Abstraction: hide the data structure
Change how a class stores its data without touching the code that uses it.
Watch the narrated video
Every step
Problem
Steps 1–3Refactor
Steps 4–8Payoff
Steps 9–10Recap
Steps 11–12The code: before and after
Before
class Warehouse {
public Map<String, Integer> stock = new HashMap<>();
}
class Receiving {
void receive(Warehouse w, String sku, int qty) {
w.stock.merge(sku, qty, Integer::sum);
}
}
class Picking {
void pick(Warehouse w, String sku, int qty) {
int have = w.stock.getOrDefault(sku, 0);
if (have >= qty) {
w.stock.put(sku, have - qty);
}
}
}
class Report {
void print(Warehouse w, String sku) {
int left = w.stock.getOrDefault(sku, 0);
System.out.println(sku + ": " + left);
}
}
class Shop {
public static void main(String[] args) {
Warehouse w = new Warehouse();
new Receiving().receive(w, "TEA-1", 20);
new Receiving().receive(w, "TEA-1", 10);
new Picking().pick(w, "TEA-1", 15);
new Report().print(w, "TEA-1"); // TEA-1: 15
}
}After
class Warehouse {
private final Map<String, Deque<Integer>> stock =
new HashMap<>();
void receive(String sku, int qty) {
stock.computeIfAbsent(sku, k -> new ArrayDeque<>())
.addLast(qty);
}
boolean pick(String sku, int qty) {
if (quantity(sku) < qty) {
return false;
}
Deque<Integer> queue = stock.get(sku);
while (qty > 0) {
int oldest = queue.pollFirst();
int used = Math.min(oldest, qty);
if (oldest > used) {
queue.addFirst(oldest - used);
}
qty -= used;
}
return true;
}
int quantity(String sku) {
return stock.getOrDefault(sku, new ArrayDeque<>())
.stream().mapToInt(Integer::intValue).sum();
}
List<Integer> deliveries(String sku) {
return List.copyOf(stock.getOrDefault(sku, new ArrayDeque<>()));
}
}
class Receiving {
void receive(Warehouse w, String sku, int qty) {
w.receive(sku, qty);
}
}
class Picking {
void pick(Warehouse w, String sku, int qty) {
w.pick(sku, qty);
}
}
class Report {
void print(Warehouse w, String sku) {
int left = w.quantity(sku);
System.out.println(sku + ": " + left);
}
}
class Shop {
public static void main(String[] args) {
Warehouse w = new Warehouse();
new Receiving().receive(w, "TEA-1", 20);
new Receiving().receive(w, "TEA-1", 10);
new Picking().pick(w, "TEA-1", 15);
new Report().print(w, "TEA-1"); // TEA-1: 15
System.out.println(w.deliveries("TEA-1")); // [5, 10]
}
}Transcript, step by step
Problem
Step 1: A shop's warehouse stock Before
A small shop's warehouse keeps its stock in a public map, from product code to quantity. Receiving adds a delivery to the map, Picking takes items out when there are enough, and Report prints what's left. Two deliveries of tea come in, 20 and then 10, and after 15 are picked, the report prints 15.
Step 2: Oldest delivery first Code smell
The tea goes stale, so picking must take the oldest delivery first, and ops need the stock kept per delivery. The quick way turns each value in the map into a queue of delivery quantities. Receiving now appends to the queue, and Picking sums it, then takes from the front in a loop. Report sums the queue too, and the run still prints 15.
Step 3: Every caller knows the map Code smell
Every caller works on the map directly, so each one depends on how the warehouse stores its stock. Receiving uses the map here, Picking here, and the report here. One change to the storage meant rewriting three classes that only wanted to receive, pick and count. The sum over the queue now exists twice, written one way in Picking and another way in Report. And when the stock moves to a database next year, all three get rewritten again.
Refactor
Step 4: Refactor first Back it out
Let's take the queue out again. We'll refactor first, so that the warehouse owns its stock, and then we'll change the storage in one place.
Step 5: Receive moves in Move Method
First, the merge moves into Warehouse, as a method called receive. Receiving now calls receive on the warehouse, and it no longer touches the map.
Step 6: Pick moves in Move Method
The check and the put move in next, as pick, which returns true when there was enough stock. Picking calls pick on the warehouse, and the run still prints 15.
Step 7: Quantity moves in Move Method
The lookup moves in last, as quantity, which returns zero for a product the warehouse has never seen. Report asks the warehouse for the quantity, and then prints it.
Step 8: Make the map private Encapsulate Collection
No caller touches the map any more, so the field becomes private, and the compiler stops any class outside Warehouse from reaching it. That's abstraction, where callers use what the warehouse does, receive, pick and quantity, and never see how it stores the stock. Encapsulation hides a single field, and abstraction goes one step further, because it hides how a whole collection is stored.
Payoff
Step 9: Keep stock per delivery Substitute Algorithm
Now the change request comes back, and this time it stays inside Warehouse. The private map now holds a queue of deliveries per product, and receive appends to it. Quantity sums the queue, and pick checks the quantity, then takes from the oldest delivery first. Each method keeps its signature and gets a new body, and that move is called Substitute Algorithm. Receiving, Picking and Report didn't change, and Shop gains one line that prints the deliveries.
Step 10: One class knows the storage Payoff
The map calls are gone from the callers, so Receiving calls receive here, Picking calls pick here, and Report calls quantity here. The sum over the queue is written once, in quantity, and pick reuses it for its check. The 15 picked came out of the oldest delivery of 20, so 5 of that one are left, next to the newer 10. The three callers depend only on receive, pick and quantity, so changing how Warehouse stores the stock stayed inside Warehouse.
Recap
Step 11: Where we started, where we ended Before / after
On the left, the stock is a public map, and Receiving, Picking and Report each call map methods on it. On the right, the map is private and holds a queue per product, behind receive, pick and quantity. A caller like Picking makes one call, and the warehouse decides which delivery the items come from.
Step 12: When to reach for it Takeaways
Reach for it when several classes read or change one collection, and the way it's stored may change. Skip it when one short function owns the map and nothing outside reads it, because methods around it add lines and hide nothing. You'll spot the need by get, put or merge on another object's collection, or by the same lookup copied into several classes. Java's own Array List works this way, because you call add and get, and it grows its hidden array by itself.