CODEWITHSUNDEEP
javapass-by-reference
djoomla
djoomla

Reputation: 19

java - string and array referencing

please help me understand this. Shouldn't the output be the same if they both work by passing a reference? Sorry if it's a stupid question.

String A="john";
String B=A;
A="mary";
System.out.println(B);

int[] a=new int[]{5};
int[] b=a;
a[0]=6;
System.out.println(b[0]);

a=john          a=5
b=a               b=a
a=mary         a=6
print b           print b
---------         ---------
john              6

Upvotes: 0

Views: 59

Answers (2)

Jason C
Jason C

Reputation: 40315

First, "passing by reference" refers to passing parameters to methods. This is not the correct terminology here.

Non-primitive variables store references to objects.

When you set A="john" you create a string, "john", and the variable A references that.

When you set B=A you set B to refer to the same object that A also refers to at that point in time.

When you set A="mary" you create a new string, "mary", and the variable A now references that. But this has no effect on that "john" string, which still exists, which B references.

It is exactly the same idea as with primitives:

int x = 10;
int y = x;  // y now holds the value, 10
x = 11;     // x now holds the value, 11, but y is still 10.

The reference to the string "john" or "mary" is the value of your variables. The value isn't the string itself, it's a reference to the existing string object (it's a memory location, really).

Note that a convenient feature of Java may be making this less obvious for strings; in Java String str = "john"; is essentially a shortcut for:

 char data[] = {'j', 'o', 'h', 'n'};
 String str = new String(data);

This is documented clearly here.

You really probably just want to watch the classic Binky's Pointer Fun video, Java version.

Upvotes: 1

Mike Ruder
Mike Ruder

Reputation: 33

When doing

String B=A

you are copying the reference from A to B, so they are not pointing to the same place in the memory.
When you assign "mary" to A, you are changing the reference of var A to a new place in memory.
Now A and B are pointing to different places in memory (Or different Strings to simplify).

When you change value in an array, you do not change the reference of the array, You are actually changing the content of the array.

Upvotes: 2

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