回答編集履歴
3
ソースコードの不備(import)を修正
test
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@@ -4,13 +4,23 @@
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```Java
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import java.util.ArrayList;
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import java.util.Arrays;
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import java.util.Comparator;
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import java.util.
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import java.util.List;
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import java.util.function.BiFunction;
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import java.util.function.Consumer;
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import java.util.function.Function;
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import java.util.stream.Collectors;
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import java.util.stream.IntStream;
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@@ -45,6 +55,18 @@
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.findFirst()
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.orElse(0);
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static Function<Integer,Comparator<long[]>> genComparatorChain = size ->
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IntStream.range(0,size)
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.mapToObj(i -> Comparator.<long[],Long>comparing(x -> x[i]))
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.reduce((a,b) -> 0, Comparator::thenComparing);
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static Comparator<long[]> comparator14 = genComparatorChain.apply(2);
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@@ -88,7 +110,9 @@
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dump_.accept(sorter.apply(comparator13,a));
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dump_.accept(sorter.apply(comparator12,a));
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dump_.accept(sorter.apply(comparator12,a));
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dump_.accept(sorter.apply(comparator14,a));
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}
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2
配列のサイズに合わせたComparator<long[]>(追記)
test
CHANGED
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@@ -131,3 +131,29 @@
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Comparator<long[]> comparator13 = Comparator.comparing(getElement.apply(0)).thenComparing(getElement.apply(1));
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```
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###
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**配列のサイズに合わせたComparator<long[]>(追記)**
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結論の３番目を変形して、配列のサイズに応じた`Comparator<long[]>`が作れることがわかりました。
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```Java
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Function<Integer,Comparator<long[]>> genComparatorChain = size ->
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IntStream.range(0,size)
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.mapToObj(i -> Comparator.<long[],Long>comparing(x -> x[i]))
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.reduce((a,b) -> 0, Comparator::thenComparing);
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Comparator<long[]> comparator14 = genComparatorChain.apply(2);
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```
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結論：Comparatorに限らず、Functionインターフェイスでもreduce()を使って、compose()、andThen()をチェインできることがわかり有益でした。
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1
結論(追記)
test
CHANGED
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@@ -25,6 +25,26 @@
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static Comparator<long[]> comparator3 = Comparator.<long[], Long>comparing(v -> v[0]).thenComparing(v -> v[1]);
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static Comparator<long[]> comparator4 = Comparator.<long[], Long>comparing(v -> v[0]).thenComparingLong(v -> v[1]);
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static Function<Integer,Function<long[],Long>> getElement = i -> (array) -> array[i];
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static Comparator<long[]> comparator13 = Comparator.comparing(getElement.apply(0)).thenComparing(getElement.apply(1));
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static Comparator<long[]> comparator12 = (x,y) ->
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IntStream.range(0, Math.min(x.length,y.length))
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.map(i -> Long.compare(x[i], y[i]))
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.dropWhile(i -> i == 0)
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.findFirst()
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.orElse(0);
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@@ -64,10 +84,50 @@
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dump_.accept(sorter.apply(comparator3,a));
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dump_.accept(sorter.apply(comparator4,a));
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dump_.accept(sorter.apply(comparator4,a));
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dump_.accept(sorter.apply(comparator13,a));
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dump_.accept(sorter.apply(comparator12,a));
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}
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}
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```
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###
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**結論(追記)**
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103
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104
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`thenComparing`のkyExtractorの型が推論されない問題の解決として、以下の３つの方法がありました。
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- Comparator.comparingに指定するラムダ式の、引数に型`long[]`を指定する
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- staticメソッドComparator.comparingにkeyExtractorの型`<long[], Long>`を指定する
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- あらかじめkeyExtractor関数`Function<long[],Long>`を定義しておき渡す。
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```Java
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118
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// 1
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Comparator.comparing((long[] v) -> v[0]).thenComparing(v -> v[1]);
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// 2
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Comparator.<long[], Long>comparing(v -> v[0]).thenComparing(v -> v[1]);
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// 3
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Function<Integer,Function<long[],Long>> getElement = i -> (array) -> array[i];
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Comparator<long[]> comparator13 = Comparator.comparing(getElement.apply(0)).thenComparing(getElement.apply(1));
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```
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