Get to the abbreviations from 5 examples of iterating Java lists

Let's look at how to write a list iterator using a simple list. This time we will use the following list of strings.

list

final List<String> months = 
Arrays.asList("January", "February", "March", "April", "May", "June", "July", "Augast", "September", "October", "November", "December");

I just want these elements to be standard output in sequence.

Imperative style

First, let's use a for loop. The for loop is an imperative style, an external iterator. In the imperative style, the programmer has to think a lot about "how" to do it.

1. for loop (index specification)

I don't think it's particularly difficult. Outputs in order from the first element. You need to specify the index by incrementing it one by one.

for-with-index

for (int i = 0; i < months.size(); i++) {
  System.out.println(months.get(i));
}

2. for loop (no index specified)

If you don't need an index, Java provides more advanced control syntax than a simple for loop. Behind the scenes, the Iterator interface is applied and the hasNext () and next () methods are called. Therefore, it is not necessary for the programmer to specify the end of the loop using the size of the list.

for-without-index

for (final String month : months) {
  System.out.println(month);
}

Functional style

Java8 has added a forEach method to the Iterable interface. It's a functional style, an internal iterator. The functional style makes it easier to focus on thinking about what to do.

3. foreach() Consumer foreach () takes a Consumer type as an argument, but this instance processes the argument received by the accept () method. Each element in the list is passed to the accept () method one by one, so it is output as standard in that.

foreach-comsumer

months.forEach(new Consumer<String>() {
  public void accept(final String month) {
    System.out.println(month);
  }
});

4. foreach() Lambda You can use a Lambda expression to replace an anonymous inner class (below a Consumer instance).

foreach-lambda

months.forEach((final String month) -> System.out.println(month));

Since the Java compiler can perform type inference, it can be omitted as follows. However, in this case, final is missed and it is no longer an immutable argument.

foreach-lambda

months.forEach(month -> System.out.println(month));

5. See foreach () method

Method references are a feature from Java8, and the class name :: method name is the basic form. Since the method that has already been defined can be executed without specifying the argument, it will be further omitted.

foreach-method-reference

months.forEach(System.out::println);