Java 8 - How to reduce the size (lines) of this map operation?

I need to get value map for a key, if its not there create new map and insert the key/value. How can i reduce the number of lines of this operation using java 8 ?

Map<String, Map<String, String> map = new HashMap();
Map<String, String> valueMap = map.get(key_1);
if(valueMap == null) {
	valueMap = new HashMap();
} 
valueMap.put(key_2, value);
map.put(key_1, valueMap);

You can use computeIfAbsent method of Map See The Documentation

map.computeIfAbsent(key_1, k -> new HashMap<>()).put(key_2, value);

Note that you won't need to create a new HashMap instance (unlike getOrDefault method for example), if there is a key, only if there is nothing (corresponds to if(valueMap == null) in your example.

Looking at the documentation for Map: https://docs.oracle.com/javase/8/docs/api/java/util/Map.html

It looks like computeIfAbsent does what you want.

First, declare a function newMap() that simply returns a new HashMap(). Then:

Map<String, Map<String, String> map = new HashMap();
map.computeIfAbsent(key_1, newMap);
map.get(key_1).put(key_2, value);

EDIT: Rather than declaring a named function newMap, you can use an anonymous function k -> new HashMap<>(), as suggested in Mark Bramnik's answer.

Do not use .putIfAbsent(key_1, new HashMap()), or .getOrDefault, or any other non-lazy function that would evaluate its default value even if the default value is not needed. These functions are appropriate if the default value is a simple value, for instance an int; they are not appropriate if the default value requires a call to new to be built.