Extending singleton (singleton inheritance)

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In this example, base class Singleton provides getMessage() method that returns "Hello world!" message.

The program demonstrates creating a base singleton class with two extended types. The user can choose between the types by setting the type variable. The extended types use reflection to create instances, as the constructors are private inner classes.

import java.lang.reflect.*;
 
enum SingletonType {
    UNKNOWN,
    FIRST_TYPE,
    SECOND_TYPE
}
 
class SingletonBase {
    private class FirstTypeSingleton extends SingletonBase {
        private FirstTypeSingleton() {
            super();
        }
 
        @Override
        public String getMessage() {
            return super.getMessage().toUpperCase();
        }
    }
 
    private class SecondTypeSingleton extends SingletonBase {
        private SecondTypeSingleton() {
            super();
        }
 
        @Override
        public String getMessage() {
            return super.getMessage().toLowerCase();
        }
    }
 
    private static SingletonType type = SingletonType.UNKNOWN;
    private static SingletonBase instance;
 
    public static void setType(SingletonType type) {
        SingletonBase.type = type;
    }
 
    public static SingletonBase getInstance() throws NoSuchMethodException, IllegalAccessException, 
	InvocationTargetException, InstantiationException {
        if (instance == null) {
            synchronized (SingletonBase.class) {
                if (instance == null) {
                    SingletonBase singleton = new SingletonBase();
                    switch (type) {
                        case UNKNOWN:
                            instance = singleton;
                            break;
                        case FIRST_TYPE:
                            Constructor<FirstTypeSingleton> firstTypeConstructor = FirstTypeSingleton.class.
										getDeclaredConstructor(SingletonBase.class);
                            firstTypeConstructor.setAccessible(true);
                            instance = firstTypeConstructor.newInstance(singleton);
                            break;
                        case SECOND_TYPE:
                            Constructor<SecondTypeSingleton> secondTypeConstructor = SecondTypeSingleton.
										class.getDeclaredConstructor(SingletonBase.class);
                            secondTypeConstructor.setAccessible(true);
                            instance = secondTypeConstructor.newInstance(singleton);
                            break;
                    }
                }
            }
        }
        return instance;
    }
 
    protected String message;
 
    private SingletonBase() {
        message = "Hello world!";
    }
 
    public String getMessage() {
        return message;
    }
}
 
public class CustomSingletonExample {
    public static void main(String args[]) {
        SingletonBase singleton = null;
        try {
            singleton = SingletonBase.getInstance();
        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException | InstantiationException e) {
            e.printStackTrace();
        }
        System.out.println(singleton.getMessage());
    }
}

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• Creating Your First Java Program
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StringBuffer and StringBuilder

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Date Class

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BigDecimal

• Comparing and Using BigDecimals
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BigInteger

• Initialization and Generating random BigIntegers
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super keyword

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Calendar and its Subclasses

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Using the static keyword

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Properties Class

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Lambda Expressions

• Introduction to Java lambdas
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• return only returns from the lambda and Execute-around Pattern

Basic Control Structures

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BufferedWriter

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New File I/O

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Scanner

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Regular Expressions

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Java Floating Point Operations

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Currency and Money

• Add custom currency

Object Cloning

• Cloning performing a deep copy and copy factory
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Recursion

• Basic idea of Recursion
• StackOverflowError & recursion to loop
• Computing the Nth Fibonacci Number and power of a number

Converting to and from Strings

• Converting String to other datatypes
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Random Number Generation

• Pseudo Random Numbers in Specific Range
• Generating Random Numbers with a Specified Seed

Singletons

• Enum Singleton and Singleton without use of Enum, Thread safe Singleton
• Extending singleton (singleton inheritance)

Autoboxing

• Using int and Integer interchangeably and Auto-unboxing may lead to NullPointerException
• Different Cases When Integer and int can be used interchangeably

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