Getting my ArrayList & HashMap to respond to the following methods - methods

Question

I'm just looking for some leeway into how I would get the empty methods below to respond to my harcoded arrayList (and HashMap if needed.).
I'll understand if no one can help me out directly, just some good advice.
import java.io.IOException;
import java.text.ParseException;
import java.util.Comparator;
import java.util.InputMismatchException;
import java.util.List;
import java.util.Map.Entry;
import java.util.Scanner;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.ListIterator;
import java.util.Collections;
import java.util.Set;
#SuppressWarnings("unused")
public class Inventory
{
Scanner in = new Scanner(System.in);
ArrayList<Sellable> groceries;
HashMap<String, Integer> stock;
public Inventory()
{
groceries = new ArrayList<Sellable>();
stock = new HashMap <String,Integer> ();
//HARDCODING...:
Sellable n1 = new Produce("Corn", 3, 3.00);
Sellable n2 = new Snack("Natural Popcorn Seeds", 2.50);
Sellable n3 = new Produce("Potatoes", 3, 3.00);
Sellable n4 = new Snack("Organic Potato Chips", 2.50);
Sellable n5 = new Produce("Apples", 5, 1.75);
Sellable n6 = new Snack("Apple Juice - 128 oz.", 3.50);
Sellable n7 = new Produce("Oranges", 5, 1.75);
Sellable n8 = new Snack("Orange Juice - 128 oz.", 3.50);
//ADD TO HASHMAP
groceries.add(n1);
groceries.add(n2);
groceries.add(n3);
groceries.add(n4);
groceries.add(n5);
groceries.add(n6);
groceries.add(n7);
groceries.add(n8);
//PUT UP FOR PRINTING
stock.put(n1.getName(), 50);
stock.put(n2.getName(), 100);
stock.put(n3.getName(), 50);
stock.put(n4.getName(), 100);
stock.put(n5.getName(), 50);
stock.put(n6.getName(), 100);
stock.put(n7.getName(), 50);
stock.put(n8.getName(), 100);
}
public void add(Sellable SE)
{
}
public boolean decrementStock(String name)
{
}
public boolean decrementStock(Sellable SE)
{
}
public boolean incrementStock(String SE)
{
}

Answer 1

public void add(Sellable SE)
{
groceries.add(SE);
}
public void decrementStock(String name)
{
Integer val = stock.get(name);
if(val != null) stock.put(name, val--);
}
public void decrementStock(Sellable SE)
{
decrementStock(SE.getName());
}
public void incrementStock(String SE)
{
Integer val = stock.get(name);
if(val != null) stock.put(name, val++);
}
I switched the boolean methods to void because I didn't know which are the specification about how a boolean value should be chosen as return value.