As the code shows:
val map = scala.collection.mutable.Map[Int, Int]().withDefaultValue(0) println(map(1)) map(1) = 10 println(map(1)) map(1) += 10 println(map(1)) and the output:
0 10 20 However, in my opinion, "map(1) += 10" is invalid as in java, and even valid, the result after this line, map(1) should be 10.
So Why??? and what exactly operation "+=" do in map ??
First, += is an assignment operator, because it ends in = and isn't a comparison operator. This means that if there is no suitable method (and Int doesn't have a += method), map(1) += 10 is converted to map(1) = map(1) + 10. By the rules in Assignments, this becomes map.update(1, map(1) + 10) and this becomes map.update(1, map.apply(1) + 10) because map is an object rather than a method.
The question asked 'what exactly operation "+=" do in map ??' is not relevant to the situation you described. Map's method += adds entry (key -> value) to Map, so that map(key) == value.
So the question is: what does map(1) += 10 does.
This simple lines
map(1) += 10 compiles into this javacode
24: invokestatic #36 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer; 27: aload_2 28: iconst_1 29: invokestatic #36 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer; 32: invokeinterface #43, 2 // InterfaceMethod scala/collection/mutable/Map.apply:(Ljava/lang/Object;)Ljava/lang/Object; 37: invokestatic #47 // Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I 40: bipush 10 42: iadd 43: invokestatic #36 // Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer; 46: invokeinterface #51, 3 // InterfaceMethod scala/collection/mutable/Map.update:(Ljava/lang/Object;Ljava/lang/Object;)V Remove boxing and unboxing ops - we are not interested in them(line 24, 29, 37, 43). Review the rest:
We get value that was in map
27: aload_2 28: iconst_1 32: invokeinterface #43, 2 // InterfaceMethod scala/collection/mutable/Map.apply:(Ljava/lang/Object;)Ljava/lang/Object; Add 10 to it:
40: bipush 10 42: iadd And accomplish update:
46: invokeinterface #51, 3 // InterfaceMethod scala/collection/mutable/Map.update:(Ljava/lang/Object;Ljava/lang/Object;)V Seems map(1) += 10 is desugared to map.update(1, map.apply(1) + 10)
In Scala Map. += means
ms += (k -> v) Adds mapping from key k to value v to map ms as a side effect and returns ms itself. ms += (k -> v, l -> w) Adds the given mappings to ms as a side effect and returns ms itself. 
apply + update or in this case map.update(1, map.apply(1) + 10)
map(1).