Refuse empty strings in Wikidata schema

extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringConstant.java

@@ -37,6 +37,8 @@ public class WbStringConstant implements WbExpression<StringValue> {
     @JsonCreator
     public WbStringConstant(@JsonProperty("value") String value) {
         Validate.notNull(value);
+        Validate.isTrue(!value.isEmpty()); // for now we don't accept empty strings
+        // because in the variable counterpart of this expression, they are skipped
         this.value = value;
     }
 
@@ -49,4 +51,17 @@ public class WbStringConstant implements WbExpression<StringValue> {
     public String getValue() {
         return value;
     }
+    
+    @Override
+    public boolean equals(Object other) {
+        if(other == null || !WbStringConstant.class.isInstance(other)) {
+            return false;
+        }
+        return value.equals(((WbStringConstant)other).getValue());
+    }
+    
+    @Override
+    public int hashCode() {
+        return value.hashCode();
+    }
 }

extensions/wikidata/tests/src/org/openrefine/wikidata/schema/WbStringConstantTest.java

@@ -0,0 +1,27 @@
+package org.openrefine.wikidata.schema;
+
+import org.openrefine.wikidata.testing.JacksonSerializationTest;
+import org.testng.annotations.Test;
+import org.wikidata.wdtk.datamodel.helpers.Datamodel;
+import org.wikidata.wdtk.datamodel.interfaces.StringValue;
+
+public class WbStringConstantTest extends WbExpressionTest<StringValue> {
+    
+    private WbStringConstant constant = new WbStringConstant("hello world");
+    
+    @Test
+    public void testSerialize() {
+        JacksonSerializationTest.canonicalSerialization(WbExpression.class, constant,
+                "{\"type\":\"wbstringconstant\",\"value\":\"hello world\"}");
+    }
+    
+    @Test
+    public void testEvaluate() {
+        evaluatesTo(Datamodel.makeStringValue("hello world"), constant);
+    }
+    
+    @Test(expectedExceptions = IllegalArgumentException.class)
+    public void testEmpty() {
+        new WbStringConstant("");
+    }
+}