#653 - Polishing.

Expose nullability in Identifiable, ResourceSupport and Links via Optional.

src/main/java/org/springframework/hateoas/Identifiable.java

@@ -16,6 +16,7 @@
 package org.springframework.hateoas;
 
 import java.io.Serializable;
+import java.util.Optional;
 
 /**
  * Interface to mark objects that are identifiable by an ID of any type.
@@ -27,7 +28,7 @@ public interface Identifiable<ID extends Serializable> {
 	/**
 	 * Returns the id identifying the object.
 	 * 
-	 * @return the identifier or {@literal null} if not available.
+	 * @return the identifier or {@link Optional#empty()} if not available.
 	 */
-	ID getId();
+	Optional<ID> getId();
 }

src/main/java/org/springframework/hateoas/PagedResources.java

@@ -18,6 +18,7 @@ package org.springframework.hateoas;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Optional;
 
 import javax.xml.bind.annotation.XmlAttribute;
 import javax.xml.bind.annotation.XmlRootElement;
@@ -105,7 +106,7 @@ public class PagedResources<T> extends Resources<T> {
 	 * @return
 	 */
 	@JsonIgnore
-	public Link getNextLink() {
+	public Optional<Link> getNextLink() {
 		return getLink(Link.REL_NEXT);
 	}
 
@@ -115,7 +116,7 @@ public class PagedResources<T> extends Resources<T> {
 	 * @return
 	 */
 	@JsonIgnore
-	public Link getPreviousLink() {
+	public Optional<Link> getPreviousLink() {
 		return getLink(Link.REL_PREVIOUS);
 	}
 
@@ -251,8 +252,8 @@ public class PagedResources<T> extends Resources<T> {
 		 */
 		@Override
 		public String toString() {
-			return String.format("Metadata { number: %d, total pages: %d, total elements: %d, size: %d }", number,
-					totalPages, totalElements, size);
+			return String.format("Metadata { number: %d, total pages: %d, total elements: %d, size: %d }", number, totalPages,
+					totalElements, size);
 		}
 
 		/* 

src/main/java/org/springframework/hateoas/ResourceSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2017 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,7 +18,8 @@ package org.springframework.hateoas;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
-import java.util.stream.Collectors;
+import java.util.Optional;
+import java.util.stream.Collectors;
 
 import javax.xml.bind.annotation.XmlElement;
 
@@ -37,14 +38,14 @@ public class ResourceSupport implements Identifiable<Link> {
 	private final List<Link> links;
 
 	public ResourceSupport() {
-		this.links = new ArrayList<>();
+		this.links = new ArrayList<>();
 	}
 
 	/**
 	 * Returns the {@link Link} with a rel of {@link Link#REL_SELF}.
 	 */
 	@JsonIgnore
-	public Link getId() {
+	public Optional<Link> getId() {
 		return getLink(Link.REL_SELF);
 	}
 
@@ -65,7 +66,7 @@ public class ResourceSupport implements Identifiable<Link> {
 	 */
 	public void add(Iterable<Link> links) {
 		Assert.notNull(links, "Given links must not be null!");
-		links.forEach(this::add);
+		links.forEach(this::add);
 	}
 
 	/**
@@ -94,7 +95,7 @@ public class ResourceSupport implements Identifiable<Link> {
 	 * @return
 	 */
 	public boolean hasLink(String rel) {
-		return getLink(rel) != null;
+		return getLink(rel).isPresent();
 	}
 
 	/**
@@ -119,13 +120,23 @@ public class ResourceSupport implements Identifiable<Link> {
 	 * Returns the link with the given rel.
 	 * 
 	 * @param rel
-	 * @return the link with the given rel or {@literal null} if none found.
+	 * @return the link with the given rel or {@link Optional#empty()} if none found.
 	 */
-	public Link getLink(String rel) {
+	public Optional<Link> getLink(String rel) {
+		return getLinks(rel).stream().findFirst();
+	}
 
-		return getLinks(rel).stream()
-				.findFirst()
-				.orElse(null);
+	/**
+	 * Returns the link with the given rel.
+	 * 
+	 * @param rel
+	 * @return the link with the given rel.
+	 * @throws IllegalArgumentException in case no link with the given rel can be found.
+	 */
+	public Link getRequiredLink(String rel) {
+
+		return getLink(rel) //
+				.orElseThrow(() -> new IllegalArgumentException(String.format("No link with rel %s found!", rel)));
 	}
 
 	/**
@@ -135,9 +146,8 @@ public class ResourceSupport implements Identifiable<Link> {
 	 */
 	public List<Link> getLinks(String rel) {
 
-		return links.stream()
-				.filter(link -> link.getRel().equals(rel))
-				.collect(Collectors.toList());
+		return links.stream() //
+				.filter(link -> link.getRel().equals(rel)).collect(Collectors.toList());
 	}
 
 	/* 

src/main/java/org/springframework/hateoas/core/LinkBuilderSupport.java

@@ -19,6 +19,7 @@ import static org.springframework.hateoas.core.EncodingUtils.*;
 import static org.springframework.web.util.UriComponentsBuilder.*;
 
 import java.net.URI;
+import java.util.Optional;
 
 import org.springframework.hateoas.Identifiable;
 import org.springframework.hateoas.Link;
@@ -68,6 +69,8 @@ public abstract class LinkBuilderSupport<T extends LinkBuilder> implements LinkB
 	 */
 	public T slash(Object object) {
 
+		object = Optional.class.isInstance(object) ? ((Optional<?>) object).orElse(null) : object;
+
 		if (object == null) {
 			return getThis();
 		}