Generics サンプル

generics class sample

paramをgenericsで定義
instance化する際に、concreteなclassを指定する

Putlog.java

public class Putlog <T>{
    private T param;
    public Putlog(T p1) {
        this.param = p1;
    }
    public void setPlog(T p1) {
        this.param = p1;
    }
    public void outlog() {
        System.out.println(param);
    }
}

sample.java

    public static void main(String[] args) {
        Putlog p = new Putlog("a1");
        p.outlog();
        p = new Putlog(1);
        p.outlog();
    }

実行結果

a1
1

generics methodは、return valueの前に型パラメータを置く

    static <T> Optional<T> get(T[] array, int index) {
        if(array == null) {
            return Optional.empty();
        } else {
            try {
                return Optional.ofNullable(array[index]);
            } catch (ArrayIndexOutOfBoundsException ex) {
                return Optional.empty();
            }
        }
    }