如何使 ArrayList 在Java中只读

给定一个 ArrayList，任务是使这个 ArrayList 在Java中只读。

例子：

Input: ArrayList: [1, 2, 3, 4, 5]
Output: Read-only ArrayList: [1, 2, 3, 4, 5] 

Input: ArrayList: [geeks, for, geeks]
Output: Read-only ArrayList: [geeks, for, geeks]

借助 Collections.unmodifiableList() 方法，可以轻松地将 ArrayList 设为只读。该方法以可修改的ArrayList为参数，返回该ArrayList的只读不可修改视图。

句法：

readOnlyArrayList = Collections.unmodifiableList(ArrayList);

下面是上述方法的实现：

// 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');
            list.add('Z');
  
            // printing the list
            System.out.println("Initial list: "
                               + list);
  
            // getting readonly list
            // using unmodifiableList() method
            List
                immutablelist = Collections
                                    .unmodifiableList(list);
  
            // printing the list
            System.out.println("ReadOnly ArrayList: "
                               + immutablelist);
  
            // Adding element to new Collection
            System.out.println("\nTrying to modify"
                               + " the ReadOnly ArrayList.");
            immutablelist.add('A');
        }
  
        catch (UnsupportedOperationException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出：

Initial list: [X, Y, Z]
ReadOnly ArrayList: [X, Y, Z]

Trying to modify the ReadOnly ArrayList.
Exception thrown : java.lang.UnsupportedOperationException