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object-pool/README.md
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@ -10,16 +10,18 @@ tags:
---
## Intent
When objects are expensive to create and they are needed only for
short periods of time it is advantageous to utilize the Object Pool pattern.
The Object Pool provides a cache for instantiated objects tracking which ones
are in use and which are available.
When objects are expensive to create and they are needed only for short periods of time it is
advantageous to utilize the Object Pool pattern. The Object Pool provides a cache for instantiated
objects tracking which ones are in use and which are available.
## Explanation
Real world example
> In our war game we need to use oliphaunts, massive and mythic beasts, but the problem is that they are extremely expensive to create. The solution is to create a pool of them, track which ones are in-use, and instead of disposing them re-use the instances.
> In our war game we need to use oliphaunts, massive and mythic beasts, but the problem is that they
> are extremely expensive to create. The solution is to create a pool of them, track which ones are
> in-use, and instead of disposing them re-use the instances.
In plain words
@ -27,11 +29,12 @@ In plain words
Wikipedia says
> The object pool pattern is a software creational design pattern that uses a set of initialized objects kept ready to use – a "pool" – rather than allocating and destroying them on demand.
> The object pool pattern is a software creational design pattern that uses a set of initialized
> objects kept ready to use – a "pool" – rather than allocating and destroying them on demand.
**Programmatic Example**
Here's the basic Oliphaunt class. These are very expensive to create.
Here's the basic `Oliphaunt` class. These giants are very expensive to create.
```java
public class Oliphaunt {
@ -60,7 +63,7 @@ public class Oliphaunt {
}
```
Next we present the Object Pool and more specifically Oliphaunt Pool.
Next we present the `ObjectPool` and more specifically `OliphauntPool`.
```java
public abstract class ObjectPool<T> {
@ -100,7 +103,7 @@ public class OliphauntPool extends ObjectPool<Oliphaunt> {
}
```
And finally here's how we utilize the pool.
Finally, here's how we utilize the pool.
```java
var pool = new OliphauntPool();
@ -113,11 +116,30 @@ And finally here's how we utilize the pool.
var oliphaunt5 = pool.checkOut();
```
Program output:
```
Pool available=0 inUse=0
Checked out Oliphaunt id=1
Pool available=0 inUse=1
Checked out Oliphaunt id=2
Checked out Oliphaunt id=3
Pool available=0 inUse=3
Checking in Oliphaunt id=1
Checking in Oliphaunt id=2
Pool available=2 inUse=1
Checked out Oliphaunt id=2
Checked out Oliphaunt id=1
Pool available=0 inUse=3
```
## Class diagram
![alt text](./etc/object-pool.png "Object Pool")
## Applicability
Use the Object Pool pattern when
* The objects are expensive to create (allocation cost)
* You need a large number of short-lived objects (memory fragmentation)
* The objects are expensive to create (allocation cost).
* You need a large number of short-lived objects (memory fragmentation).

70
observer/README.md
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@ -10,17 +10,21 @@ tags:
---
## Also known as
Dependents, Publish-Subscribe
## Intent
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified
and updated automatically.
Define a one-to-many dependency between objects so that when one object changes state, all its
dependents are notified and updated automatically.
## Explanation
Real world example
> In a land far away lives the races of hobbits and orcs. Both of them are mostly outdoors so they closely follow the changes in weather. One could say that they are constantly observing the weather.
> In a land far away lives the races of hobbits and orcs. Both of them are mostly outdoors so they
> closely follow the changes in weather. One could say that they are constantly observing the
> weather.
In plain words
@ -28,11 +32,13 @@ In plain words
Wikipedia says
> The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
> The observer pattern is a software design pattern in which an object, called the subject,
> maintains a list of its dependents, called observers, and notifies them automatically of any state
> changes, usually by calling one of their methods.
**Programmatic Example**
Let's first introduce the weather observer interface and our races, orcs and hobbits.
Let's first introduce the `WeatherObserver` interface and our races, `Orcs` and `Hobbits`.
```java
public interface WeatherObserver {
@ -58,11 +64,12 @@ public class Hobbits implements WeatherObserver {
public void update(WeatherType currentWeather) {
switch (currentWeather) {
LOGGER.info("The hobbits are facing " + currentWeather.getDescription() + " weather now");
}
}
}
```
Then here's the weather that is constantly changing.
Then here's the `Weather` that is constantly changing.
```java
public class Weather {
@ -109,38 +116,47 @@ Here's the full example in action.
var weather = new Weather();
weather.addObserver(new Orcs());
weather.addObserver(new Hobbits());
weather.timePasses();
weather.timePasses();
weather.timePasses();
weather.timePasses();
```
weather.timePasses();
// The weather changed to rainy.
// The orcs are facing rainy weather now
// The hobbits are facing rainy weather now
weather.timePasses();
// The weather changed to windy.
// The orcs are facing windy weather now
// The hobbits are facing windy weather now
weather.timePasses();
// The weather changed to cold.
// The orcs are facing cold weather now
// The hobbits are facing cold weather now
weather.timePasses();
// The weather changed to sunny.
// The orcs are facing sunny weather now
// The hobbits are facing sunny weather now
Program output:
```
The weather changed to rainy.
The orcs are facing rainy weather now
The hobbits are facing rainy weather now
The weather changed to windy.
The orcs are facing windy weather now
The hobbits are facing windy weather now
The weather changed to cold.
The orcs are facing cold weather now
The hobbits are facing cold weather now
The weather changed to sunny.
The orcs are facing sunny weather now
The hobbits are facing sunny weather now
```
## Class diagram
![alt text](./etc/observer.png "Observer")
## Applicability
Use the Observer pattern in any of the following situations
* When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in separate objects lets you vary and reuse them independently
* When a change to one object requires changing others, and you don't know how many objects need to be changed
* When an object should be able to notify other objects without making assumptions about who these objects are. In other words, you don't want these objects tightly coupled
Use the Observer pattern in any of the following situations:
* When an abstraction has two aspects, one dependent on the other. Encapsulating these aspects in
separate objects lets you vary and reuse them independently.
* When a change to one object requires changing others, and you don't know how many objects need to
be changed.
* When an object should be able to notify other objects without making assumptions about who these
objects are. In other words, you don't want these objects tightly coupled.
## Typical Use Case
* Changing in one object leads to a change in other objects
* Changing in one object leads to a change in other objects.
## Real world examples

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observer/src/main/java/com/iluwatar/observer/generic/Observable.java
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@ -35,7 +35,7 @@ import java.util.concurrent.CopyOnWriteArrayList;
*/
public abstract class Observable<S extends Observable<S, O, A>, O extends Observer<S, O, A>, A> {
protected List<O> observers;
protected final List<O> observers;
public Observable() {
this.observers = new CopyOnWriteArrayList<>();