add abstract class to read in all files

src/lib/src/main/java/gov/usgs/earthquake/nshmp/netcdf/NetcdfDataFiles.java

+package gov.usgs.earthquake.nshmp.netcdf;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import gov.usgs.earthquake.nshmp.gmm.NehrpSiteClass;
+
+/**
+ * Read in all NetCDF files associated with a {@link NetcdfDataType},
+ * holds {@link Netcdf} objects associated with each NetCDF file.
+ *
+ * @author U.S. Geological Survey
+ */
+public abstract class NetcdfDataFiles<T extends Netcdf<?>> extends ArrayList<T> {
+  private final Path netcdfPath;
+  private final NetcdfDataType dataType;
+
+  public NetcdfDataFiles(Path netcdfPath, NetcdfDataType dataType) {
+    super();
+    this.netcdfPath = netcdfPath;
+    this.dataType = dataType;
+  }
+
+  /**
+   * Returns the path to the directory where all NetCDF files are.
+   */
+  public Path netcdfPath() {
+    return netcdfPath;
+  }
+
+  /**
+   * Returns the data type of the NetCDF files.
+   */
+  public NetcdfDataType dataType() {
+    return dataType;
+  }
+
+  /**
+   * Returns the set of NEHRP site classes for all data files.
+   */
+  public Set<NehrpSiteClass> siteClasses() {
+    return stream()
+        .flatMap(netcdf -> netcdf.netcdfData().siteClasses().stream())
+        .sorted()
+        .collect(Collectors.toSet());
+  }
+
+  /**
+   * Find NetCDF files (.nc) with specific {@link NetcdfDataType}.
+   *
+   * @param netcdfPath Path to NetCDF files
+   * @param dataType The data type to filter by
+   * @throws IOException
+   */
+  protected Stream<Path> walkFiles(Path netcdfPath, NetcdfDataType dataType) throws IOException {
+    return Files.walk(netcdfPath)
+        .filter(file -> file.getFileName().toString().endsWith(".nc"))
+        .filter(file -> NetcdfDataType.getDataType(file) == dataType);
+  }
+
+  /**
+   * Returns the list of {@link Netcdf} objects associated with a NetCDF file
+   * and {@link NetcdfDataType}.
+   *
+   * @param netcdfPath The path to NetCDF files
+   * @param dataType The data type to read in
+   * @throws IOException
+   */
+  protected abstract List<T> readFiles(Path netcdfPath, NetcdfDataType dataType);
+}