Hamcrest - Matchers.hasProperty: how to check if a List of objects contains an object with a concrete value

You need the matcher hasItem

assertThat(
  x,
  hasItem(allOf(
    Matchers.<Employee>hasProperty("name", is("Mateusz")),
    Matchers.<Employee>hasProperty("age", is(27))
  ))
);

You can also use hasItems, contains or containsInAnyOrder :

assertThat(
  x,
  hasItems( // or contains or containsInAnyOrder 
    Matchers.<Employee>hasProperty("name", is("Mateusz")),
    Matchers.<Employee>hasProperty("age", is(27))
  )
);