Java Collections emptyList() 方法

2025年3月21日 | 阅读 4 分钟

Java Collections 类的 emptyList() 方法用于获取一个不包含任何元素的 List。这些空列表本质上是不可变的。

Java 中的 Collections.emptyList() 方法，它是 java.util.Collections 类的一部分，返回一个不可变的空 List。这意味着列表在创建后不能被修改，确保它在其生命周期内保持为空。

emptyList() 的主要用例是当你需要从方法返回一个列表或将一个空列表分配给一个变量，但又想确保列表不会被意外修改时。通过使用一个不可变的空列表，你可以保证列表将保持为空且不变。

语法

以下是 emptyList() 方法的声明

参数

该方法不接受任何参数。

返回值

emptyList() 方法返回一个空的不可变列表。

异常

不适用

兼容版本

Java 1.5 及以上版本

示例 1

CollectionsEmptyListExample1.java

import java.util.*;
public class CollectionsEmptyListExample1 {
	public static void main(String[] args) {		
	     //Create an empty List    
	     List<String> EmptyList = Collections.<String>emptyList();
	     System.out.println("Empty list: "+EmptyList);
	     }	  
}

输出

Empty list: []

emptyList() 的一个重要特性是，它每次调用时都返回同一个空列表实例。这意味着如果你多次调用 emptyList()，你将始终获得同一个列表对象。这种行为有助于节省内存，因为你的程序只需要一个空列表的实例。

由于返回的列表是不可变的，尝试使用 add() 或 remove() 等方法修改它将导致 UnsupportedOperationException。这确保了空列表保持为空且不可更改，进一步保证了其不可变性。

示例 2

CollectionsEmptyListExample2.java

import java.util.*;
public class CollectionsEmptyListExample2 {
	public static void main(String[] args) {		
		//Create an empty List  
	      List<String> emptylist = Collections.emptyList();
	      System.out.println("Created empty immutable list: "+emptylist);
	      //Try to add elements
	      emptylist.add("A");
	      emptylist.add("B");
	      }	  
}

输出

Created empty immutable list: []
Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractList.add(AbstractList.java:153)
	at java.base/java.util.AbstractList.add(AbstractList.java:111)
	at myPackage.CollectionsEmptyListExample1.main(CollectionsEmptyListExample1.java:9)

示例 3

CollectionsEmptyListExample3.java

import java.util.*;
public class CollectionsEmptyListExample3 {
	public static void main(String[] args) {		
		//Create an empty List    
	      List<Integer> empList = Collections.emptyList();
	      empList.add(1);
	      empList.add(2);    
	      System.out.println("Created empty immutable List: "+empList);	     
	      }	  
}

输出

Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractList.add(AbstractList.java:153)
	at java.base/java.util.AbstractList.add(AbstractList.java:111)
	at myPackage.CollectionsEmptyListExample3.main(CollectionsEmptyListExample3.java:8)

让我们通过几个 Java 示例程序来详细了解 Java Collections emptyList() 方法。

示例 1

文件名： EmptyListExample.java

// Java program for demonstrating the features and functionalities of Java Collections emptyList() method. 
import java.util.Collections;
import java.util.List;
public class EmptyListExample {
    public static void main(String[] args) {
        // Create an empty list using the emptyList() method from the Collections class
        List<String> emptyList = Collections.emptyList();
        // Output a message indicating that an empty list has been created
        System.out.println("An immutable empty list has been created: " + emptyList);
        try {
            // Attempt to add an element to the empty list, which should raise an UnsupportedOperationException
            emptyList.add("item"); // This operation should throw an exception
        } catch (UnsupportedOperationException e) {
            // Output a message indicating that an attempt to modify the empty list has failed
            System.out.println("Failed to modify the empty list: " + e.getMessage());
        }
        // Check if the empty list is still empty
        System.out.println("Is the empty list still empty? " + emptyList.isEmpty());
        try {
            // Attempt to clear the empty list, which should raise an UnsupportedOperationException
            emptyList.clear(); // This operation should throw an exception
        } catch (UnsupportedOperationException e) {
            // Output a message indicating that an attempt to clear the empty list has failed
            System.out.println("Failed to clear the empty list: " + e.getMessage());
        }
        // Demonstrate that the emptyList() method returns the same instance of an empty list
        List<String> anotherEmptyList = Collections.emptyList();
        System.out.println("Are both empty lists the same instance? " + (emptyList == anotherEmptyList));
        try {
            // Attempt to modify the second empty list, which should raise an UnsupportedOperationException
            anotherEmptyList.add("another item"); // This operation should throw an exception
        } catch (UnsupportedOperationException e) {
            // Output a message indicating that an attempt to modify the second empty list has failed
            System.out.println("Failed to modify the second empty list: " + e.getMessage());
        }
        // Check if the second empty list is still empty
        System.out.println("Is the second empty list still empty? " + anotherEmptyList.isEmpty());
        // Output the second empty list after attempted modifications
        System.out.println("Second empty list after attempted modifications: " + anotherEmptyList);
    }
}

输出

An immutable empty list has been created: []
Failed to modify the empty list: null
Is the empty list still empty? true
Are both empty lists the same instance? true
Failed to modify the second empty list: null
Is the second empty list still empty? true
Second empty list after attempted modifications: []

总而言之，Collections.emptyList() 方法是获取 Java 中不可变空列表的便捷方法，适用于需要一个不可修改的空列表的场景。

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Java Collections emptyList() 方法

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Java Collections emptyList() 方法

语法

参数

返回值

异常

兼容版本

示例 1

示例 2

示例 3

示例 1

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