java immutable object question

Question

String abc[]={"abc"};
String def[]={};

def=abc;
def[0]=def[0]+"changed";
System.out.println(abc[0]);

by changing "def" object, my abc object is changed as well. Beside String[] array has this characteristic what other java object has similar characteristic? can explain more? in order to prevent abc from changed when i changed def, i will have to do def = abc.clone();

Answer 1

You are confusing object mutability/immutability with copying of reference values.

In these diagrams, [var/index] is a reference variable, and {{an Object}} is an object.

String abc[]={"abc"};
String def[]={};

   [abc] ------> {{a String[1]}}
                 [0] --------------> {{a String "abc"}}

   [def] ------> {{a String[0]}}

Now you make def reference variable points to the same object as abc reference variable:

def=abc;

   [abc] ------> {{a String[1]}}
              /  [0] --------------> {{a String "abc"}}
             /
   [def] ---/    {{a String[0]}}

At this point, the array of length zero is unreferenced, and should be garbage-collectable. We can narrow our discussion to the array of length one. Note that a String[] is an array of references. With this next line, you changed what the only element in the length one array points to.

def[0]=def[0]+"changed";

   [abc] ------> {{a String[1]}}
              /  [0] ---------      {{a String "abc"}}
             /                 
   [def] ---/                   --> {{a String "abcchanged"}}

Note that {{a String "abc"}} itself was not mutated. [abc] and [def] now points to the same {{a String[1]}}, which is mutable (i.e. you can make the elements of the array, which are references to String objects, to point to anything).

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in order to prevent abc from changed when i changed def, i will have to do def = abc.clone();

Actually, that's not quite accurate. Let's see what happens if you clone() an array of references to a mutable type StringBuilder.

    StringBuilder[] abc = new StringBuilder[] { new StringBuilder("Hello") };
    StringBuilder[] def = abc.clone();
    def[0].append(" world!");
    System.out.println(abc[0]); // prints "Hello world!"

I won't make the diagrams for you this time, but you can easily draw it out on paper. What's happening here is that even though clone() makes a second {{a StringBuilder[1]}} object with its own element (i.e. def != abc), that element is pointing to the same {{a StringBuilder}} object (i.e. def[0] == abc[0]).

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In short:

• 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

If you want more in-depth understanding of the issues, I recommend the following:

• Is Java pass by reference? -- NO!
• Are Java function parameters always passed-by-value? -- YES!