如何使用迭代器和 for 循环中的流/过滤器/lambdas？

Question

I'm not too familiar with streams, filters, and lambdas as I'm used to writing older Java code. I wanted to try using new things in newer versions of Java. I can't seem to get anything started without getting compiling problems. Any help is appreciated, thank you.

List<Obj> a = getListForA();    // ['werdsegs', 'wsghnmrst', 'vaasdfdas', 'iujhgfds']
List<Obj> b = getListForB();    // ['aalaksdjf', 'erftghjuk', 'werdsejfksd', 'asdklgjaklgj', 'poijgndf']

a.forEach((item) -> {
    String foo = item.substring(0,5);
    for(Iterator<Obj> i = b.iterator(); i.hasNext();) {
        Obj o = i.next();
        if(foo.equals(o.substring(0,5)) {
            i.remove();
        }
    }
});

I expect for any item that contains the substring, remove it from the list for b.

After the code runs, list b should show the list below with werdsejfksd missing from ['aalaksdjf', 'erftghjuk', 'asdklgjaklgj', 'poijgndf']

Answer 1

Your code runs fine if you replace Obj with String and fix the compilation errors:

List<String> a = Arrays.asList("werdsegs", "wsghnmrst", "vaasdfdas", "iujhgfds");
List<String> b = new ArrayList<>(Arrays.asList("aalaksdjf", "erftghjuk", "werdsejfksd", "asdklgjaklgj", "poijgndf"));

a.forEach((item) -> {
    String foo = item.substring(0,5);
    for (Iterator<String> i = b.iterator(); i.hasNext(); ) {
        String o = i.next();
        if (foo.equals(o.substring(0,5))) {
            i.remove();
        }
    }
});

System.out.println(b); // prints: [aalaksdjf, erftghjuk, asdklgjaklgj, poijgndf]

As suggested by jonathan Heindl in another answer , you can simplify the code by using removeIf(...) . I'd build a Set first, though, for better performance:

List<String> a = Arrays.asList("werdsegs", "wsghnmrst", "vaasdfdas", "iujhgfds");
List<String> b = new ArrayList<>(Arrays.asList("aalaksdjf", "erftghjuk", "werdsejfksd", "asdklgjaklgj", "poijgndf"));

Set<String> set = a.stream().map(s -> s.substring(0,5)).collect(Collectors.toSet());
b.removeIf(s -> set.contains(s.substring(0,5)));

System.out.println(b); // prints: [aalaksdjf, erftghjuk, asdklgjaklgj, poijgndf]

If you want a new list, instead of modifying b , you can use the filter(...) method of Stream :

List<String> a = Arrays.asList("werdsegs", "wsghnmrst", "vaasdfdas", "iujhgfds");
List<String> b = Arrays.asList("aalaksdjf", "erftghjuk", "werdsejfksd", "asdklgjaklgj", "poijgndf");

Set<String> set = a.stream().map(s -> s.substring(0,5)).collect(Collectors.toSet());
List<String> c = b.stream().filter(s -> ! set.contains(s.substring(0,5))).collect(Collectors.toList());

System.out.println(c); // prints: [aalaksdjf, erftghjuk, asdklgjaklgj, poijgndf]

Answer 2

I guess like this

List<String> a = getListForA();    // ['werdsegs', 'wsghnmrst', 'vaasdfdas', 'iujhgfds']
List<String> b = getListForB();    // ['aalaksdjf', 'erftghjuk', 'werdsejfksd', 'asdklgjaklgj', 'poijgndf']

b.removeIf(bObj->a.stream().anyMatch(aObj->aObj.substring(0,5).equals(bObj.substring(0,5))));