Java中的集合 unmodifiableList() 方法和示例

Java.util.Collections类的unmodifiableList()方法用于返回指定列表的不可修改视图。此方法允许模块为用户提供对内部列表的“只读”访问权限。对返回列表的查询操作“通读”到指定列表，并尝试修改返回的列表，无论是直接还是通过其迭代器，都会导致 UnsupportedOperationException。

如果指定的列表是可序列化的，则返回的列表将是可序列化的。同样，如果指定的列表执行，则返回的列表将实现 RandomAccess。

句法：

public static  ListT> 
    unmodifiableList(List list)

参数：此方法将列表作为参数，为其返回不可修改的视图。

返回值：此方法返回指定列表的不可修改视图。

下面是说明unmodifiableList()方法的示例

示例 1：

// Java program to demonstrate
// unmodifiableList() method
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {
        try {
  
            // creating object of ArrayList
            List list = new ArrayList();
  
            // populate the list
            list.add('X');
            list.add('Y');
  
            // printing the list
            System.out.println("Initial list: " + list);
  
            // getting unmodifiable list
            // using unmodifiableList() method
            List immutablelist = Collections
                                                .unmodifiableList(list);
  
            // printing the list
            System.out.println("Unmodifiable list: "
                               + immutablelist);
        }
  
        catch (UnsupportedOperationException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

Initial list: [X, Y]
Unmodifiable list: [X, Y]

示例 2：对于UnsupportedOperationException

// Java program to demonstrate
// unmodifiableList() method
  
import java.util.*;
  
public class GFG1 {
    public static void main(String[] argv)
        throws Exception
    {
        try {
  
            // creating object of ArrayList
            List list = new ArrayList();
  
            // populate the list
            list.add('X');
            list.add('Y');
  
            // printing the list
            System.out.println("Initial list: " + list);
  
            // getting unmodifiable list
            // using unmodifiableList() method
            List immutablelist = Collections
                                                .unmodifiableList(list);
  
            // Adding element to new Collection
            System.out.println("\nTrying to modify"
                               + " the unmodifiablelist");
            immutablelist.add('Z');
        }
  
        catch (UnsupportedOperationException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

Initial list: [X, Y]

Trying to modify the unmodifiablelist
Exception thrown : java.lang.UnsupportedOperationException