Java 数组asList()方法及示例

java.util.Arrays 类的 asList() 方法是用来返回一个由指定数组支持的固定大小的列表。这个方法作为 基于数组和基于集合的API之间的桥梁 ，与Collection.toArray()结合使用。返回的列表是可序列化的，并且实现了RandomAccess。

提示： 该方法在O(1)时间内运行。

语法

public static List asList(T... a)

参数： 该方法接收需要转换为List的 数组a 。这里……被称为 varargs ，它是一个参数数组，工作原理类似于对象数组参数。

特别注意： 如果是原始数据类型（int, float,等），数组的类型必须是一个封装类（Integer,Float,等），即你不能传递int a[]，但你可以传递Integer a[]。如果你传递int a[]，这个函数将返回一个List <int a[]>，而不是List <Integer>，因为在这种情况下 “autoboxing “不会发生，int a[]本身被识别为一个对象，并且返回一个int数组的List，而不是整数的List，这将在各种Collection函数中引起错误。

返回值： 该方法返回一个指定数组的 列表视图 。

例1 :

// 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<String> 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]

例2 :

// 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<Integer> 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]

例3 :

// 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<Integer> 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

Java中的数组asList方法及示例