java.lang.NullPointerException: 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference [duplicate]

Question

if (chat_message.getReceiver().equals("myid") 
    && chat_message.getSender().equals("userid") 
    || chat_message.getReceiver().equals("userid") 
    && chat_message.getSender().equals("myid")
) 

I get this error?

I did everything right, firebase is also right.

When I enter the application, it closes immediately. Does anyone know the reason?

The error I got:

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference

Answer 1

If the IDs of sender/receiver are constant strings "myid"/"userid" it is better to use Joda (inverted) conditions to prevent NullPointerException:

final String sender = chat_message.getSender();
final String receiver = chat_message.getReceiver();

if (("myid".equals(sender) && "userid".equals(receiver)) 
    || ("myid".equals(receiver) && "userid".equals(sender)) 
)