Java中的数组 asList() 方法及示例

Java.util.Arrays类的asList()方法用于返回由指定数组支持的固定大小的列表。此方法与 Collection.toArray() 结合，充当基于数组和基于集合的 API 之间的桥梁。返回的列表是可序列化的并实现了 RandomAccess。

Tip: This runs in O(1) time. 

句法：

public static List asList(T... a)

参数：该方法接受需要转换为List的数组a 。这里……被称为可变参数，它是一个参数数组，其工作方式类似于对象数组参数。

Special Note: The type of array must be a Wrapper Class(Integer,Float, etc) in case of primitive data types(int, float,etc) , i.e you can’t pass int a[] but you can pass Integer a[]. If you pass int a[], this function will return a  List and not List , as “autoboxing” doesn’t happen in this case and int a[] is itself identified as an object and a List of int array is returned, instead of list of integers , which will give error in various Collection functions .

返回值：此方法返回指定数组的列表视图。

示例 1：

Java

// Java program to Demonstrate asList() method
// of Arrays class for a string value
 
// Importing utility classes
import java.util.*;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] argv) throws Exception
    {
 
        // Try block to check for exceptions
        try {
 
            // Creating Arrays of String type
            String a[]
                = new String[] { "A", "B", "C", "D" };
 
            // Getting the list view of Array
            List list = Arrays.asList(a);
 
            // Printing all the elements in list object
            System.out.println("The list is: " + list);
        }
 
        // Catch block to handle exceptions
        catch (NullPointerException e) {
 
            // Print statement
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出

The list is: [A, B, C, D]

示例 2：

Java

// Java program to Demonstrate asList() method
// of Arrays class For an integer value
 
// Importing utility classes
import java.util.*;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] argv) throws Exception
    {
        // Try block to check for exceptions
        try {
 
            // Creating Arrays of Integer type
            Integer a[] = new Integer[] { 10, 20, 30, 40 };
 
            // Getting the list view of Array
            List list = Arrays.asList(a);
 
            // Printing all the elements inside list object
            System.out.println("The list is: " + list);
        }
 
        // Catch block to handle exceptions
        catch (NullPointerException e) {
 
            // Print statements
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出

The list is: [10, 20, 30, 40]

示例 3：

Java

// Java Program to demonstrate asList() method
// Which returns fixed size list and
// throws UnsupportedOperationException
// if any element is added using add() method
 
// Importing required classes
import java.util.*;
 
// Main class
public class GFG {
 
    // Main driver method
    public static void main(String[] argv) throws Exception
    {
        // Try block to check for exceptions
        try {
 
            // Creating Arrays of Integer type
            Integer a[] = new Integer[] { 10, 20, 30, 40 };
 
            // Getting the list view of Array
            List list = Arrays.asList(a);
 
            // Adding another int to the list
            // As Arrays.asList() returns fixed size
            // list, we'll get
            // java.lang.UnsupportedOperationException
            list.add(50);
 
            // Printing all the elements of list
            System.out.println("The list is: " + list);
        }
 
        // Catch block to handle exceptions
        catch (UnsupportedOperationException e) {
 
            // Display message when exception occurs
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：