Verify map with custom values

I have the following map: ``` Map<String, MyCustomObject> ``` My goal is to verify capacity of this map using hamcrest matchers. I've tried the following approach: ``` assertThat(map, hasEntry("key", (MyCustomObject)hasItem(hasProperty("propertyName", equalTo("value"))))); ``` But looks like that `hasItem` method works only with collections. Is there any alternative method for verifying custom objects? `new MyCustomObject()` does not work in my case because test fails by hashcode equality. And, another thing is that I cannot modify `MyCustomObject class`.