java - Mocking a List and attempting to iterate over it -

currently using mockito test method 1 of classes. class contains list, , method takes in object of same class. issue when attempt iterate on list object. i'm getting null pointer list. below see code snippets.

private shipment shipment; private shipment shipment2; @mock private order mockorder1; @mock private order mockorder2; @mock private order mockorder3; @mock private arraylist<order> mockshipmentorders; @mock private arraylist<order> mockshipmentorders2;  @before public void setup(){     mockitoannotations.initmocks(this);     mockshipmentorders.add(mockorder1);     mockshipmentorders.add(mockorder2);     mockshipmentorders2.add(mockorder3);     shipment = new shipment(1, mockshipmentorders);     shipment2 = new shipment(2, mockshipmentorders2); }  @test public void test_mergeshipments_increasesbyonewhenashipmentofoneaddedtoashipmentordersizeoftwo(){     shipment.mergeshipments(shipment2);     assertequals(3, shipment.getshipmentorders().size()); } 

above can see mockito test , below class method:

public class shipment {

private long shipmentid; private list<order> shipmentorders;  public shipment(long shipmentid, list<order> shipmentorders){     this.shipmentid = shipmentid;     this.shipmentorders = shipmentorders; }  public list<order> getshipmentorders(){     return shipmentorders; }  public void mergeshipments(shipment shipment2){          list<order> existingshipment = shipment2.getshipmentorders();     (order order : existingshipment){         shipmentorders.add(order);     } } 

when run test getting java.lang.nullpointerexception line: (order order : existingshipment){ in mergeshipemts();

the question is; possible mock list, call list , run foreach on mocked list?

there fundamental issues why example not work , throws nullpointerexception.

1. the call add() on mocked list doesn't anything. void methods on mocks "no-ops" default
2. iterating on list using for-each syntax calls collection.iterator() under hood. returns null, because you've not setup mockito return else.

instead, not mock list , instead pass actual list. arrays.aslist() convenient testing.

@before public void setup(){     mockitoannotations.initmocks(this);     shipment = new shipment(1, arrays.aslist(mockorder1, mockorder2));     shipment2 = new shipment(2, arrays.aslist(mockorder3)); } 

if you're determined mock list you'll have mock behaviour, i.e. making add() store , .iterator() return iterator. can done rather painfully follows. i've included demonstrate principle.

@mock private list<string> mockedlist;  @before public void init() {     mockitoannotations.initmocks(this);      list<string> reallist = new arraylist<>();     doanswer(new answer<string>() {         @override         public string answer(invocationonmock invocation) throws throwable {             reallist.add(invocation.getargumentat(0, string.class));             return null;         }      }).when(mockedlist).add(any());      when(mockedlist.iterator()).thenanswer(new answer<iterator<string>>() {          @override         public iterator<string> answer(invocationonmock invocation) throws throwable {             return reallist.iterator();         }     });      mockedlist.add("bar");     mockedlist.add("baz"); }  @test public void iterateovermockedlist() {     (string each : mockedlist) {         system.out.println(each);     } }