java: Add sample nucleotide-count implementation

assignments/java/nucleotide-count/example.java

@@ -0,0 +1,43 @@
+import java.util.HashMap;
+import java.util.Map;
+
+public final class DNA {
+  private final String sequence;
+
+  public DNA(String sequence) {
+    this.sequence = sequence;
+  }
+
+  public int count(char base) {
+    if (isCountable(base))
+      throw new IllegalArgumentException(base + " is not a nucleotide");
+
+    try {
+      return nucleotideCounts().get(base);
+    } catch (NullPointerException e) {
+      return 0;
+    }
+  }
+
+  private static boolean isCountable(char base) {
+    final String COUNTABLE_NUCLEOTIDES = "ACGTU";
+    return COUNTABLE_NUCLEOTIDES.indexOf(base) == -1;
+  }
+
+  public Map<Character, Integer> nucleotideCounts() {
+    Map<Character, Integer> counts = emptyCounts();
+    for (char c : sequence.toCharArray()) {
+      counts.put(c, counts.get(c) + 1);
+    }
+    return counts;
+  }
+
+  private static Map<Character, Integer> emptyCounts() {
+    Map<Character, Integer> counts = new HashMap<Character, Integer>();
+    counts.put('A', 0);
+    counts.put('C', 0);
+    counts.put('T', 0);
+    counts.put('G', 0);
+    return counts;
+  }
+}