Use IdentityHashSet for cycle detection

Fixes https://github.com/stleary/JSON-java/issues/650

src/main/java/org/json/JSONObject.java

@@ -37,9 +37,10 @@ import java.lang.reflect.Modifier;
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.Enumeration;
 import java.util.HashMap;
-import java.util.HashSet;
+import java.util.IdentityHashMap;
 import java.util.Iterator;
 import java.util.Locale;
 import java.util.Map;
@@ -1526,7 +1527,7 @@ public class JSONObject {
      *            the bean
      */
     private void populateMap(Object bean) {
-        populateMap(bean, new HashSet<Object>());
+        populateMap(bean, Collections.newSetFromMap(new IdentityHashMap<Object, Boolean>()));
     }
 
     private void populateMap(Object bean, Set<Object> objectsRecord) {

src/test/java/org/json/junit/JSONObjectTest.java

@@ -74,6 +74,7 @@ import org.json.junit.data.MyNumber;
 import org.json.junit.data.MyNumberContainer;
 import org.json.junit.data.MyPublicClass;
 import org.json.junit.data.RecursiveBean;
+import org.json.junit.data.RecursiveBeanEquals;
 import org.json.junit.data.Singleton;
 import org.json.junit.data.SingletonEnum;
 import org.json.junit.data.WeirdList;
@@ -3311,6 +3312,21 @@ public class JSONObjectTest {
         new JSONObject(ObjD);
         new JSONObject(ObjE);
     }
+    @Test(expected=JSONException.class)
+    public void testRecursiveEquals() {
+        RecursiveBeanEquals a = new RecursiveBeanEquals("same");
+        a.setRef(a);
+        new JSONObject(a);
+    }
+    @Test
+    public void testNotRecursiveEquals() {
+        RecursiveBeanEquals a = new RecursiveBeanEquals("same");
+        RecursiveBeanEquals b = new RecursiveBeanEquals("same");
+        RecursiveBeanEquals c = new RecursiveBeanEquals("same");
+        a.setRef(b);
+        b.setRef(c);
+        new JSONObject(a);
+    }
 
 
     @Test

src/test/java/org/json/junit/data/RecursiveBeanEquals.java

@@ -0,0 +1,33 @@
+package org.json.junit.data;
+
+/** test class for verifying if recursively defined bean can be correctly identified */
+public class RecursiveBeanEquals {
+  private final String name;
+  private Object reference;
+
+  public RecursiveBeanEquals(String name) {
+    this.name = name;
+  }
+
+  public String getName() {
+    return name;
+  }
+
+  public Object getRef() {
+    return reference;
+  }
+
+  public void setRef(Object refObj) {
+    reference = refObj;
+  }
+
+  @Override
+  public boolean equals(Object other) {
+    return other instanceof RecursiveBeanEquals && name.equals(((RecursiveBeanEquals) other).name);
+  }
+
+  @Override
+  public int hashCode() {
+    return name.hashCode();
+  }
+}