如何使用Java Lambda创建/初始化ArrayList的ArrayList

Question

How to convert the following code to lambda in java8???

    List<List<String>> my2dList = new ArrayList<List<String>>();
    int counter = 0;
    for (int i = 0; i < 5; i++) {
        my2dList.add(new ArrayList<String>());
        for (int j = 0; j < 10; j++) {
            System.out.println("Counter: " +counter);
            my2dList.get(i).add(new String(""+counter));
            counter++;
        }
    }

expected result:

[[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [10, 11, 12, 13, 14, 15, 16, 17, 18, 19], [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], [30, 31, 32, 33, 34, 35, 36, 37, 38, 39], [40, 41, 42, 43, 44, 45, 46, 47, 48, 49]]

Answer 1

You can use IntStream.range(int startInclusive, int endExclusive) to generate a stream of integers.

You can then use mapToObj(IntFunction<? extends U> mapper) to process those integers.

Finally, you can use collect(Collector<? super T,A,R> collector) to collect the values, eg to a List by using Collectors.toList() .

List<List<String>> my2dList =
        IntStream.range(0, 5)
                 .mapToObj(i -> IntStream.range(0, 10)
                                         .mapToObj(j -> Integer.toString(i * 10 + j))
                                         .collect(Collectors.toList()))
                 .collect(Collectors.toList());

---

UPDATE

If you want to print the values as they are streamed, use peek(Consumer<? super T> action) .

If the peek() method should see the value as an int , you can split the expression in the mapToObj so you can peek at the intermediate value, before it is converted to String .

The conversion to String can then be done with a method reference, instead of a lambda.

                 .mapToObj(i -> IntStream.range(0, 10)
                                         .map(j -> i * 10 + j)
                                         .peek(val -> System.out.println("Counter: " + val))
                                         .mapToObj(Integer::toString)
                                         .collect(Collectors.toList()))