mockito check argument

Channel channel = Mockito.mock(XxxClass.class);
  
Mockito.when(channel.writeAndFlush((XxxClass) Mockito.argThat(new ArgMatcher()))).
  
thenReturn((ChannelFuture) anyObject());

private class ArgMatcher extends ArgumentMatcher {
  
@Override
  
public boolean matches(Object o) {
  
String arg = o.toString();
  
Assert.xxx

return true;
  
}
  
}