How to get field from Object

I have a class:

public class resultJTO extends HashMap<String, Object> {

    private static final long serialVersionUID = 1L;
}

I would like to get particular fields of this Hashmap. I mean I'm doing get on resultJTO

 Object airport = resultJTO.get("AIRPORT");

I was trying to do it by this way:

Object airport = resultJTO.get("AIRPORT");
        Field field = searchJTO.class.getDeclaredField("city");
        field.setAccessible(true);
        field.get(airport);

but getting

ava.lang.IllegalArgumentException: Can not set java.lang.String field AirportSearchJTO.city to java.util.HashSet

at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:58)
at sun.reflect.UnsafeObjectFieldAccessorImpl.get(UnsafeObjectFieldAccessorImpl.java:36)
at java.lang.reflect.Field.get(Field.java:393)
at SearchServiceTest.shouldReturnProperMergedAirportIfAvailableInBothSources(SearchServiceTest.java:191)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.mockito.internal.runners.DefaultInternalRunner$1$1.evaluate(DefaultInternalRunner.java:44)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:74)
at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:80)
at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:39)
at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

what should I do to fetch for example city?

Inspect the error message. It says: Can not set java.lang.String field flightmap.service.module.search.jto.AirportSearchJTO.city to java.util.HashSet. So it tries to set field of type String to value of type HashSet. It is not only about wrong types, but also about something different than you think — it is not about getting the value, but setting it. So the stacktrace shows something different than you think.

Now inspect this line: at flightmap.service.module.search.SearchServiceTest.shouldReturnProperMergedAirportIfAvailableInBothSources(SearchServiceTest.java:191)

Open your SearchServiceTest.java class, go to line 191 and see what's there. That's the reason for this stacktrace. And again: it sets value, not gets it.

Your searchResultJTO object has HashSet of AirportSearchJTO as values and String as keys.
You need to access the values in a loop like this

Object airportCollection = searchResultJTO.get("AIRPORT");
Field field = AirportSearchJTO.class.getDeclaredField("city");
field.setAccessible(true);
if (airportCollection instanceof Collection) {
   for(Object airport : (Collection)airportCollection){
      field.get(airport);
   }
}