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Java Language Unmodifiable Collection


Example

Sometimes it's not a good practice expose an internal collection since it can lead to a malicious code vulnerability due to it's mutable characteristic. In order to provide "read-only" collections java provides its unmodifiable versions.

An unmodifiable collection is often a copy of a modifiable collection which guarantees that the collection itself cannot be altered. Attempts to modify it will result in an UnsupportedOperationException exception.

It is important to notice that objects which are present inside the collection can still be altered.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class MyPojoClass {
    private List<Integer> intList = new ArrayList<>();

    public void addValueToIntList(Integer value){
        intList.add(value);
    }
    
    public List<Integer> getIntList() {
        return Collections.unmodifiableList(intList);
    }
}

The following attempt to modify an unmodifiable collection will throw an exception:

import java.util.List;

public class App {

    public static void main(String[] args) {
        MyPojoClass pojo = new MyPojoClass();
        pojo.addValueToIntList(42);
        
        List<Integer> list = pojo.getIntList();
        list.add(69);
    }
}

output:

Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.Collections$UnmodifiableCollection.add(Collections.java:1055)
    at App.main(App.java:12)