# 
## What is the collectors class?

**Collectors** is a utility class that provides various implementations of reduction operations such as grouping elements, collecting elements to different collections, summarizing elements according to various criteria, etc. The different functionalities in the Collectors class are usually used as final operations on streams.

## The toUnmodifiableList() method

**`toUnmodifiableList()`** is a static method of the `Collectors` used to collect/accumulate all the elements to a new unmodifiable list. The method returns a `Collector` that collects all the input elements into an unmodifiable list, and which disallows `null` values, raising a `NullPointerException` if it's supplied with `null` values. This method was introduced in Java 10.

The `toUnmodifiableList` method is defined in the `Collectors` class. The `Collectors` class is defined in the `java.util.stream` package. To import the `Collectors` class, check the following import statement.

public static <T> Collector<T, ?, List<T>> toUnmodifiableList()

# 
#  Parameters 

The method has no parameters.

##
## Return value

This method returns a `Collector` that collects all the input elements into an unmodifiable list in the encounter order.

## Code

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;


public class main {

    public static void main(String[] args) {
        List<String> stringList = Arrays.asList("educative", "io", "edpresso");

        System.out.println("Stream before modification - " + stringList);

        Stream<String> stringStream = stringList.stream();

        List<String> uppercaseList = stringStream.map(String::toUpperCase).collect(Collectors.toUnmodifiableList());

        System.out.println("Resulting list after modification - " + uppercaseList);

        uppercaseList.add("test");
    }
}

java9

#      
## Explanation

- Lines 1-4: We import the relevant packages.
- Line 10: We define a list of strings called `stringList`.
- Line 12: We print the `stringList`.
- Line 14: We create a stream out of the `stringList`.
- Line 16: We convert the elements of the stream to uppercase using the `map` method and collect the resulting elements to a new unmodifiable list using the `toUnmodifiableList` method.
- Line 18: We print the new list of uppercase elements.
- Line 20: We try to add an element to the returned unmodifiable list which throws an exception.

java9.tar.gz

What is Collectors.toUnmodifiableList() in Java?

It collects stream elements into an unmodifiable list, disallows nulls, and throws exceptions when modified. Introduced in Java 10.

What is Collectors.toUnmodifiableList() in Java?

What is the collectors class?

The toUnmodifiableList() method

Syntax

Parameters

Return value

Code

Explanation