Java 8 Streams映射和过滤器之间的区别

Question

Result of both approach are same which is best practise to use. I am new bee to java 8. i am little bit confused on stream.map and stream.filter

List<String> alpha =
        Arrays.asList("a", "b", "csddddddddddd",
                "d", "ssdddddddddd", "sw", "we", "wew");

// Java 8
List<String> collect = alpha.stream()
        .map(String::toUpperCase)
        .collect(Collectors.toList());

List<Integer> collect2 = alpha.stream()
        .map(s -> s.length())
        .collect(Collectors.toList());

List<Integer> collect3 = collect2.stream()
        .filter(s -> s > 10)
        .collect(Collectors.toList());

List<Integer> collect4 = collect2.stream()
        .map(s -> {
            Integer temp = 0;
            if (s > 10) {
                temp = s;
            }
            return temp;
        })
        .filter(s -> s > 10)
        .collect(Collectors.toList());

Result of both List are same :

[13, 12]
[13, 12]

what is best approach. and what is best approach in this regards.

Answer 1

If you want to filter the elements of a Stream by some condition (ie remove elements that don't satisfy the condition), use filter , not map .

The purpose of map is to convert an element of one type to an element of another type.

The only reason you get the same results in collect3 and collect4 is that after applying map in collect4 , you apply a filter, which removes all the 0 s produced by map .

The entire .map(s ->{ Integer temp = 0;if(s>10) {temp=s;} return temp;}) call is redundant. That's a very unreadable and inefficient way to write code.