Java Examples

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

References

• Wikiversity: Computer Programming