BackedVirtualMemory.java

public abstract class BackedVirtualMemory extends SimpleVirtualMemory {

    protected int currentPageIndex = 0;
    protected int pageSize;
    protected int maxPages;

    public BackedVirtualMemory(){
        this(100, 10);
    }

    public BackedVirtualMemory(int pageSize, int maxPages){
        super.currentPage = new String[pageSize];
        this.pageSize = pageSize;
        this.maxPages = maxPages;
    }

    public String read(int address){
        handlePageFault(address);
        return super.read(address % pageSize);
    }
    
    public void write(int address, String value){
        handlePageFault(address);
        super.write(address % pageSize, value);
    }
    
    protected void handlePageFault(int address){
        int correctPageIndex = address / pageSize;

        if(correctPageIndex == currentPageIndex){
            return; // Nothing to do as current page is correct page
        }
        
        writeOut(currentPageIndex, super.currentPage);
        super.currentPage = readIn(correctPageIndex);
        
        currentPageIndex = correctPageIndex;
    }

    protected abstract void writeOut(int pageNumber, String[] page);
    protected abstract String[] readIn(int pageNumber);
}

FileBackedVirtualMemory.java

public class FileBackedVirtualMemory extends BackedVirtualMemory {

    @Override
    protected void writeOut(int pageNumber, String[] page) {
        // Write this page out to a file, doesn't matter how or where so long as we can
        // use the pageNumber to find it again.
    }

    @Override
    protected String[] readIn(int pageNumber) {
        // Read the correct file into an array and return it.
        return null;
    }

}

MemoryBackedVirtualMemory.java

public class MemoryBackedVirtualMemory extends BackedVirtualMemory {

    String[][] pages;
    
    public MemoryBackedVirtualMemory(){
        super();
        pages = new String[super.maxPages][];
    }
    
    public MemoryBackedVirtualMemory(int pageSize, int maxPages){
        super(pageSize, maxPages);
        pages = new String[maxPages][];
    }
    
    @Override
    protected void writeOut(int pageNumber, String[] page) {
        pages[pageNumber] = page;
    }

    @Override
    protected String[] readIn(int pageNumber) {
        if(pages[pageNumber] == null){
            pages[pageNumber] = new String[super.pageSize];
        }
        return pages[pageNumber];
    }
}

SimpleVirtualMemory.java

public class SimpleVirtualMemory implements VirtualMemory{
    protected static final int DEFAULT_PAGE_SIZE = 100;
    protected String[] currentPage;

    public SimpleVirtualMemory(){
        currentPage = new String[DEFAULT_PAGE_SIZE * 10];
    }

    public SimpleVirtualMemory(int size){
        currentPage = new String[size];
    }

    public String read(int address){
        return currentPage[address];
    }

    public void write(int address, String value){
        currentPage[address] = value;
    }
}

VirtualMemory.java

public interface VirtualMemory {

    String read(int address);
    void write(int address, String value);
    
}

VirtualMemoryTest.java

public class VirtualMemoryTest {

    public static void main(String[] args) {
        int pageSize = 5;
        int maxPages = 10;
        
        VirtualMemory memory = new MemoryBackedVirtualMemory(pageSize, maxPages);
        for(int i = 0; i < pageSize * maxPages; i++){
            memory.write(i, "Value at " + i);
        }
        for(int i = 0; i < pageSize * maxPages; i++){
            System.out.println(memory.read(i));
        }
    }
}