Implement caching & fix minor bugs

src/main/java/at/pcgamingfreaks/mkvaudiosubtitlechanger/impl/Cache.java

@@ -0,0 +1,19 @@
+package at.pcgamingfreaks.mkvaudiosubtitlechanger.impl;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.function.Function;
+
+public class Cache<Key, Value> {
+    private final Map<Key, Value> cache = new HashMap<>();
+
+    /**
+     * Retrieve {@link Value} from Cache or run creationFunction and return its value.
+     * @param key key of cache map
+     * @param creationFunction function to create missing values
+     * @return {@link  Value} from Cache, or if missing result from creationFunction.
+     */
+    public synchronized Value retrieve(Key key, Function<Key, Value> creationFunction) {
+        return cache.computeIfAbsent(key, creationFunction::apply);
+    }
+}