[JAVA] How to sort the List of SelectItem

A List of SelectItems that are often used to set choices. You may want to sort by using the value obtained from the database. For such a case.

--Environment - CentOS Linux release 7.8.2003 (Core) - openjdk version "11.0.7" 2020-04-14 LTS - JSF 2.3.9

For example, when there is a List of SelectItem like this

/**List of SelectItems. */
@Getter
private List<SelectItem> items;

/**Set a list of SelectItems. */
private void setItems() {
    this.items = new ArrayList<SelectItem>();
    this.items.add(new SelectItem(1, "Dog"));
    this.items.add(new SelectItem(0, "Bear"));
    this.items.add(new SelectItem(3, "Monkey"));
    this.items.add(new SelectItem(2, "Cat"));
}

Sort in ascending order of value

Reference: [Collections (Java Platform SE 8)](https://docs.oracle.com/javase/jp/8/docs/api/java/util/Collections.html#sort-java.util.List-java. util.Comparator-)

Collections.sort(this.items, new Comparator<SelectItem>() {
    @Override
    public int compare(SelectItem item1, SelectItem item2) {
        if (item1 != null && item2 != null) {
            if (item1.getValue() != null && item2.getValue() != null) {
                return item1.getValue().toString().compareTo(item2.getValue().toString());
            }
        }
        return 0;
    }
});

(Bonus) Get the minimum value of value

SelectItem minItem = Collections.min(this.items, new Comparator<SelectItem>() {
    //The content is the same as sorting
});
var min = Integer.valueOf(minItem.getValue().toString());

Sort by descending value

Reference: Comparator (Java Platform SE 8)

//I just reversed item1 and item2 in the ascending return
Collections.sort(this.items, new Comparator<SelectItem>() {
    @Override
    public int compare(SelectItem item1, SelectItem item2) {
        if (item1 != null && item2 != null) {
            if (item1.getValue() != null && item2.getValue() != null) {
                return item2.getValue().toString().compareTo(item1.getValue().toString());
            }
        }
        return 0;
    }
});

Sort by label ascending order

Reference: Sort the list of hiragana and alphabets --Qiita

Comparator.How to use comparing

this.items = this.items.stream().sorted(Comparator.comparing(SelectItem::getLabel)).collect(Collectors.toList());

Collator.How to use getInstance

Collections.sort(this.items, new Comparator<SelectItem>() {
    @Override
    public int compare(SelectItem item1, SelectItem item2) {
        if (item1 != null && item2 != null) {
            if (item1.getLabel() != null && item2.getLabel() != null) {
                return Collator.getInstance(Locale.JAPANESE).compare(item1.getLabel(), item2.getLabel());
            }
        }
        return 0;
    }
});

Sort in descending order of label

Comparator.How to use comparing

// reversed()In descending order
this.items = this.items.stream().sorted(Comparator.comparing(SelectItem::getLabel).reversed()).collect(Collectors.toList());

Collator.How to use getInstance

//I just reversed item1 and item2 in the ascending return
Collections.sort(this.items, new Comparator<SelectItem>() {
    @Override
    public int compare(SelectItem item1, SelectItem item2) {
        if (item1 != null && item2 != null) {
            if (item1.getLabel() != null && item2.getLabel() != null) {
                return Collator.getInstance(Locale.JAPANESE).compare(item2.getLabel(), item1.getLabel());
            }
        }
        return 0;
    }
});

What failed when trying to sort

I found a useful thing called Comparator, so I tried to use it immediately and failed. A compilation error occurs around Comparator.comparing (SelectItem :: getValue). Since the value of SelectItem is Object, can't it be used as a sort key?

this.items = this.items.stream().sorted(Comparator.comparing(SelectItem::getValue)).collect(Collectors.toList());