Previously, we checked the adapter pattern, the use cases, and the bridge pattern. The pattern we will examine in this post is the composite pattern.

By following the composite pattern, we “compose” objects into tree structures, representing part-whole hierarchies. Therefore, clients can treat individual objects and compositions uniformly.

By referencing the word hierarchies, the first thing that comes to mind is organizations. A military organization is a great example of the composite pattern.

One of the most basic actions of military personnel is to execute orders. Thus, we will make an interface that specifies the ability to execute orders.

package com.gkatzioura.design.structural.composite;

public interface MilitaryPersonnel {

    void executeOrder();

}

The private is the lowest rank in a military organisation. A private cannot delegate a task or give orders. Therefore, they will execute an order.

package com.gkatzioura.design.structural.composite;

public class Private implements MilitaryPersonnel {

    @Override
    public void executeOrder() {

    }

}

Above the private, there are other ranks, like Major, Lieutenant, Colonel, etc. Those are officer ranks. Officers execute orders, but they can also assign orders. So, the Officer interface will specify the ability to assign orders.

package com.gkatzioura.design.structural.composite;

public interface Officer {

    void assignOrder();

}

Be aware that an officer, in order to execute an order, he will take some actions on his own, and he might as well assign some orders to lower rank personnel.

Next, the lieutenant will be able to execute orders and assign orders to ranks lower than him.

package com.gkatzioura.design.structural.composite;

import java.util.ArrayList;
import java.util.List;

public class Lieutenant implements MilitaryPersonnel, Officer {

    private List<MilitaryPersonnel> lowerRankersonel = new ArrayList<>();

    public Lieutenant(List<MilitaryPersonnel> lowerRankersonel) {
        this.lowerRankersonel = lowerRankersonel;
    }

    public void addPrivateUnderCommand(Private soldier) {
        lowerRankersonel.add(soldier);
    }

    @Override
    public void executeOrder() {

        //other actions

        assignOrder();

        //other actions.
    }

    @Override
    public void assignOrder() {

        lowerRankersonel.forEach(lr->lr.executeOrder());
    }
}

The same that applies to the lieutenant applies to the major, who is able to execute orders and assign orders to lower ranks.

package com.gkatzioura.design.structural.composite;

import java.util.ArrayList;
import java.util.List;

public class Colonel implements MilitaryPersonnel, Officer {

    private List<MilitaryPersonnel> lowerRankersonel = new ArrayList<>();

    public Colonel(List<MilitaryPersonnel> lowerRankersonel) {
        this.lowerRankersonel = lowerRankersonel;
    }

    public void addPrivateUnderCommand(Private soldier) {
        lowerRankersonel.add(soldier);
    }

    public void addLieutenantUnderCommand(Lieutenant lieutenant) {
        lowerRankersonel.add(lieutenant);
    }

    @Override
    public void executeOrder() {
        //other actions

        assignOrder();

        //other actions
    }

    @Override
    public void assignOrder() {
        lowerRankersonel.forEach(lr->lr.executeOrder());
    }
}

In our scenario, the general is the highest rank, and, thus, when he assigns an order, this order will be executed by the composite that we implemented.

package com.gkatzioura.design.structural.composite;

import java.util.ArrayList;
import java.util.List;

public class General implements  Officer {

    private List<MilitaryPersonnel> lowerRankersonel = new ArrayList<>();

    public General(List<MilitaryPersonnel> lowerRankersonel) {
        this.lowerRankersonel = lowerRankersonel;
    }

    @Override
    public void assignOrder(MilitaryPersonnel militaryPersonnel) {
        militaryPersonnel.executeOrder();
    }
}

And, a main class will display the composite’s functionality:

package com.gkatzioura.design.structural.composite;

import java.util.Collections;

public class CompositeScenario {

    public static void main(String[] args) {

        Private ryan = new Private();
        Lieutenant lieutenant = new Lieutenant(Collections.singletonList(ryan));
        Major major = new Major(Collections.singletonList(lieutenant));
        General general = new General();
        general.assignOrder(major);
    }
}

As you can see, the general object is the client treated as all the objects uniformly. The whole hierarchy is represented in a tree structure. The private is the leaf, and the major and the lieutenant represent the composite that forwards the requests to the corresponding child components.

You can find the source code here on GitHub.