PASS OBJECT AS PARAMETER BY AAMIR JAMIL
OBJECTIVE
Study Pass
THEORY
In previous lab we have only
been using simple types as parameters to methods. However, it is both correct
and common to pass objects to methods. For example, consider the following short
program:
// Objects may be passed to methods.
class Test {
int a, b; //made by aamirjamil
Test(int i, int j)
{
a = i;
b = j;
}
// return true if o
is equal to the invoking object
boolean equals(Test
o) {
if(o.a == a
&& o.b == b) return true;
else return false;
}
}
class PassOb {
public static void
main(String args[]) {
Test ob1 = new
Test(100, 22);
Test ob2 = new
Test(100, 22);
Test ob3 = new
Test(2,1);
System.out.println("ob1
== ob2: " + ob1.equals(ob2));
System.out.println("ob1
== ob3: " + ob1.equals(ob3));
}
}
This program generates the
following output:
As you can see, the equals(
) method inside Test compares two objects for equality
and returns the result. That
is, it compares the invoking object with the one that it is
passed. If they contain the
same values, then the method returns true. Otherwise,
it returns false. Notice
that the parameter o in equals( ) specifies Test as its
type.
Although Test is a class
type created by the program, it is used in just the same way
as Java’s built-in types.
A Closer Look at Argument Passing
In general, there are two ways
that a computer language can pass an argument to a subroutine. The first way is
call-by-value, the second way
an argument can be passed is call-by-reference.
Passed By Value
class Test {
void meth(int i,
int j) {
i *= 2;
j /= 2;
}
}
class CallByValue {
public static void
main(String args[]) {
Test ob = new
Test();
int a = 15, b = 20;
System.out.println("a
and b before call: " +
a + " " +
b);
ob.meth(a, b);
System.out.println("a
and b after call: " +
a + " " +
b);
}
}
The output from this program is
shown here:
Passed By Reference
class Test {
int a, b;
Test(int i, int j)
{
a = i;
b = j;
}
// pass an object
void meth(Test o) {
o.a *= 2;
o.b /= 2;
}
}
class CallByRef
{public static void main(String args[]) {
Test ob = new
Test(15, 20);
System.out.println("ob.a
and ob.b before call: " +
ob.a + "
" + ob.b);
ob.meth(ob);
System.out.println("ob.a
and ob.b after call: " +
ob.a + "
" + ob.b);
}
}
This program generates the
following output:
Returning Objects
A method can return any type of
data, including class types that you create. For
example, in the following
program, the incrByTen( ) method returns an object in
which the value of a is
ten greater than it is in the invoking object.
// Returning an
object.
class Test {
int a;
Test(int i) {
a = i;
}
Test incrByTen() {
Test temp = new
Test(a+10);
return temp;
}
}
class RetOb {
public static void
main(String args[]) {
Test ob1 = new
Test(2);
Test ob2;
ob2 = ob1.incrByTen();
System.out.println("ob1.a:
" + ob1.a);
System.out.println("ob2.a:
" + ob2.a);
ob2 =
ob2.incrByTen();
System.out.println("ob2.a
after second increase: "
+ ob2.a);
}
}
The output generated by this
program is shown here:
As you can see, each time incrByTen(
) is invoked, a new object is created, and a
reference
to it is returned to the calling routine
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