Add convenience methods for fetching optional values

asdf-core/src/main/java/org/asdfformat/asdf/node/AsdfNode.java

@@ -7,6 +7,7 @@
 import java.time.Instant;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 
 
 /**
@@ -138,6 +139,34 @@ public interface AsdfNode extends Iterable<AsdfNode> {
      */
     AsdfNode get(AsdfNode key);
 
+    /**
+     * Get an optional mapping value as AsdfNode, indexed by String key.
+     * @param key mapping key
+     * @return value
+     */
+    Optional<AsdfNode> getOptional(String key);
+
+    /**
+     * Get an optional sequence value or mapping value as AsdfNode, indexed by long key.
+     * @param key sequence index or mapping key
+     * @return value
+     */
+    Optional<AsdfNode> getOptional(long key);
+
+    /**
+     * Get an optional mapping value as AsdfNode, indexed by boolean key.
+     * @param key mapping key
+     * @return value
+     */
+    Optional<AsdfNode> getOptional(boolean key);
+
+    /**
+     * Get an optional mapping value as AsdfNode, indexed by AsdfNode key.
+     * @param key mapping key
+     * @return value
+     */
+    Optional<AsdfNode> getOptional(AsdfNode key);
+
     /**
      * Get a NUMBER mapping value as BigDecimal, indexed by String key.
      * @param key mapping key

asdf-core/src/main/java/org/asdfformat/asdf/node/impl/AsdfNodeBase.java

@@ -11,6 +11,7 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
+import java.util.Optional;
 
 public abstract class AsdfNodeBase implements AsdfNode {
 
@@ -99,6 +100,42 @@ public AsdfNode get(final AsdfNode key) {
         throw new IllegalStateException(makeGetErrorMessage("AsdfNode"));
     }
 
+    @Override
+    public Optional<AsdfNode> getOptional(final String key) {
+        if (containsKey(key)) {
+            return Optional.of(get(key));
+        } else {
+            return Optional.empty();
+        }
+    }
+
+    @Override
+    public Optional<AsdfNode> getOptional(final long key) {
+        if (containsKey(key)) {
+            return Optional.of(get(key));
+        } else {
+            return Optional.empty();
+        }
+    }
+
+    @Override
+    public Optional<AsdfNode> getOptional(final boolean key) {
+        if (containsKey(key)) {
+            return Optional.of(get(key));
+        } else {
+            return Optional.empty();
+        }
+    }
+
+    @Override
+    public Optional<AsdfNode> getOptional(final AsdfNode key) {
+        if (containsKey(key)) {
+            return Optional.of(get(key));
+        } else {
+            return Optional.empty();
+        }
+    }
+
     @Override
     public BigDecimal getBigDecimal(final String key) {
         return get(key).asBigDecimal();