Question

I have list (array list)that can contain many instances (between 500-3000 instances ).
during the program some function need to access to this list (many times)and search for specific instance or more ,to get the instance\s they need loop on the list and provide parentName and name (which is string) and are not uniqe key .

my question is since the list need to be accessed many time there is a way to define/design it better that the access to the list can be more efficient?

Please keep in mind that the functions that need to get instance/s from the list
cannot provide full key the can provide only name and parentName which can have more that one instance.

List<Obj>   myList = new ArrayList<Obj>();

class obj
{
parentName
Name
type 
curr
....

Answer 1

Use a Map<MyEntry, List<Obj>> where MyEntry is a class enclosing parent name and name as such:

public final class MyEntry
{
    private final String parentName;
    private final String name;
    private final int hashCode;

    public MyEntry(final String parentName, final String name)
    {
        this.parentName = parentName;
        this.name = name;
        hashCode = 31 * parentName.hashCode() + name.hashCode();
    }

    // Override .equals() and .hashCode()
    @Override
    public int hashCode()
    {
        return hashCode;
    }

    @Override
    public boolean equals(final Object o)
    {
        if (this == o)
            return true;
        if (o == null)
            return false;
        if (getClass() != o.getClass())
            return false;
        final MyEntry other = (MyEntry) o;
        return parentName.equals(other.parentName)
            && name.equals(other.name);
    }

    // Have a nice string representation
    @Override
    public String toString()
    {
        return "parent name: " + parentName + ", name: " + name;
    }
}

You can, for instance, have a method in your Obj which returns the matching MyEntry object. Also, if you use Guava, have a look at MultiMap.

You will notice that the hash code is precomputed: this can be done since the MyEntry class is immutable. This allows for very fast usage as keys for a Map.

(edit: added .toString())