I have a Java Map:

Map<String,String> mapThings = new HashMap<>();
updateMapThings(mapThings, sourceMap); // sourceMap is the original Map which has all the 
                                       // values I want to populate mapThings with.

So now mapTHings is like below

{
    "a" : "A",
    "b" : "B",
    "c" : "C"
}

With a,A,b,B,c,C (all the key/values) populated from sourceMap

Now I want to populate mapThings with more key/values from sourceMap2, so mapThings will become like

{
    "a" : "A",
    "b" : "B",
    "c" : "C",
    "d" : "D",
    "e" : "E",
    "f" : "F"
}

where d,D,e,E,f,F are all key/values populated from sourceMap2.

Should I call

updateMapThings(mapThings, sourceMap2);

This is where I get confused whether Java is PassBy Value or Reference. How do I cleanly update the same map with different key/values from more than one source?