mapをkey・value双方向から参照したい場合に以下のライブラリが使える。

GuavaのBiMap

<!-- https://mvnrepository.com/artifact/com.google.guava/guava -->
<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>22.0</version>
</dependency>

import com.google.common.collect.BiMap;
import com.google.common.collect.HashBiMap;

Map<String, String> m = new HashMap<>();
m.put("a", "あ");
m.put("i", "い");
m.put("u", "う");

BiMap<String, String> bimap = HashBiMap.create(m);
System.out.println(bimap.get("a"));//あ
System.out.println(bimap.inverse().get("あ"));//a

Apache Commons CollectionsのBidiMap

<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-collections4 -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-collections4</artifactId>
    <version>4.1</version>
</dependency>

import org.apache.commons.collections4.BidiMap;
import org.apache.commons.collections4.bidimap.DualHashBidiMap;

BidiMap<String, String> bidimap = new DualHashBidiMap<>(m);
System.out.println(bidimap.get("a"));//あ
System.out.println(bidimap.inverseBidiMap().get("あ"));//a