Add abstract class Tag, extend Tag with SingleTag and PairedTag

Complete inheritance task.

java-oop-ru/inheritance/src/main/java/exercise/PairedTag.java

@@ -2,8 +2,26 @@
 
 import java.util.Map;
 import java.util.List;
-import java.util.stream.Collectors;
 
 // BEGIN
+public class PairedTag extends Tag {
+    private final String body;
+    private final List<Tag> singleTags;
 
+    public PairedTag(String tag, Map<String, String> attributes, String body, List<Tag> singleTags) {
+        super(tag, attributes);
+        this.body = body;
+        this.singleTags = singleTags;
+    }
+
+    public String toString() {
+        var result = new StringBuilder(String.format("<%s", this.getTag()));
+        this.getAttributes().forEach((k, v) -> result.append(String.format(" %s=\"%s\"", k, v)));
+        result.append(">");
+        singleTags.forEach(tag -> result.append(tag.toString()));
+        result.append(body);
+        result.append(String.format("</%s>", this.getTag()));
+        return result.toString();
+    }
+}
 // END

java-oop-ru/inheritance/src/main/java/exercise/SingleTag.java

@@ -3,5 +3,16 @@
 import java.util.Map;
 
 // BEGIN
+public class SingleTag extends Tag{
+    public SingleTag(String tag, Map<String, String> attributes) {
+        super(tag, attributes);
+    }
 
+    public String toString() {
+        var result = new StringBuilder(String.format("<%s", this.getTag()));
+        this.getAttributes().forEach((k, v) -> result.append(String.format(" %s=\"%s\"", k, v)));
+        result.append(">");
+        return result.toString();
+    }
+}
 // END

java-oop-ru/inheritance/src/main/java/exercise/Tag.java

@@ -1,8 +1,25 @@
 package exercise;
 
-import java.util.stream.Collectors;
 import java.util.Map;
 
 // BEGIN
+public abstract class Tag {
+    private final String tag;
+    private final Map<String, String> attributes;
 
+    public Tag(String tag, Map<String, String> attributes) {
+        this.tag = tag;
+        this.attributes = attributes;
+    }
+
+    public abstract String toString();
+
+    public String getTag() {
+        return tag;
+    }
+
+    public Map<String, String> getAttributes() {
+        return attributes;
+    }
+}
 // END