Java 8 lambda doesn't change data

Question

Well I'm digging through Java8 lambda and I'm facing following problem - lambda doesn't change data:

DamnLambda.class:

public static void inc ( List<Integer> list, Funtion<Integer,Integer> func) {

    for(Integer intr : list) {
        intr = func.apply(intr);
    }

Let's try to invoke:

List<Integer> l = Arrays.asList(1,2,3);

DamnLambda.inc(l, x -> x+=1);
System.out.println(l); //[1,2,3] ? Why not [2,3,4] ?

I can't understad why it doesn't change any data. I tried also different version of same functionality:

l.forEach(x -> x+1); //same thing, doesn't change the data.

What am I missing here?

Assigning a value to the loop iteration variable will have no effect and, in fact, should not be done. — Steve
– Steve, Commented Jul 6, 2014 at 23:55
You're missing the point of functional programming. You're not supposed to change the input. You're supposed to generate output that is a transformation of the input. — Steve
– Steve, Commented Jul 6, 2014 at 23:56
Well guys u got me, I was blindly trying to perform some lambda operations on the list and forgotten about one of the primal rules. Btw. code compiles. — krzakov
– krzakov, Commented Jul 6, 2014 at 23:57

Brian Goetz · Accepted Answer · 2014-07-07 00:17:39Z

8

How about:

list.replaceAll(x -> x+1);

answered Jul 7, 2014 at 0:17

Brian Goetz

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1 Comment

krzakov Over a year ago

It's even better than previous one, but I marked previous as an answer because he reflected the situation I had in mind. I was trying to understand usage lambda in practice.

Community · Accepted Answer · 2017-05-23 11:43:41Z

4

Brian Goetz' answer is clearly the one to use if you want to mutate a list in-place. An alternative, which alfasin's answer seems to be describing, is to create a new list containing the modified values. The way to do with using lambda and streams is:

List<Integer> result =
    list.stream()
        .map(i -> i + 1)
        .collect(Collectors.toList());

edited May 23, 2017 at 11:43

CommunityBot

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answered Jul 7, 2014 at 3:34

Stuart Marks

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1 Comment

krzakov Over a year ago

veru useful, I'm planning to go through streams today :)

Nir Alfasi · Accepted Answer · 2014-07-06 23:58:09Z

3

Change inc() to:

public static List<Integer> inc ( List<Integer> list, Funtion<Integer,Integer> func) {

    List<Integer> result = new ArrayList<Integer>();
    for(Integer intr : list) {
        result.add(func.apply(intr));
    }
    return result;
}

and then call:

l = DamnLambda.inc(l, x -> x+=1);

answered Jul 6, 2014 at 23:58

Nir Alfasi

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