Are String Arrays mutable?

Question

I wonder if String arrays in Java are mutable ? I know that Strings are immutable, but how about string Arrays ?

If I have a string array, and change the content, will a new string object be created ? Or will the actual value just be changed ?

Thanks in advance

Solution 1

The Strings contained in the String[] are indeed immutable, but the array is mutable.

This is well explained in this answer:

• Immutability means that objects of a certain type can not change in any meaningful way to outside observers
  • Integer, String, etc are immutable
  • Generally all value types should be
• Array objects are mutable
  • It may be an array of references to immutable types, but the array itself is mutable
    • Meaning you can set those references to anything you want
    • Also true for array of primitives
  • An immutable array will not be practical
• References to objects can be shared
  • If the object is mutable, mutation will be seen through all these references

EDIT:

Somewhat related: Why can't strings be mutable in Java and .NET?

Solution 2

As far as i remember the field in your array will reference another String

String[] array {"I","like","rain"};
array[2] = "sun"

your array can be changed. the Strings themselves not.

Solution 3

The String[] array does not contain references to string variables, as some people might say. The String[] array contains values (or more exactly references to values), but not references to variables.

public class Main {
    public static void main(String[] args) {

        String one="1";
        String two="2";

        String[] arr = {one, two};

        System.out.println(arr[1]);
        // Result is "2"

        two="3";

        System.out.println(arr[1]);
        // Result is still "2"
    }
}

So, as you can see, the arr[1] is not a reference to String two. It got the value from the variable two and that's it. Changing the variable two did not affect arr[1].

The same thing about ArrayList:

//...

    String one="1";
        String two="2";
    List arr2 = new ArrayList<String>();
        
        arr2.add(one);
        arr2.add(two);
        
        System.out.println(arr2.get(0));
   // Result is "1"

        one = "one";
        System.out.println(arr2.get(0));
   // Result is still "1", not "one"

//...

Therefore the array String elements are immutable (which is logical, because Strings are immutable).

The mutability occurs when you pass the arrays arr or arr2 themselves to a procedure, not their immutable String elements.

For example:

   change(arr);
   // where arr is a String[] array of {"1","2"}
   // see the procedure below

     System.out.println(arr[0]);
  // Result is "one"
  
        
  // ...
    
    static void change(String[] someArray){
        someArray[0]="one";
    }

In other words, the array is passed by reference (=mutable), but its string elements are passed by value (immutable).

Solution 4

In Arrays, each element is just a pointer to an object. So, when you do something like

String one = "1";
String two = "2";
String three = "3";
String four = "4";
String[] myStringArray = {one, two, three};
myStringArray[2] = four;

Then the pointer that was at the 3rd element of the array now points to the four instead of three.

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