Interpretation of Java documentation: Detailed description of the asList() method of the Arrays class

In Java applications, it is often necessary to operate and convert arrays. The Arrays class provides many convenient methods, one of which is the asList() method. This article will explain the meaning and usage of the asList() method in detail, and will also provide some code examples to help readers better understand this method.

1. asList() method overview

The asList() method is a static method that returns a List object that contains all the elements in the specified array. The syntax of this method is as follows:

public static <T> List<T> asList(T... a)

The return value type of this method is List, where T is any reference or basic type. The asList() method accepts any number of T type parameters, that is, variable length parameters, which means that any number of T type elements can be passed, and an array of T type can also be passed.

2. Practical application of asList() method

Through the asList() method, an array can be converted into a List. In the Java collection framework, a List is a linear data structure that can contain repeated elements. Unlike arrays, Lists have the characteristics of dynamic expansion, and elements can be added and deleted easily.

In the following example, we can first create an array containing some elements and then convert it to a List:

String[] strArray = {"Hello","Java","World"};
List<String> strList = Arrays.asList(strArray);

In this example, we use the asList() method to An array of type String is converted into a List object. It can be noticed that we do not need to manually create the List object, but generate it using the return value of Arrays.asList().

3. Usage restrictions of asList() method

It should be noted that the parameters in the asList() method should be reference types. This is because the basic type data does not implement the java.lang.Object interface and therefore cannot be used as elements in the List. If you want to convert a basic type array to a List, you need to manually encapsulate it into the corresponding wrapper class.

The following is a sample code to convert an int type array to a List:

int[] intArray = {1, 2, 3};
List<Integer> intList = Arrays.asList(Arrays.stream(intArray).boxed().toArray(Integer[]::new));

In this example, we use Arrays.stream() to convert an int type array into an IntStream stream, Then use the boxed() method to unbox it and convert it to an Integer type, and toArray() it into an array of Integer type. Finally, we use the asList() method to convert the Integer type array to a List.

It should be noted that when converting a basic type array to the corresponding wrapper class, it needs to be implemented manually before Java 8. In Java 8, we can use Stream API to accomplish this task.

4. Notes

When using the asList() method, you need to pay attention to some characteristics of the List object. Since the converted List object is essentially just a wrapping process, operations such as add() and remove() cannot be performed. If you try to perform these operations on the converted List object, an UnsupportedOperationException will be thrown. In addition, since List and array are reference types, the original array may be affected when operating on List.

The following is a usage example:

Integer[] intArray = {1, 2, 3};
List<Integer> intList = Arrays.asList(intArray);
intList.set(0, 10);

In this example, we first convert an array of Integer type into a List object, and then use the set() method to set the The first element is set to 10. At this point, we can notice that the first element in the original array intArray is also set to 10.

5. Conclusion

By reading this article, we can understand the usage and limitations of the asList() method in the Arrays class. The asList() method is a convenient tool for converting between arrays and Lists, but it should be noted that List objects do not support adding and removing elements. It is worth mentioning that when converting a basic type array to a List, you need to manually encapsulate it into the corresponding wrapper class. In actual use, we need to choose between List and array according to specific needs.

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