What is Pass by Value and Pass by reference?
These are the two way for passing
parameters to the method or functions in programming languages.
Pass by Value or call by value
If we
call a method by passing values to it, it is known as call by value.
Whatever changes you are doing in the called
method, will not affect the calling method.(unless and until you return that
value from the method).
i.e In case of call by value
original value is not changed
E.g
Public class passByValue {
Public static Void increment(int i)
{
i
= i + 1; // i =11;
}
Calling part:
public static void main(String[] args)
{
int i =
10;
increment(i);
System.out.println("
i: " +i ); // i = 10;
}
Here value of ‘i ‘remain constant in
outside increment. That is we are just passing values to the method.
Pass by Reference :
In the pass by reference we are
passing the reference (reference to memory location) of the object not the
value. So the calling parameter and method arguments both are pointing to same
memory location, so change made to the values will affect both the caller and
callee.
i.e In case of call by reference original value is changed if we
made changes in the called method
In java there is no Pass by Reference , But for c
and c++ there are Pass by Reference ,so
taking the example from c to explain
E.g from C. – swap variables.
void swap(int *x, int *y) {
int temp;
temp = *x; /* assigning temp to address of x */
*x = *y; /* assign x to y */
*y = temp; /* assigning y to temp */
return;
}
int main () {
/* local variable definition */
int a = 30;
int b = 50;
printf("Before swap, value
of a : %d\n", a );
printf("Before swap, value
of b : %d\n", b );
/* * &a -> address of variable a
* &b -> address of
variable b.
*/
swap(&a, &b);
printf("After swap, value
of a : %d\n", a );
printf("After swap, value
of b : %d\n", b );
return 0;
}
O/P :
Before swap, value
of a :30
Before swap, value of b :50
After swap, value of a
:50
After
swap, value of b
:30
In the above example we will be able
to see the how to do a pass by reference. By passing ‘&a’ we are passing
the reference of the ‘a’(address location of the a), so whatever changes made
to this ‘a’ will reflect to the memory location. So if we don’t return also the
same will reflect for calling program also.
But we know that in java there is no
concept of pointers and passing the variables with ‘&’. So there is no concept
of pass by reference in java.
There Exists only Pass by value in JAVA
In java we can do only pass by values
(call by values) and passes the
references by value.
If it is primitive type or if it is an
object, everything we are passing to functions using values.
Java passes the
references by value.
While hearing
this, java don’t have pass by reference, we will be quite surprised!!!! Because
every time we are hearing only, object , reference and instance in java, also all object variables are references. But java
doesn’t pass method parameters by references, it passes that by the references
by value.
"Java
manipulates objects 'by reference,' but it passes object references to methods
'by value.'" – From O'Reilly's Java in a
Nutshell by David
Flanagan
In java
they are 2 types of parameters we are passing to the methods.
Primitive data types and Reference
data types.
Primitive data type in
Java are passed by value.
Reference data types are
passed by object
references by value
For Primitive we have already seen example in pass by value.
Pass object references
by value – using Reference data types
Java copies and
passes the reference by value, not the object.
But these passed reference are just copies of
the object references, not the original Object.
public class Employee {
int number;
String name;
public static void increment(Employee emp) {
emp.number = emp.number +1;
}
public static void main(String[] args) {
Employee e = new Employee();
e.number = 1001;
increment(e);
System.out.println(e.number);
}
}
When an object
reference is passed to a method, the method gets a copy of the object
reference.
So the
passes and received arguments refer to same object.
Original
reference and the parameter copy both will refer the same Java object.
So
change in values of the reference from method reflect to both original and
copy.
But from method (inside
the method), we will be able to change
the state of object, but that will not get reflected to the passed object,
since we are passing only copy of reference.
public static void increment(Employee emp) {
Employee emp1 = new Employee();
emp1.number = 2000;
emp = emp1;
System.out.println(emp.number);
}
Consider the above
example, here we are changing the state of object emp to emp1.So now change in
emp.number will not reflect to e.number.
 |
Value of e.number doesn’t
change. But the value of Emp.number changed to ‘2000’.
That is reference by value
are not actual object reference.
Swap will not
work in java.
In the above
in ‘Pass by Reference’ section you
have seen a swap program, from C.
But this will not work in java, since java doesn’t
have pass by reference.
public class Employee {
int number;
String name;
public static void swap(Employee emp1,Employee emp2) {
Employee temp = emp1;
emp1 = emp2;
emp2 = temp;
System.out.println("At the end of swap
method, value of e1.number : "+
emp1.number );
System.out.println("At the end of swap method,
value of e2.number : "+ emp2.number );
}
public static void main(String[] args) {
Employee e1 = new Employee();
e1.number = 1001;
Employee e2 = new Employee();
e2.number = 3001;
System.out.println("Before swap, value of e1.number :"+ e1.number );
System.out.println("Before swap, value of e2.number : "+ e2.number );
swap(e1,e2);
System.out.println("After swap, value of e1.number :"+ e1.number );
System.out.println("After swap, value of e2.number : "+ e2.number );
}
}
O/P:
Before
swap, value of e1.number :1001
Before
swap, value of e2.number : 3001
At
the end of swap method, value of
e1.number : 3001
At
the end of swap, value of e2.number :
1001
After
swap, value of e1.number :1001
After
swap, value of e2.number : 3001
From the
main() method we passed two objects e1 and e2 to swap()
method and those are received by swap() method as emp1 and emp2 references
respectively. Then inside swap() method emp1 and emp2 are
getting interchanged.
You
will be able to see that inside the swap method both are interchanged.
At the end
of swap method, value of e1.number :
3001
At the end
of swap, value of e2.number : 1001
Inside
the swap() , the state of object changed,
so that will not reflected to the value of the reference.
So in main()
program if we print after the swap() it is giving old values only
After swap,
value of e1.number :1001
After swap,
value of e2.number : 3001
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