Count number of items with property

Question

I have list List where Custom is like

class Custom{
    public int id;
    public String name;
}

How to

Answer 1

What about this? :

package test;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;

public class CustomArrayBuilder extends ArrayList {

    Map namesMap = new HashMap();

    public CustomArrayBuilder(Collection c) {
        super(c);
        this.prepareAddAll(c);
    }

    public int getDifferentNamesAmount() {
        return this.namesMap.size();
    }

    public int getNameAmount(String name) {
        Integer integer = this.namesMap.get(name);
        return (integer != null) ? integer : 0;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Custom set(int index, Custom element) {
        Custom custom = super.set(index, element);
        prepareSet(custom, element);
        return custom;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean add(Custom e) {
        this.prepareAdd(e);
        return super.add(e);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void add(int index, Custom element) {
        this.prepareAdd(element);
        super.add(index, element);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Custom remove(int index) {
        Custom custom = super.remove(index);
        this.prepareRemove(custom);
        return custom;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void clear() {
        super.clear();
        this.namesMap.clear();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean addAll(Collection c) {
        this.prepareAddAll(c);
        return super.addAll(c);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean addAll(int index, Collection c) {
        this.prepareAddAll(c);
        return super.addAll(index, c);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean remove(Object o) {
        if (super.remove(o)) {
            this.prepareRemove((Custom) o);
            return true;
        } else {
            return false;
        }
    }

    private void prepareSet(Custom oldCustom, Custom newCustom) {
        if (oldCustom != null && !oldCustom.name.equals(newCustom.name)) {
            this.prepareRemove(oldCustom);
            this.prepareAdd(newCustom);
        }
    }

    private void prepareAdd(Custom custom) {
        if (custom != null) {
            Integer integer = this.namesMap.get(custom.name);
            this.namesMap.put(custom.name, (integer != null) ? integer + 1 : 1);
        }
    }

    private void prepareAddAll(Collection c) {
        for (Custom custom : c) {
            this.prepareAdd(custom);
        }
    }

    private void prepareRemove(Custom custom) {
        if (custom != null) {
            Integer integer = this.namesMap.get(custom.name);
            this.namesMap.put(custom.name, (integer != null && integer > 0) ? integer - 1 : 0);
        }
    }
}

Usage:

package test;

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

public class Test {

    public static void main(String[] args) {

        List list = new ArrayList() {{
            add(new Custom("A"));
            add(new Custom("B"));
            add(new Custom("C"));
            add(new Custom("A"));
            add(new Custom("A"));
            add(new Custom("B"));
        }};

        CustomArrayBuilder customs = new CustomArrayBuilder(list);
        Custom custom = new Custom("B");
        customs.add(custom);
        customs.add(custom);
        customs.remove(custom);
        customs.remove(custom);
        customs.remove(custom);

        System.out.println("A: " + customs.getNameAmount("A"));
        System.out.println("B: " + customs.getNameAmount("B"));
        System.out.println("C: " + customs.getNameAmount("C"));
        System.out.println("Z: " + customs.getNameAmount("Z"));
        System.out.println("Total different names: " + customs.getDifferentNamesAmount());
    }
}

Output:

A: 3
B: 2
C: 1
Z: 0
Total different names: 3

It could be usefull when you often use your count operations. Note: You shouldn't change name of custom object, it should be final:

package test;

class Custom {
    public int id;
    final public String name;

    public Custom(String name) {
        this.name = name;
    }
}

Or you must do something with list too, when you are changing name of some Custom object from the list.