jMonkeyEngine-wiki/docs/modules/contributions/pages/es/entitysystem/beginner.adoc

= Zay-ES Beginner Tutorial
:revnumber: 2.0
:revdate: 2020/07/25


In this article we explain the first steps how to work with Zay-ES and what you should know about the background.

If you are new Entity Systems please read the xref:es/entitysystem/introduction.adoc[Entity System Introduction] first.


== Sample Code

[source,java]
----

import com.simsilica.es.Entity;
import com.simsilica.es.EntityData;
import com.simsilica.es.EntityId;
import com.simsilica.es.base.DefaultEntityData;

public class Main {

    public static void main(String[] args) {

        //Creating the EntityData
        EntityData entityData = new DefaultEntityData();

        //Creates a new EntityId, the id is handled as an object to prevent botching
        EntityId entityId = entityData.createEntity();
        //A new TestComponent is added to the Entity
        entityData.setComponent(entityId, new TestComponent("Hello World"));

        //Get a new Entity Object with TestComponents
        Entity entity = entityData.getEntity(entityId, TestComponent.class);
        //Get the Component and display the value
        TestComponent testComponent = entity.get(TestComponent.class);
        System.out.println(testComponent.getValue());

        //Overwrite the existing component
        entity.set(new TestComponent("New Value"));
        System.out.println(testComponent.getValue());

        //Remove the Entity from the data
        entityData.removeEntity(entity.getId());
    }
}

----


=== Description of the sample


== Create a Component Class

You simply create a component by implementing the EntityComponent interface:

[source,java]
----
public class TestComponent implements EntityComponent {

    private String value;

    public TestComponent(String value)
    {
        this.value=value;
    }

    public String getValue()
    {
        return value;
    }
}
----

But there are some rules you must consider:
Components only have a constructor and getter.
It is important that you only store pure data in the components because otherwise you would no longer know where you can find the logic and the approach of a clear software design would get lost.
Besides an entity can only have one component of the same class.

[IMPORTANT]
====

Beginners tend to add large objects like spatials and geometries to a component.
This is a big mistake because such objects contain logic and in our approach components are data only. Abstract it to a general level or store it completely in the systems.
====



[IMPORTANT]
====

Never subclass Component classes.
====



== Initialize the Entity System

[source,java]
----
 EntityData entityData = new DefaultEntityData();
----

The EntityData is the main class of the entity system.  All the data are stored here.

How the entityData works:

For every component class a new Hashmap is created which contains the entityId as a key and the component as a value. Therefore, if you need to know all entities which own a component of a certain type, the system will search in these Hashmaps for the required entityIds.


== Creating Entitys and adding Components

[source,java]
----
//Creates a new EntityId, the id is handled as an object to prevent botching
EntityId entityId = entityData.createEntity();
//A new TestComponent is added to the Entity
entityData.setComponent(entityId, new TestComponent("Hello World"));
----

EntityIds are a objects which contain a long value. Zay-ES uses this objects to prevent users from writing dirty code.

Every entity has a unique id which is a long.
Ids are not reused because, if they were, they would not be unique anymore, which would cause a huge penalty.


[TIP]
====

Often people are scared that they will run out of ids:
If you create a new entity every nano second, you would need roughly 585 years before it wraps.

====



== The Entity Class

[source,java]
----
//Get a new Entity Object with TestComponents
Entity entity = entityData.getEntity(entityId, TestComponent.class);
//Get the Component and display the value
TestComponent testComponent = entity.get(TestComponent.class);
System.out.println(testComponent.getValue());
----

If you want to work with entities, the EntityData is able to create Entity objects. This objects contains
all the Components of the classes you are interested in. In this example it is only the TestComponent.class.
You can have multiple Entity objects for the same entity.


[IMPORTANT]
====
The data of this Entity objects will not be updated if other classes change the components for this entity
====



== Replacing a component

[source,java]
----

//Overwrite the existing component
entity.set(new TestComponent("New Value"));
System.out.println(testComponent.getValue());
----


== Delete an entity

[source,java]
----

//Remove the Entity from the data
entityData.removeEntity(entity.getId());

----


=== Entity Sets

The most important feature of Zay-ES are the Entity Sets.
It is strongly recommended that you read the xref:es/entitysystem/entityset.adoc[Entity Set tutorial] after reading this article.


[IMPORTANT]
====
Read the xref:es/entitysystem/entityset.adoc[tutorial] about entity sets
====