How to get the key in Collectors.toMap merge function?

问题

When a duplicate key entry is found during Collectors.toMap(), the merge function (o1, o2) is called.

Question: how can I get the key that caused the duplication?

String keyvalp = "test=one\ntest2=two\ntest2=three";

Pattern.compile("\n")
    .splitAsStream(keyval)
    .map(entry -> entry.split("="))
    .collect(Collectors.toMap(
        split -> split[0],
        split -> split[1],
        (o1, o2) -> {
            //TODO how to access the key that caused the duplicate? o1 and o2 are the values only
            //split[0]; //which is the key, cannot be accessed here
        },
    HashMap::new));

Inside the merge function I want to decide based on the key which if I cancel the mapping, or continue and take on of those values.

回答1:

You need to use a custom collector or use a different approach.

Map<String, String> map = new Hashmap<>();
Pattern.compile("\n")
    .splitAsStream(keyval)
    .map(entry -> entry.split("="))
    .forEach(arr -> map.merge(arr[0], arr[1], (o1, o2) -> /* use arr[0]));

Writing a custom collector is rather more complicated. You need a TriConsumer (key and two values) is similar which is not in the JDK which is why I am pretty sure there is no built in function which uses. ;)

回答2:

The merge function has no chance to get the key, which is the same issue, the builtin function has, when you omit the merge function.

The solution is to use a different toMap implementation, which does not rely on Map.merge:

public static <T, K, V> Collector<T, ?, Map<K,V>>
    toMap(Function<? super T, ? extends K> keyMapper,
          Function<? super T, ? extends V> valueMapper) {
    return Collector.of(HashMap::new,
        (m, t) -> {
            K k = keyMapper.apply(t);
            V v = Objects.requireNonNull(valueMapper.apply(t));
            if(m.putIfAbsent(k, v) != null) throw duplicateKey(k, m.get(k), v);
        },
        (m1, m2) -> {
            m2.forEach((k,v) -> {
                if(m1.putIfAbsent(k, v)!=null) throw duplicateKey(k, m1.get(k), v);
            });
            return m1;
        });
}
private static IllegalStateException duplicateKey(Object k, Object v1, Object v2) {
    return new IllegalStateException("Duplicate key "+k+" (values "+v1+" and "+v2+')');
}

^{(This is basically what Java 9’s implementation of toMap without a merge function will do)}

So all you need to do in your code, is to redirect the toMap call and omit the merge function:

String keyvalp = "test=one\ntest2=two\ntest2=three";

Map<String, String> map = Pattern.compile("\n")
        .splitAsStream(keyvalp)
        .map(entry -> entry.split("="))
        .collect(toMap(split -> split[0], split -> split[1]));

(or ContainingClass.toMap if its neither in the same class nor static imports)<\sup>

The collector supports parallel processing like the original toMap collector, though it’s not very likely to get a benefit from parallel processing here, even with more elements to process.

If, if I get you correctly, you only want to pick either, the older or newer value, in the merge function based on the actual key, you could do it with a key Predicate like this

public static <T, K, V> Collector<T, ?, Map<K,V>>
    toMap(Function<? super T, ? extends K> keyMapper,
          Function<? super T, ? extends V> valueMapper,
          Predicate<? super K> useOlder) {
    return Collector.of(HashMap::new,
        (m, t) -> {
            K k = keyMapper.apply(t);
            m.merge(k, valueMapper.apply(t), (a,b) -> useOlder.test(k)? a: b);
        },
        (m1, m2) -> {
            m2.forEach((k,v) -> m1.merge(k, v, (a,b) -> useOlder.test(k)? a: b));
            return m1;
        });
}

Map<String, String> map = Pattern.compile("\n")
        .splitAsStream(keyvalp)
        .map(entry -> entry.split("="))
        .collect(toMap(split -> split[0], split -> split[1], key -> condition));

There are several ways to customize this collector…

回答3:

There is, of course, simple and trivial trick - saving the key in the 'key mapper' function and getting the key in the 'merge' function. So, the code may look like the following (assuming the key is Integer):

final AtomicInteger key = new AtomicInteger(); 
...collect( Collectors.toMap( 
   item -> { key.set(item.getKey()); return item.getKey(); }, // key mapper 
   item -> ..., // value mapper
   (v1, v2) -> { log(key.get(), v1, v2); return v1; } // merge function
);

Note: this is not good for parallel processing.

来源：https://stackoverflow.com/questions/44407112/how-to-get-the-key-in-collectors-tomap-merge-function

标签

java

java-8

collectors