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Code Block

language	java

package SingletonPatternDemo;

// Class to handle the singleton instance of ReportHeader
public class ReportHeader {
    // Static variable to hold the single instance, marked as volatile to ensure visibility
    // of changes across threads.
    private static volatile ReportHeader instance;

    // Private variable to hold the header text
    private String header;

    // Private constructor to prevent instantiation from outside the class
    private ReportHeader() {
        // Initializes the Codeheader tostring initializewith thespecific headerdetails
        this.header = "ABC Programming Company\nSan Antonio Office\n210-837-1234\n\n";
    }

    // Public method to get the singleton instance
    public static ReportHeader getInstance() {
        // First check to see if an instance already exists without locking
        if (instance == null) {
            // Synchronize on the class object to ensure only one thread can enter
            // this block at a time.
            synchronized (ReportHeader.class) {
                // Double check to make sure the instance was not created in another thread
                // while the current thread was waiting on the class' lock.
                if (instance == null) {
                    // If the instance is still null, create a new one
                    instance = new ReportHeader();
                }
            }
        }
        // Return the instance
        return instance;
    }

    // Getter method for the header string
    public String getHeader() {
        return this.header;
    }

    // New printHeader method Method to print the header to the console
    public void printHeader() {
        System.out.println(this.header);
    }
}

Key Points of the Singleton Pattern:

Single Instance: Ensures that a class has only one instance and provides a global point of access to it.
Thread Safety: The use of volatile and double-checked locking helps to keep the singleton instance updates visible and synchronized across threads while minimizing locking overhead.
Lazy Initialization: The instance of ReportHeader is not created until the getInstance() method is called. This is beneficial if the instance creation is resource-intensive and/or the instance is optional for some time after the program starts.

Code Block

language	java

package SingletonPatternDemo;

public class SingletonDemo {
    public static void main(String[] args) {
        ReportHeader reportHeader = ReportHeader.getInstance();
        reportHeader.printHeader(); // Using the new printHeader method

        // Write report
    }
}

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Let's consider a Report Header Singleton class that provides a consistent header for all reports generated by the system. The Singleton ensures that we only create one instance of the header for efficiency and consistency.

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Key

Key Points of the Singleton Pattern: