Java 8 Lambda, filter HashMap, cannot resolve method

Question

I'm kinda new to Java 8's new features. I am learning how to filter a map by entries. I have looked at this tutorial and this post for my problem, but I am unable to solve.

@Test
public void testSomething() throws Exception {
    HashMap<String, Integer> map = new HashMap<>();
    map.put("1", 1);
    map.put("2", 2);
    map = map.entrySet()
            .parallelStream()
            .filter(e -> e.getValue()>1)
            .collect(Collectors.toMap(e->e.getKey(), e->e.getValue()));
}

However, my IDE (IntelliJ) says "Cannot resolve method 'getKey()'", thus unable to complile: enter image description here

Neither does this help: enter image description here
Can anyone help me to solve this issue? Thanks.

if i may ask.. what is e exactly.. dont see it declared anywhere.. — Zuko
– Zuko, Commented Jan 8, 2015 at 13:28
@tobias_k, should not a problem regarding generics. It should work according to this leveluplunch.com/java/examples/filter-map-by-value — Daniel
– Daniel, Commented Jan 8, 2015 at 13:31
@FlorentBayle adding casting (HashMap<String, Integer>) helps! — Daniel
– Daniel, Commented Jan 8, 2015 at 13:34

assylias · Accepted Answer · 2015-01-08 13:32:00Z

34

The message is misleading but your code does not compile for another reason: collect returns a Map<String, Integer> not a HashMap.

If you use

Map<String, Integer> map = new HashMap<>();

it should work as expected (also make sure you have all the relevant imports).

answered Jan 8, 2015 at 13:32

assylias

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5 Comments

Marko Topolnik Over a year ago

Apparently it will take some time for the compiler to get smart enough about the true cause of an error. Quite frustrating at the moment...

Daniel Over a year ago

If we down cast what is returned by .collect by (HashMap<String, Integer>), it appears to work. But is there any issue regarding casting?

Zuko Over a year ago

I for one always prefer to generate data structures from the interfaces, List, Map e.t.c.

assylias Over a year ago

@Daniel The collector does not give you any guarantee on the type returned - so it may work now and break in the future.

Sachin Over a year ago

The message is misleading: This saved my day. I was using toSet(), where I should have used toList(). The message is indeed very confusing.

sol4me · Accepted Answer · 2015-01-08 13:37:44Z

5

Your are returning Map not hashMap so you need to change map type to java.util.Map. Moreover you can use method reference rather then calling getKey, getValue. E.g.

Map<String, Integer> map = new HashMap<>();
        map.put("1", 1);
        map.put("2", 2);
        map = map.entrySet()
                .parallelStream()
                .filter(e -> e.getValue() > 1)
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

You could solve it by using some intellij help as well for e.g. if you press ctrl+alt+v in front of

new HashMap<>();
            map.put("1", 1);
            map.put("2", 2);
            map = map.entrySet()
                    .parallelStream()
                    .filter(e -> e.getValue() > 1)
                    .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

The variable created by intellij will be of exact type and you will get.

Map<String, Integer> collect = map.entrySet()
        .parallelStream()
        .filter(e -> e.getValue() > 1)
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

answered Jan 8, 2015 at 13:37

sol4me

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1 Comment

Daniel Over a year ago

ctrl+alt+v is awesome

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