#355 document, abstract base, traits and example domain

abstract-document/src/main/java/com/iluwatar/abstractdocument/AbstractDocument.java

@@ -0,0 +1,38 @@
+package com.iluwatar.abstractdocument;
+
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.function.Function;
+import java.util.stream.Stream;
+
+public abstract class AbstractDocument implements Document {
+
+	private final Map<String, Object> properties;
+
+	protected AbstractDocument(Map<String, Object> properties) {
+		Objects.requireNonNull(properties, "properties map is required");
+		this.properties = properties;
+	}
+
+	@Override
+	public Void put(String key, Object value) {
+		properties.put(key, value);
+		return null;
+	}
+
+	@Override
+	public Object get(String key) {
+		return properties.get(key);
+	}
+
+	@Override
+	public <T> Stream<T> children(String key, Function<Map<String, Object>, T> constructor) {
+		return Stream.of(get(key))
+				.filter(el -> el != null)
+				.map(el -> (List<Map<String, Object>>) el)
+				.findFirst().get().stream()
+				.map(constructor);
+	}
+
+}