The left hand side must be a variable (ArrayList and Array)

I'm a little confused between an ArrayList and an Array, I understand what a variable is but I don't understand why I can't convert the ArrayList to an Array and have the same effect.

This piece of code I currently have when using an ArrayList

import java.util.Objects;

import com.google.gson.Gson;
import com.google.gson.JsonObject;


public final class NpcDefinitionLoader extends JsonLoader {

    /**
     * Creates a new {@link NpcDefinitionLoader}.
     */
    public NpcDefinitionLoader() {
        super(DefinitionEditor.fileChooser.getSelectedFile().getAbsolutePath());
    }

    @Override
    public void load(JsonObject reader, Gson builder) {
        int index = reader.get("id").getAsInt();
        String name = Objects.requireNonNull(reader.get("name").getAsString());
        String description = Objects.requireNonNull(reader.get("examine").getAsString());
        int combatLevel = reader.get("combat").getAsInt();
        int size = reader.get("size").getAsInt();
        boolean attackable = reader.get("attackable").getAsBoolean();
        boolean aggressive = reader.get("aggressive").getAsBoolean();
        boolean retreats = reader.get("retreats").getAsBoolean();
        boolean poisonous = reader.get("poisonous").getAsBoolean();
        int respawnTime = reader.get("respawn").getAsInt();
        int maxHit = reader.get("maxHit").getAsInt();
        int hitpoints = reader.get("hitpoints").getAsInt();
        int attackSpeed = reader.get("attackSpeed").getAsInt();
        int attackAnim = reader.get("attackAnim").getAsInt();
        int defenceAnim = reader.get("defenceAnim").getAsInt();
        int deathAnim = reader.get("deathAnim").getAsInt();
        int attackBonus = reader.get("attackBonus").getAsInt();
        int meleeDefence = reader.get("defenceMelee").getAsInt();
        int rangedDefence = reader.get("defenceRange").getAsInt();
        int magicDefence = reader.get("defenceMage").getAsInt();

        NPCDefinitions.definitions.get(index) = new NPCDefinitions(index, name, description, combatLevel, size, attackable, aggressive, retreats,
            poisonous, respawnTime, maxHit, hitpoints, attackSpeed, attackAnim, defenceAnim, deathAnim, attackBonus, meleeDefence,
            rangedDefence, magicDefence);
    }
}

I get an error here

NPCDefinitions.definitions.get(index) = new NPCDefinitions(index, name, description, combatLevel, size, attackable, aggressive, retreats,
        poisonous, respawnTime, maxHit, hitpoints, attackSpeed, attackAnim, defenceAnim, deathAnim, attackBonus, meleeDefence,
        rangedDefence, magicDefence);

Saying the left side must be a variable. When I use an Array like this

NPCDefinitions.definitions[index] = new NPCDefinitions(index, name, description, combatLevel, size, attackable, aggressive, retreats,
        poisonous, respawnTime, maxHit, hitpoints, attackSpeed, attackAnim, defenceAnim, deathAnim, attackBonus, meleeDefence,
        rangedDefence, magicDefence);

Code works perfect.

I'm confused about this, how would I make this work by using an ArrayList?