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I need to convert mutable list object to immutable list. What is the possible way in java?

public void action() { List<MutableClass> mutableList = Arrays.asList(new MutableClass("san", "UK", 21), new MutableClass("peter", "US", 34)); List<MutableClass> immutableList = immutateList(mutableList); } public List<MutableClass> immutateList(List<MutableClass> mutableList){ //some code here to make this beanList immutable //ie. objects and size of beanList should not be change. //ie. we cant add new object here. //ie. we cant remove or change existing one. }

MutableClass

final class MutableClass { final String name; final String address; final int age; MutableClass(String name, String address, int age) { this.name = name; this.address = address; this.age = age; } }
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11
  • What you have there is not immutable, because you can very easily call the .add(Object o) method. Make a subclass of ArrayList, override the .add(..) method(s) and have it either have an empty implementation, or throw an unchecked exception. And, make that class final, so you can't subclass it and change the implementation Commented May 20, 2015 at 11:28
  • You could Wrap list in another object and only implement the Iterable interface Commented May 20, 2015 at 11:29
  • 3
    Answer to your question : stackoverflow.com/questions/10750791/… Commented May 20, 2015 at 11:30
  • 2
    @dubey-theHarcourtians, you should look at the entire answer of this link, giving you the answer 3 lines under Commented May 20, 2015 at 11:36
  • 2
    I am sure that this question has been answered before, but it is not a duplicate of stackoverflow.com/questions/10750791/… Commented May 20, 2015 at 18:18

8 Answers 8

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85

Once your beanList has been initialized, you can do

beanList = Collections.unmodifiableList(beanList);

to make it unmodifiable. (See Immutable vs Unmodifiable collection)

If you have both internal methods that should be able to modify the list, and public methods that should not allow modification, I'd suggest you do

// public facing method where clients should not be able to modify list public List<Bean> getImmutableList(int size) { return Collections.unmodifiableList(getMutableList(size)); } // private internal method (to be used from main in your case) private List<Bean> getMutableList(int size) { List<Bean> beanList = new ArrayList<Bean>(); int i = 0; while(i < size) { Bean bean = new Bean("name" + i, "address" + i, i + 18); beanList.add(bean); i++; } return beanList; }

(Your Bean objects already seem immutable.)

As a side-note: If you happen to be using Java 8+, your getMutableList can be expressed as follows:

return IntStream.range(0, size) .mapToObj(i -> new Bean("name" + i, "address" + i, i + 18)) .collect(Collectors.toCollection(ArrayList::new));
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25

In JDK 8:

List<String> stringList = Arrays.asList("a", "b", "c"); stringList = Collections.unmodifiableList(stringList);

In JDK 9:

List stringList = List.of("a", "b", "c");

reference

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17

From Java 10 on, List.copyOf(Collection) can be used to return an unmodifiable list from the given collection. From source code of List.copyOf method:

  • if the given collection is an unmodifiable List, List.copyOf() will not create a copy.

  • if the given collection is mutable and modified, the returned list will not reflect such modifications. Meaning they are independent.

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What do you mean by your second point? By using List.copyOf(Collection), are you making a defensive copy, similar to what stackoverflow.com/a/47461151/107158 suggests?
Indeed. See docs.oracle.com/javase/10/docs/api/java/util/… for more info.
12

Use Collections.unmodifiableList(). You pass in your original ArrayList and it returns a list that throws an exception if you try to add, remove or shift elements. For example, use return Collections.unmodifiableList(beanList); instead of return beanList; at the end of getImmutableList(). main() will throw an exception. The Collections class has methods for all of the other common collection types besides List as well.

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6

Creating empty immutable list before java SE 9

Prior to Java 9, we have to use unmodifiableList() method of Collections class to create immutable list.

List<String> noElementList = new ArrayList<String>(); List<String> immuList = Collections.unmodifiableList(noElementList);

Creating Non-empty immutable list before Java SE 9

List<String> list = new ArrayList<String>(); list.add("Atto"); list.add("Rick"); list.add("Shalini"); List<String> immuList = Collections.unmodifiableList(list);

Java 9 – Creating Immutable list using static factory method of()

List<String> immuList = List.of();

Java 9 – Creating Non-empty immutable list

List<String> immuList = List.of("Atto", "Rick", "Shalini");
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5

If are open to using a third party library, Eclipse Collections lets you convert from MutableList to ImmutableList and back again.

MutableList<String> mutable = Lists.mutable.with("a", "b", "c"); ImmutableList<String> immutable = mutable.toImmutable(); MutableList<String> mutableAgain = immutable.toList();

This also works with primitive collections.

MutableCharList mutable = CharLists.mutable.with('a', 'b', 'c'); ImmutableCharList immutable = mutable.toImmutable(); MutableCharList mutableAgain = immutable.toList();

If you have an ArrayList as the mutable List, the following will work.

List<String> mutable = new ArrayList<>(Arrays.asList("a", "b", "c")); ImmutableList<String> immutable = Lists.immutable.withAll(mutable); List<String> mutableAgain = immutable.toList();

Note: I am a committer for Eclipse Collections.

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the other answers are correct but lack the essential element: The TYPING (if it's immutable then it should look like it's a regular, mutable list). This answer, while going the extra mile, is the most robust one.
3

Make it immutable instead of using directly unmodifiableList on list as otherwise still original list can be changed.

Basically unModifiable Collection is a view, So indirectly it could still be 'modified' from some other reference that is modifiable. Also as its just a readonly view of annother collection , When the source collection changes unModifiable Collection will always present with latest values.

However immutable Collection can be treated as a readonly copy of another collection and can not be modified. In this case when the source collection changes , immutable Collection do not reflect the changes

List<String> immutableList=Collections.unmodifiableList( new ArrayList<String>(modifiableList));

Using guava:

import java.util.*; import com.google.common.collect.ImmutableList; ImmutableList<String> iList = ImmutableList.copyOf(list);
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2

Below solution is for making list as Immutable without using any API.

Immutable Object with ArrayList member variable

public final class Demo { private final List<String> list = new ArrayList<String>(); public Demo() { list.add("A"); list.add("B"); } public List<String> getImmutableList() { List<String> finalList = new ArrayList<String>(); list.forEach(s -> finalList.add(s)); return finalList; } public static void main(String[] args) { Demo obj = new Demo(); System.out.println(obj.getImmutableList()); obj.getImmutableList().add("C"); System.out.println(obj.getImmutableList()); } }

So the actual list will not change, always output will be [A,B]

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So getImmutableList() essentially creates a defensive copy of the list so that the client cannot modify the original list?
Yes, Client can not modify list