java - Mockito Mock when field changes -

i have class

class a{  map<string, string> obj; b b;  public void method1(str1, str2, str3){   obj = (new map dependent on str1 , str2)   string str = b.method2(obj, str3)   return str; } 

how can mock b.method2. mean want test method1 , check correct string returned. purpose want when(b.method2()).thenreturn("mystr").

here complete, working example:

public class b {     public string method2(map<string, string> obj, string str3) {         // irrelevant our test of         return null;     } }  public class {      private map<string, string> obj;     private b b;      public a(b b) {         this.b = b;     }      /**      * replaces internal obj map new map containing str1 key , str2 value,      * , calls b.method2() map , str3.      * @return value returned b.method2()      */     public string method1(string str1, string str2, string str3) {         obj = new hashmap<>();         obj.put(str1, str2);         string str = b.method2(obj, str3);         return str;     }      public map<string, string> getobj() {         return obj;     } }  public class atest {     @test     public void method1shouldcallmethod2withcreatedmap() {         b mockb = mock(b.class);          map<string, string> expectedmap = new hashmap<>();         expectedmap.put("hello", "world");          when(mockb.method2(expectedmap, "!")).thenreturn("ok");         a = new a(mockb);         string result = a.method1("hello", "world", "!");         assertequals("ok", result);         assertequals(expectedmap, a.getobj());     }      // same test, other technique     @test     public void method1shouldcallmethod2withcreatedmap2() {         b mockb = mock(b.class);           when(mockb.method2(mockito.<map<string, string>>any(), anystring())).thenreturn("ok");         a = new a(mockb);         string result = a.method1("hello", "world", "!");         assertequals("ok", result);          map<string, string> expectedmap = new hashmap<>();         expectedmap.put("hello", "world");          assertequals(expectedmap, a.getobj());         verify(mockb).method2(expectedmap, "!");     } }