Java中的最终数组

Question

The following code in Java uses a final array of String and there is no question about it.

final public class Main {
  public static final String[] CONSTANT_ARRAY = {"I", "can", "never", "change"};

  public static void main(String[] args) {
    for (int x = 0; x < CONSTANT_ARRAY.length; x++) {
      System.out.print(CONSTANT_ARRAY[x] + " ");
    }
  }
}

It displays the following output on the console.

I can never change

---

The following code also goes without any question.

final public class Main {
  public static final String[] CONSTANT_ARRAY = {"I", "can", "never", "change"};

  public static void main(String[] args) {
    //CONSTANT_ARRAY={"I", "can", "never", "change"}; //Error - can not assign to final variable CONSTANT_ARRAY.
    for (int x = 0; x < CONSTANT_ARRAY.length; x++) {
      System.out.print(CONSTANT_ARRAY[x] + " ");
    }
  }
}

Obviously, the commented line causes the specified error because we are trying to reassign the declared final array of type String .

---

What about the following code.

final public class Main {
  public static final String[] CONSTANT_ARRAY = {"I", "can", "never", "change"};

  public static void main(String[] args) {
    CONSTANT_ARRAY[2] = "always";  //Compiles fine.
    for (int x = 0; x < CONSTANT_ARRAY.length; x++) {
      System.out.print(CONSTANT_ARRAY[x] + " ");
    }
  }
}

and it displays I can always change means that we could manage to modify the value of the final array of type String . Can we ever modify the entire array in this way without violating the rule of final ?

Answer 1

final in Java affects the variable , it has nothing to do with the object you are assigning to it.

final String[] myArray = { "hi", "there" };
myArray = anotherArray; // Error, you can't do that. myArray is final
myArray[0] = "over";  // perfectly fine, final has nothing to do with it

Edit to add from comments: Note that I said object you are assigning to it . In Java an array is an object. This same thing applies to any other object:

final List<String> myList = new ArrayList<String>():
myList = anotherList; // error, you can't do that
myList.add("Hi there!"); // perfectly fine. 

Answer 2

You are misinterpreting the final implementation. final applies to the array object reference, which means once it is initiated, the reference can never change but the array its self can be populated. "Its not violating the rules" you have specified only one rule about the reference change which is working accordingly. If you want the values should also never change you should go for Immutable lists ie

List<String> items = Collections.unmodifiableList(Arrays.asList("I", "can", "never", "change"));

Answer 3

You can only make it so the array reference can't be changed. If you want the elements to be unable to be changed, you need to use an unmodifiable collection of some kind.

Answer 4

当你声明一个数组为final时，你可以改变数组中的元素，但是你不能改变这个数组的引用。

Answer 5

final only guarantees immutability of primitives. And also guarantees that a variable is assigned only once. If an object is mutable you can change the content of it event it defined as final. You may check immutable collections for your needs. Such as Collections.unmodifiableList() http://docs.oracle.com/javase/6/docs/api/java/util/Collections.html#unmodifiableList(java.util.List )

Answer 6

The reference to the array object is final (can not change eg in case you would attempt to associate a different Java array object (instance of String[]) to the same final variable...you'd get a compile time error).

BUT the fields of the final array object in your example are not final, and so you can modify their value. ...while the Java object you created, CONSTANT_ARRAY, after receiving an initial value, will have that value "forever" == until the JVM stops. :) It will be the same String Array instance "forever".

Final variables in Java are not a big deal, just spend some time to digest the topic/idea carefully. :-)
I suggest to all of those who are uncertain to meditate over this page, for example: https://docs.oracle.com/javase/specs/jls/se7/html/jls-4.html#jls-4.12.4

" Once a final variable has been assigned, it always contains the same value. If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object.

This applies also to arrays, because arrays are objects; if a final variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array. "

Answer 7

The value of the variable CONSTANT_ARRAY cannot change. That variable contains a (reference to an) array. However, the contents of the array can change. Same thing happens when you declare any kind of final variable that is not a simple scalar type (eg an object).

Be careful how you name your variables. :-) Calling it a CONSTANT_ARRAY doesn't make the contents of the array unchangeable.

Here's a good reference: The final word on final

Answer 8

When a variable is declared with the final keyword, its value can't be modified, essentially, a constant. This also means that you must initialize a final variable. If the final variable is a reference , this means that the variable cannot be re-bound to reference another object , but the internal state of the object pointed by that reference variable can be changed ie you can add or remove elements from the final array or final collection.

Answer 9

    final int[] res;
    int[] res1;
    int[] res2 = new int[1];
    res2[0]=20;

    res1=res2;
    res1=res2;//no error
    System.out.println("res1:"+res1[0]);

    res = res2;//only once
   //res = res2;//error already initialised
    res2[0]=30;

    System.out.println("res:"+res[0]);     

output:: res1:20 res:30