Return empty JSON array for easier processing

acme4j-client/src/main/java/org/shredzone/acme4j/Authorization.java

@@ -272,7 +272,7 @@ public class Authorization extends AcmeResource {
      */
     private List<List<Challenge>> fetchCombinations(JSON json, List<Challenge> challenges) {
         JSON.Array jsonCombinations = json.get("combinations").asArray();
-        if (jsonCombinations == null) {
+        if (jsonCombinations.isEmpty()) {
             return Arrays.asList(challenges);
         }
 

acme4j-client/src/main/java/org/shredzone/acme4j/Metadata.java

@@ -17,10 +17,8 @@ import static java.util.stream.Collectors.toList;
 
 import java.net.URI;
 import java.util.Collection;
-import java.util.Collections;
 
 import org.shredzone.acme4j.util.JSON;
-import org.shredzone.acme4j.util.JSON.Array;
 import org.shredzone.acme4j.util.JSON.Value;
 
 /**
@@ -61,12 +59,9 @@ public class Metadata {
      * itself for the purposes of CAA record validation. Empty if not available.
      */
     public Collection<String> getCaaIdentities() {
-        Array array = meta.get("caa-identities").asArray();
+        return meta.get("caa-identities").asArray().stream()
-        if (array == null) {
+                .map(Value::asString)
-            return Collections.emptyList();
+                .collect(toList());
-        }
-
-        return array.stream().map(Value::asString).collect(toList());
     }
 
     /**

acme4j-client/src/main/java/org/shredzone/acme4j/connector/ResourceIterator.java

@@ -157,12 +157,9 @@ public class ResourceIterator<T extends AcmeResource> implements Iterator<T> {
      *            JSON map to read from
      */
     private void fillUriList(JSON json) {
-        JSON.Array array = json.get(field).asArray();
+        json.get(field).asArray().stream()
-        if (array == null) {
+                .map(JSON.Value::asURI)
-            return;
+                .forEach(uriList::add);
-        }
-
-        array.stream().map(JSON.Value::asURI).forEach(uriList::add);
     }
 
 }

acme4j-client/src/main/java/org/shredzone/acme4j/util/JSON.java

@@ -207,6 +207,13 @@ public final class JSON implements Serializable {
             return data.size();
         }
 
+        /**
+         * Returns {@code true} if the array is empty.
+         */
+        public boolean isEmpty() {
+            return data.isEmpty();
+        }
+
         /**
          * Gets the {@link Value} at the given index.
          *
@@ -298,11 +305,11 @@ public final class JSON implements Serializable {
         /**
          * Returns the value as JSON array.
          *
-         * @return {@link JSON.Array}, or {@code null} if the value was not set.
+         * @return {@link JSON.Array}, which is empty if the value was not set.
          */
         public Array asArray() {
             if (val == null) {
-                return null;
+                return new Array(path, Collections.emptyList());
             }
 
             try {

acme4j-client/src/test/java/org/shredzone/acme4j/util/JSONTest.java

@@ -102,12 +102,26 @@ public class JSONTest {
         JSON.Array array = json.get("array").asArray();
 
         assertThat(array.size(), is(4));
+        assertThat(array.isEmpty(), is(false));
         assertThat(array.get(0), is(notNullValue()));
         assertThat(array.get(1), is(notNullValue()));
         assertThat(array.get(2), is(notNullValue()));
         assertThat(array.get(3), is(notNullValue()));
     }
 
+    /**
+     * Test empty array.
+     */
+    @Test
+    public void testEmptyArray() {
+        JSON json = TestUtils.getJsonAsObject("json");
+        JSON.Array array = json.get("missingArray").asArray();
+
+        assertThat(array.size(), is(0));
+        assertThat(array.isEmpty(), is(true));
+        assertThat(array.stream().count(), is(0L));
+    }
+
     /**
      * Test all array iterator related methods.
      */