io.BufferedReader(new java.io.FileReader(file));
             String line;
 
             while(true) {
                 line = reader.readLine();
                 if (line == null) {
                     break;
                 }
                 System.out.println(line);
             }
             reader.close();
             System.out.println("");
         } catch(Exception exception) {
             System.out.println("Error reading " + filename);
             exception.printStackTrace();
         }
     }
     
     private static void appendFile(String filename)
     {
         try {
             java.io.File file = new java.io.File(filename);
             java.io.BufferedWriter writer = 
                 new java.io.BufferedWriter(new java.io.FileWriter(file, true));
             double celsius;
             double fahrenheit;
             
             for(celsius = 51; celsius <= 100; celsius++) {
                 fahrenheit = calculateFahrenheit(celsius);
                 writer.write(celsius + "," + fahrenheit + "\n");
             }
             writer.close();
         } catch(Exception exception) {
             System.out.println("Error appending to " + filename);
             exception.printStackTrace();
         }
     }
     
     private static void deleteFile(String filename) {
         java.io.File file;
         
         try {
             file = new java.io.File(filename);
             file.delete();
         } catch(Exception exception) {
             System.out.println("Error deleting " + filename);
             exception.printStackTrace();
         }
         
     }
     
     private static boolean fileExists(String filename) {
         java.io.File file;
         
         file = new java.io.File(filename);
         return file.exists();
     }
 }