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Java8 - Listにfilter,mapなどの高階関数を追加してみる例
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import java.util.ArrayList; | |
import java.util.Iterator; | |
import java.util.List; | |
import java.util.function.Function; | |
import java.util.function.Predicate; | |
// 任意のListにmap, filterメソッドを追加する。 | |
interface FunctionalList<T> extends List<T> { | |
default <R> FunctionalList<R> map(Function<T, R> f) { | |
FunctionalList<R> list; | |
try { | |
list = this.getClass().newInstance(); | |
} catch (InstantiationException | IllegalAccessException ex) { | |
throw new RuntimeException(ex); | |
} | |
for (Iterator<T> itr = this.iterator(); itr.hasNext();) { | |
list.add(f.apply(itr.next())); | |
} | |
return list; | |
} | |
default FunctionalList<T> filter(Predicate<T> pred) { | |
FunctionalList<T> list; | |
try { | |
list = this.getClass().newInstance(); | |
} catch (InstantiationException | IllegalAccessException ex) { | |
throw new RuntimeException(ex); | |
} | |
for (T t : this) { | |
if (pred.test(t)) { | |
list.add(t); | |
} | |
} | |
return list; | |
} | |
} | |
/** | |
* デフォルトメソッドとしてインターフェースに処理を切り出すことで、 | |
* オーバライド扶養でArrayList, LinkedListなど好きなリストに実装を追加できる。 | |
* が、実際にはそんなに使わないかも。streamと違って即時評価だし。 | |
*/ | |
public class FunctionalListImpl<T> extends ArrayList<T> implements FunctionalList<T>{ | |
// test | |
public static void main(String[] args) { | |
FunctionalList<String> list = new FunctionalListImpl<>(); | |
list.add("123"); | |
list.add("456"); | |
list.add("12"); | |
list.add("45"); | |
list.add("10000"); | |
FunctionalList<Integer> result = list.filter(s -> s.length() > 2) | |
.map(Integer::parseInt); | |
System.out.println(result); | |
// 通常のstreamも併用可能。 | |
System.out.println(result.stream().reduce(1, (a,b)-> a * b)); | |
} | |
} |
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