StringBuilder and StringBuffer Classes

These classes are used to creating mutable sequences of characters. StringBuilder is not thread-safe but offers better performance, while StringBuffer is thread-safe but slightly slower.

public class StringBuilderExample {
    public static void main(String[] args) {
        // Testing constructors
        StringBuilder builder1 = new StringBuilder();
        builder1.append("test");
        System.out.println("builder1: " + builder1);
        System.out.println("---");
        
        // Converting a String to StringBuilder
        StringBuilder builder2 = new StringBuilder("initial");
        System.out.println("builder2: " + builder2);
        System.out.println("---");
        
        // Appending multiple strings
        StringBuilder messageBuilder = new StringBuilder();
        messageBuilder.append("Hello");
        messageBuilder.append(" ");
        messageBuilder.append("Java");
        messageBuilder.append("!");
        System.out.println("messageBuilder: " + messageBuilder);
        System.out.println("---");
        
        // Converting back to String
        String finalMessage = messageBuilder.toString();
        System.out.println("String result: " + finalMessage);
    }
}

Date and Calendar Classes

The Date class repersents a specific instant in time, while Calendar proivdes methods to date manipulation and conversion.

Date Class Example

import java.util.Date;

public class DateExample {
    public static void main(String[] args) {
        // Default constructor uses current system time
        Date currentDate = new Date();
        System.out.println("Current date: " + currentDate);
        System.out.println("---");
        
        // Get milliseconds since epoch
        long milliseconds = currentDate.getTime();
        System.out.println("Milliseconds: " + milliseconds);
        System.out.println("---");
        
        // Create a specific date from milliseconds
        Date specificDate = new Date(1671978206541L);
        System.out.println("Specific date: " + specificDate);
    }
}

Calendar Class Example

import java.util.Calendar;

public class CalendarExample {
    public static void main(String[] args) {
        // Get calendar instance for current locale
        Calendar cal = Calendar.getInstance();
        System.out.println(cal);
        System.out.println("---");
        
        // Extract date components
        int currentYear = cal.get(Calendar.YEAR);
        int currentMonth = cal.get(Calendar.MONTH);
        int currentDay = cal.get(Calendar.DAY_OF_MONTH);
        System.out.println(currentYear + "-" + (currentMonth + 1) + "-" + currentDay);
        System.out.println("---");
        
        // Set to a specific date
        cal.set(2023, Calendar.NOVEMBER, 15);
        int setYear = cal.get(Calendar.YEAR);
        int setMonth = cal.get(Calendar.MONTH);
        int setDay = cal.get(Calendar.DAY_OF_MONTH);
        System.out.println(setYear + "-" + (setMonth + 1) + "-" + setDay);
    }
}

Primitive Wrapper Classes

Wrapper classes allow primitive types to be used as objects and provide utility methods.

public class WrapperExample {
    public static void main(String[] args) {
        int primitiveValue = 42;
        
        // Manual boxing (pre-JDK 5)
        Integer wrappedInt = new Integer(99);
        // Manual unboxing
        int extractedValue = wrappedInt.intValue();
        System.out.println(wrappedInt);
        System.out.println(extractedValue);
        System.out.println("---");
        
        // Auto-boxing and auto-unboxing (JDK 5+)
        Integer autoBoxed = 123;      // Auto-boxing
        int autoUnboxed = autoBoxed;  // Auto-unboxing
        
        // Converting String to int
        String numericString = "255";
        int convertedValue = Integer.parseInt(numericString);
        System.out.println("Converted: " + convertedValue);
        System.out.println("After addition: " + (convertedValue + 45));
    }
}