singleton example

logger.java

package com.alex.static;

class LoggerSingleton {

    private static LoggerSingleton instance = null;
    private int logCount = 0;

    private LoggerSingleton() {
        // nothing to do here :)
    }

    public static LoggerSingleton getInstance() {
        return instance == null ? instance = new LoggerSingleton() : instance;
    }

    public void log(String message) {
        System.out.println(logCount + ": " + message);
        logCount++;
    }
}

class SingletonDemo {

    public static void run() {
        doHardWork();
    }

    public static void doHardWork() {
        LoggerSingleton logger = LoggerSingleton.getInstance();
        HardProcessor processor = new HardProcessor(1);

        logger.log("Hard work started..."); // пишем в лог!
        processor.processTo(5);
        logger.log("Hard work finished!");
    }
}

class HardProcessor {
    private int start;

    public HardProcessor(int start) {
        this.start = start;
        LoggerSingleton.getInstance().log("Processor just created."); // пишем в лог!
    }

    public int processTo(int end) {
        int sum = 0;
        for (int i = start; i <= end; ++i) {
            sum += i;
            LoggerSingleton.getInstance().log("In progress..." + i);
        }
        LoggerSingleton.getInstance().log("Processor just calculated some value: " + sum);
        return sum;
    }
}

class Program {
    public static void main(String[] args) {
        SingletonDemo.run();
    }
}