回答編集履歴
2
Java8の書き方追加
test
CHANGED
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@@ -41,3 +41,23 @@
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41
41
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.collect(groupingBy(e -> e.getValue(), mapping(e -> e.getKey(), toList())));
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42
42
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43
43
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```
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44
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+
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45
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+
Java8ならこうなるか
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46
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+
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47
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+
```java
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48
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+
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49
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+
// import static java.util.stream.Collectors.*;
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50
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+
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51
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+
final int MAX = 10;
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52
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+
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53
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+
Map<Integer, List<Character>> map
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54
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+
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55
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= IntStream.rangeClosed('a', 'z')
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56
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+
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57
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.collect(toMap(c -> (char)c, c -> (int)(Math.random() * MAX + 1))
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58
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+
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59
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.entrySet().stream()
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60
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+
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61
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.collect(groupingBy(e -> e.getValue(), mapping(e -> e.getKey(), toList())));
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62
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+
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63
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```
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1
Streamでマッピング
test
CHANGED
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@@ -1,3 +1,5 @@
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1
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+
computeIfAbsentとかgetOrDefaultとかを活用
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2
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+
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1
3
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```java
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2
4
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3
5
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final int MAX = 10;
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@@ -21,3 +23,21 @@
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21
23
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}
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22
24
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23
25
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```
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26
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+
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27
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+
Java9以上ならStreamでこんな書き方もできるが、ここで使っているMap.entryじゃなくても2つの値をペアにしてオブジェクトを作れればなんでもいい
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28
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+
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29
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+
```java
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30
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+
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31
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+
// import static java.util.stream.Collectors.*;
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32
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+
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33
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+
final int MAX = 10;
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34
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+
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35
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+
Map<Integer, List<Character>> map
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36
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+
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37
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= IntStream.rangeClosed('a', 'z')
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38
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+
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39
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.map(c -> Map.entry((char)c, (int)(Math.random() * MAX + 1))
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40
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+
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41
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.collect(groupingBy(e -> e.getValue(), mapping(e -> e.getKey(), toList())));
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42
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+
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43
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+
```
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