how to use Java 8 for long function definitions? - java-8

Question

I have a an object Product,
and code as below , hashSetProducts is LinkedHashSet of Products. How can I write all below using Java 8 stream function ? I understand that value of remianing will be replaced each time. I want the final value after the for loop exits.
int getRemaining(int remaining){
for(Product P : hashSetProducts){
remaining = calculate(p.qty(), p.price(), remaining, location); //
use Java 8 stream here
}
return remaining
}
private int calculate(int qty, double price, int rem, Location location){
if(rem== 0){
return 0;
}
int avail = location.get(qty, rem);
if(avail > 0){
rem = avail - rem;
}
return rem;
}

Answer 1

mapToLong will execute arbitrary code that returns a long. Here is an MCVE that uses your calculation verbatim:
import java.util.LinkedHashSet;
public class HelloWorld{
public static class Product {
private int qty;
private double price;
private int used;
public Product(int qty, double price, int used) {
this.qty = qty;
this.price =price;
this.used = used;
}
public int qty() {return qty;}
public double price() {return price;}
public int used() {return used;}
};
public static class Location {
public long get(int qty, int used) { return 0; };
};
public static void main(String []args) {
LinkedHashSet<Product> hashSetProducts = new LinkedHashSet();
hashSetProducts.add(new Product(1,1.0,1));
hashSetProducts.add(new Product(2,2.0,2));
hashSetProducts.add(new Product(3,3.0,3));
Location location = new Location();
long remaining = hashSetProducts.stream().mapToLong(p -> {
int qty = p.qty();
int used = p.used();
if( used == 0 )
return 0;
long rem = location.get(qty, used);
if( qty > 0)
rem = used - rem;
return rem;
}).sum();
System.out.println(remaining);
}
}