Question

How to make ArrayList read only in Java

Answer 1

import java.util.*;
  
public class MyClass {
    public static void main(String args[]) {
          
        ArrayList<String> al1 = new ArrayList<String>();  
        al1.add("java");
        al1.add("c++");
        al1.add("c#");
        al1.add("python");
        al1.add("javascript");
        al1.add("c");
        System.out.println(al1); 
   
        List<String> unmodifiableList= Collections.unmodifiableList(al1);  
    
        unmodifiableList.add("php"); // Error
    }
}
  
  
/*
run:
  
[java, c++, c#, python, javascript, c]

Exception in thread "main" java.lang.UnsupportedOperationException
	at java.base/java.util.Collections$UnmodifiableCollection.add(Collections.java:1060)
	at MyClass.main(MyClass.java:17)
 
*/

 

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