Excercise

Create an Invoice class with 4 attributes: PartNumber (type int), PartDescription (type String), Quantity (type int), and Price (type double). Create a constructor that allows you to initialize all 4 attributes from values passed in as parameters. Override the toString() method to display all 4 attributes in a format of your choosing. Create a List of 10 Invoices containing data of your choosing. Use lambdas and streams to perform the following queries on the list of Invoice objects and display the results: Sort the Invoice objects by PartDescription, then display the results. Sort the Invoice objects by Price, then display the results. Map each Invoice to its PartDescription and Quantity, sort the results by Quantity, then display the results. Map each Invoice to its PartDescription and the value of the Invoice (i.e., Quantity * Price). Order the results by Invoice value. Modify Part (d) above to select the Invoice values in the range of $200 to $500 (inclusive) and display the results separately.
The code I have. Need help on this code to demonstrate the use of lambdas and streams.
code
public class Invoice {

private int partNumber ;
private String partDescription;
private int quantity;
private double price;

public Invoice(int partNumber, String partDescription, int quantity, double price) {
this.partNumber = partNumber;
this.partDescription = partDescription;
this.quantity = quantity;
this.price = price;
}

@Override
public String toString() {
return "Invoice{" +
"partNumber=" + partNumber +
", partDescription=" + partDescription +" "+
", quantity=" + quantity +
", price=" + price +
'}';
}

public int getPartNumber() {
return partNumber;
}

public void setPartNumber(int partNumber) {
this.partNumber = partNumber;
}

public String getPartDescription() {
return partDescription;
}

public void setPartDescription(String partDescription) {
this.partDescription = partDescription;
}

public int getQuantity() {
return quantity;
}

public void setQuantity(int quantity) {
this.quantity = quantity;
}

public double getPrice() {
return price;
}

public void setPrice(double price) {
this.price = price;
}
public String getDescriptionQuantity() {
return String.format("%s %s", partDescription, quantity);
}

public String getDescriptionTotalPrice() {
return String.format("%s %s", partDescription, quantity * price);
}

public double getValue() {
return quantity * price; // getting the invoice total value
}

public boolean isWithinRange(double min, double max) {
return getValue() >= min && getValue() <=max;
}
}
// Invoice Generator
import java.util.ArrayList;

public class InvoiceGenerator {

private ArrayList invoiceList = new ArrayList();

public InvoiceGenerator() {
invoiceList.add(new Invoice(101,"some desc",1,100));
invoiceList.add(new Invoice(102,"some descA",2,101));
invoiceList.add(new Invoice(103,"some descB",3,102));
invoiceList.add(new Invoice(104,"some descC",4,103));
invoiceList.add(new Invoice(105,"some descD",5,104));
invoiceList.add(new Invoice(106,"some descH",6,105));
invoiceList.add(new Invoice(107,"some descE",7,106));
invoiceList.add(new Invoice(108,"some descF",8,107));
invoiceList.add(new Invoice(109,"some descG",9,108));
invoiceList.add(new Invoice(110,"some descI",10,109));
}

public ArrayList getInvoiceList(){
return invoiceList;
}
}
// Invoice Handler
import java.util.ArrayList;

import static java.util.Comparator.comparing;

public class InvoiceHandler {

private final ArrayList invoices;

private InvoiceHandler(){
this.invoices = new InvoiceGenerator().getInvoiceList();
}

private void sortByPrice(){
invoices.stream().sorted(comparing(Invoice::getPrice)).forEach(System.out::println);
}

private void sortByDescription(){
invoices.stream().sorted(comparing(Invoice::getPartDescription)).forEach(System.out::println);
}

private void mappingDescriptionAndPrice(){
invoices.stream().sorted(comparing(Invoice::getQuantity)).map(Invoice::getDescriptionQuantity).forEach(System.out::println);

// sorted method requires a value, so we have sorted on the basis of quantity.
// we can also call it as sorted(invoice -> invoice.getQuantity())
// sorted, map, filter function take in lambda functions which are of the form functionName(input -> {return expression})
}

private void mappingDescriptionAndValue() {
invoices.stream().sorted(comparing(Invoice::getValue)).map(Invoice::getDescriptionTotalPrice).forEach(System.out::println);
}

private void filterByRange(double min, double max) {
invoices.stream().sorted(comparing(Invoice::getValue)).filter(invoice -> invoice.isWithinRange(min, max)).map(Invoice::getDescriptionTotalPrice).forEach(System.out::println);
}

public static void main(String[] args) {
InvoiceHandler invoiceHandler = new InvoiceHandler();
System.out.println("nSorted by description:n");
invoiceHandler.sortByDescription();
System.out.println("nSorted by price:n");
invoiceHandler.sortByPrice();
System.out.println("nMapping each invoice to its description and quantity:n");
invoiceHandler.mappingDescriptionAndPrice();
System.out.println("nMapping each invoice to its description and value:n");
invoiceHandler.mappingDescriptionAndValue();
System.out.println("nFilter value between range and map each Invoice to its description and value:n");
invoiceHandler.filterByRange(200,500);
}

}