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Engineering LibreTexts

7.8: Java Examples

  • Page ID
    10707
  • Strings

    // 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));
        }
    }
    

    Output

    string: Hello
    string..toLowerCase(): hello
    string.toUpperCase(): HELLO
    string.indexOf('e'): 1
    string.length(): 5
    string.replace('H', 'j'): jello
    string(substring(2,4): ll
    string.trim(): Hello
    Bob earned $123.46
    

    Files

    // 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();
        }
    }
    

    Output

    Celsius,Fahrenheit
    0.0,32.0
    1.0,33.8
    2.0,35.6
    ...
    98.0,208.4
    99.0,210.2
    100.0,212.0