Fix code quality and follow design principles

src/main/java/seedu/address/logic/parser/ParserUtil.java

@@ -141,7 +141,7 @@ public static Instrument parseInstrument(String instrument) throws ParseExceptio
     }
 
     /**
-     * Parses {@code Collection<String> instruments} into a {@code Set<Instrument>}.
+     * Parses {@code Collection<String> instruments} into a {@code Set<Tag>}.
      */
     public static Set<Tag> parseInstruments(Collection<String> instruments) throws ParseException {
         requireNonNull(instruments);
@@ -168,7 +168,7 @@ public static Genre parseGenre(String genre) throws ParseException {
     }
 
     /**
-     * Parses {@code Collection<String> genres} into a {@code Set<Genre>}.
+     * Parses {@code Collection<String> genres} into a {@code Set<Tag>}.
      */
     public static Set<Tag> parseGenres(Collection<String> genres) throws ParseException {
         requireNonNull(genres);

src/main/java/seedu/address/logic/parser/musician/FindCommandParser.java

@@ -57,6 +57,14 @@ public FindCommand parse(String args) throws ParseException {
             throw new ParseException(FindCommand.MESSAGE_MORE_THAN_ONE_WORD);
         }
 
+        HashSet<Predicate<Musician>> predicates = this.buildPredicates(names, tags, instruments, genres);
+
+        return new FindCommand(predicates);
+    }
+
+    /** Builds a set of predicates based on the given lists of keywords. If a list is empty, skip over it. */
+    private HashSet<Predicate<Musician>> buildPredicates(List<String> names, List<String> tags,
+                                                         List<String> instruments, List<String> genres) {
         HashSet<Predicate<Musician>> predicates = new HashSet<>();
         if (!names.isEmpty()) {
             predicates.add(new NameContainsKeywordsPredicate(names));
@@ -70,8 +78,6 @@ public FindCommand parse(String args) throws ParseException {
         if (!genres.isEmpty()) {
             predicates.add(new GenreMatchesPredicate(genres));
         }
-
-        return new FindCommand(predicates);
+        return predicates;
     }
-
 }

src/main/java/seedu/address/model/tag/GenreMatchesPredicate.java

@@ -1,6 +1,7 @@
 package seedu.address.model.tag;
 
 import java.util.List;
+import java.util.function.Predicate;
 
 import seedu.address.commons.util.StringUtil;
 import seedu.address.commons.util.ToStringBuilder;
@@ -9,15 +10,16 @@
 /**
  * Tests that a {@code Musician}'s {@code Genre} tag matches any of the keywords given.
  */
-public class GenreMatchesPredicate extends TagMatchesPredicate {
+public class GenreMatchesPredicate implements Predicate<Musician> {
 
-    public GenreMatchesPredicate(List<String> genre) {
-        super(genre);
+    private final List<String> genres;
+
+    public GenreMatchesPredicate(List<String> genres) {
+        this.genres = genres;
     }
 
     @Override
     public boolean test(Musician musician) {
-        List<String> genres = super.getTagNames();
         return genres.stream()
                 .anyMatch(genreToMatch -> musician.getGenres().stream().anyMatch(
                         musicianGenre -> StringUtil.containsWordIgnoreCase(musicianGenre.tagName, genreToMatch)
@@ -35,17 +37,18 @@ public boolean equals(Object other) {
             return false;
         }
 
-        return super.equals(other);
+        GenreMatchesPredicate otherGenreMatchesPredicate = (GenreMatchesPredicate) other;
+        return genres.equals(otherGenreMatchesPredicate.genres);
     }
 
     @Override
     public int hashCode() {
-        return super.hashCode();
+        return genres.hashCode();
     }
 
     @Override
     public String toString() {
-        return new ToStringBuilder(this).add("genres", super.getTagNames()).toString();
+        return new ToStringBuilder(this).add("genres", genres).toString();
     }
 
 }

src/main/java/seedu/address/model/tag/InstrumentMatchesPredicate.java

@@ -1,6 +1,7 @@
 package seedu.address.model.tag;
 
 import java.util.List;
+import java.util.function.Predicate;
 
 import seedu.address.commons.util.StringUtil;
 import seedu.address.commons.util.ToStringBuilder;
@@ -9,15 +10,16 @@
 /**
  * Tests that a {@code Musician}'s {@code Instrument} tag matches any of the keywords given.
  */
-public class InstrumentMatchesPredicate extends TagMatchesPredicate {
+public class InstrumentMatchesPredicate implements Predicate<Musician> {
 
-    public InstrumentMatchesPredicate(List<String> instrument) {
-        super(instrument);
+    private final List<String> instruments;
+
+    public InstrumentMatchesPredicate(List<String> instruments) {
+        this.instruments = instruments;
     }
 
     @Override
     public boolean test(Musician musician) {
-        List<String> instruments = super.getTagNames();
         return instruments.stream()
                 .anyMatch(instrumentToMatch -> musician.getInstruments().stream().anyMatch(
                         musicianInstrument -> StringUtil.containsWordIgnoreCase(
@@ -36,17 +38,17 @@ public boolean equals(Object other) {
             return false;
         }
 
-        return super.equals(other);
+        InstrumentMatchesPredicate otherInstrumentMatchesPredicate = (InstrumentMatchesPredicate) other;
+        return instruments.equals(otherInstrumentMatchesPredicate.instruments);
     }
 
     @Override
     public int hashCode() {
-        return super.hashCode();
+        return instruments.hashCode();
     }
 
     @Override
     public String toString() {
-        return new ToStringBuilder(this).add("instruments", super.getTagNames()).toString();
+        return new ToStringBuilder(this).add("instruments", instruments).toString();
     }
-
 }

src/main/java/seedu/address/model/tag/TagMatchesPredicate.java

@@ -25,10 +25,6 @@ public boolean test(Musician musician) {
                 ));
     }
 
-    protected List<String> getTagNames() {
-        return tagNames;
-    }
-
     @Override
     public boolean equals(Object other) {
         if (other == this) {