Does Call by reference Java supports?
This is a very basic question asked in every interview and should known to each programmer also.
Everyone says "no" as Java does not support pointers. No one actually see the address of the object in Java. No one can dream of call by reference without pointers with their experience of C/C++.
Of course, they are very correct.
In Java the method parameters can be primitive data types or object references. Both are passed as value only but small tricky explanation is here for object references. I repeat, when primitive data types are passed as method parameters, they are passed by value (a copy of the value) but incase of object references, the reference is copied (of course, here also a copy only) and passed to the called method. That is, object reference is passed as a value. So, original reference and the parameter copy both will refer the same Java object. As both refer the same object, if the calling method changes the value of its reference (passed from calling method), the original object value itself changes. Note that object itself is not passed, but it’s references is passed.
Object reference is a handle to a Java object. Do not compare this reference term with the used in C/C++. In C/C++, the reference directly points to the memory address of a variable and can be used for pointer manipulations.
Finally to say, in Java, everyone is passed by value. But with objecct, the value of the reference is passed.
Let us see two programs on call by value and call by reference.
Case 1: Call-by-value or Pass-by-value
In the following program, a data type int is passed as parameter to a method call.
public class CallByValue
{
public void display(int y)
{
y = 20;
}
public static void main(String args[])
{
CallByValue cbv = new CallByValue();
int x = 10;
cbv.display(x);
System.out.println(x); // prints 10 and not 20
}
}
The variable x value 10 is passed to parameter of y of display() method. As copy of x is passed, changing y does not change x value in main() method.
Case 2: Call by reference Java or Pass-by-reference
Here, for display() method the reference of StringBuffer object is passed.
public class CallByReference
{
public void display(StringBuffer sb2)
{
sb2.append("World");
System.out.println(sb2); // prints HelloWorld
}
public static void main(String args[])
{
CallByReference cbr = new CallByReference();
StringBuffer sb1 = new StringBuffer("Hello");
cbr.display(sb1);
System.out.println(sb1); // prints HelloWorld
}
}
The value of object reference sb1 is passed to sb2. sb1 literal value "Hello" is passed to sb2. Now sb1 reference and sb2 reference refer the same object. Changing sb2 affects sb1 also. Both sb1 and sb2 prints "HelloWorld".
What do you think about this program? Is it call by value or reference as per your post?
class Test {
public static void swap(Integer i, Integer j) {
Integer temp = new Integer(i);
i = j;
j = temp;
}
public static void main(String[] args) {
Integer i = new Integer(10);
Integer j = new Integer(20);
swap(i, j);
System.out.println(“i = ” + i + “, j = ” + j);
}
}
public class Test
{
int x=12, y=25;
public static void main()
{
int x=5, z=10;
String s= “Java is Fun”;
System.out.println(x+” “+ y + “ “ + z);
Function1(x,z);
System.out.println(x+ “ “+y + “ “+z);
System.out.println(s);
Function2(s);
System.out.println(s);
}// end of main
public static void Function2( String s)
{
System.out.println(x+” “+ y );
s=”Java is Platform Independent”;
System.out.println(s);
return;
} // end of Function2
public static void Function1(int x, int z)
{
System.out.println(x+ “ “+y + “ “+z);
x++; z++; y++;
System.out.println(x+ “ “+y + “ “+z);
return;
} // end of Function1
}// end of class
1. Identify the calling and the called methods.
2. Which function is called using CBV and which is called using CBR?
3. Identify the local variables in each method.
4. Identify the global variables.
5. Identify the composite datatypes in the above code
6. Give the output of the above code
class CallByReference
{
public void display(StringBuffer sb2)
{
sb2.append(“World”);
System.out.println(sb2); // prints HelloWorld
}
public void setData(Integer i) {
++i;
}
public static void main(String args[])
{
CallByReference cbr = new CallByReference();
StringBuffer sb1 = new StringBuffer(“Hello”);
cbr.display(sb1);
System.out.println(sb1); // prints HelloWorld
Integer a = new Integer(19);
cbr.setData(a); //It should append the value of a
System.out.println(a); //It should print 20 as per the code which you explained but its giving 19
}
}
thank u sir
but there is little confusion in pass by reference .. plz try it in more easy way
how can we use call by reference in numeric data……?
You cannot use on numeric data. If required, convert int to Integer and then use.