In Java 8 how do I transform a Map to another Map using a lambda?

Question

I\'ve just started looking at Java 8 and to try out lambdas I thought I\'d try to rewrite a very simple thing I wrote recently. I need to turn a Map of String to Column int

Answer 1

Map<String, Integer> map = new HashMap<>();
map.put("test1", 1);
map.put("test2", 2);

Map<String, Integer> map2 = new HashMap<>();
map.forEach(map2::put);

System.out.println("map: " + map);
System.out.println("map2: " + map2);
// Output:
// map:  {test2=2, test1=1}
// map2: {test2=2, test1=1}

You can use the forEach method to do what you want.

What you're doing there is:

map.forEach(new BiConsumer<String, Integer>() {
    @Override
    public void accept(String s, Integer integer) {
        map2.put(s, integer);     
    }
});

Which we can simplify into a lambda:

map.forEach((s, integer) ->  map2.put(s, integer));

And because we're just calling an existing method we can use a method reference, which gives us:

map.forEach(map2::put);