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Understanding Parameter Passing
Parameters are Passed by Value
It is important to understand how parameter passing works. When you make changes to a parameter passed to a method, a separate copy of the value is passed. Any changes made to the parameter passed into the method will have no effect on the actual parameter.
Parameters are Passed by Value
It is important to understand how parameter passing works. When you make changes to a parameter passed to a method, a separate copy of the value is passed. Any changes made to the parameter passed into the method will have no effect on the actual parameter.
Java passes all parameters to a method by value.
Parameters are Passed by Value
It is important to understand how parameter passing works. When you make changes to a parameter passed to a method, a separate copy of the value is passed. Any changes made to the parameter passed into the method will have no effect on the actual parameter.
Java passes all parameters to a method by value. This means that the current value of the actual
parameter is copied into the formal parameter in the method header.
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
Using the code at left, we will demonstrate how primitive data types are handled as parameters in method calls.
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
The program begins by assigning the value 25 to the integer variable parameter.
25parameter
value
Output :
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
The test object is created.
25parameter
value
Output :
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
The value of parameter is displayed.
25parameter
value
Output :
Initial parameter value is 25
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
The PrimitiveParamTest object method changeValue is called.
25parameter
value
Output :
Initial parameter value is 25
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
As a result of the method call, a copy of parameter is made. This copy is called the formal parameter. The formal parameter is sent to the method.
25parameter
value
Output :
Initial parameter value is 25
25formal parameter
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
When the method refers to parameter, it uses the formal parameter. The formal parameter is changed to 100.
25parameter
value
Output :
Initial parameter value is 25
100formal parameter
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
The value of the formal parameter is displayed.
25parameter
value
Output :
Initial parameter value is 25
Inside method changeValue, parameter value changed to 100
100formal parameter
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
As the method is left, the formal parameter is destroyed.
25parameter
value
Output :
Initial parameter value is 25
Inside method changeValue, parameter value changed to 100
100formal parameter
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
Control is returned to the calling method. The formal parameter no longer exists.
25parameter
value
Output :
Initial parameter value is 25
Inside method changeValue, parameter value changed to 100
An ExampleUsing Primitive Data Types
public class PrimitiveParamTest {
public void changeValue (int parameter)
{
parameter = 100;
System.out.println("Inside method changeValue,parameter value changed to " + parameter);
}
public static void main (String args[])
{
int parameter = 25;
PrimitiveParamTest test = new PrimitiveParamTest();
System.out.println("Initial parameter value is " + parameter);
test.changeValue(parameter);
System.out.println("After method changeValue call,parameter value is " + parameter);
}
}
The value of parameter is displayed.
25parameter
value
Output :
Initial parameter value is 25
Inside method changeValue, parameter value changed to 100
After method changeValue call, parameter value is 25
Passing Objects as Parameters
How does Java pass objects by value?
Passing Objects as Parameters
How does Java pass objects by value?
When an object is passed to a method, we are actually passing a reference to that object.
Passing Objects as Parameters
How does Java pass objects by value?
When an object is passed to a method, we are actually passing a reference to that object.
The value that is copied is the address of the object.
Referencing Objects
Since a reference to an object actually contains the address of the object, the name of the object can been viewed as a “pointer” to the actual object. Remember that an object doesn’t “exist” until it is instantiated.
Shape myObject;myObject
null
Instantiating Objects
The object comes into “existence” when it is instantiated. For example the following code will create the object below:
myObject = new Shape();
myObject
Copying Objects
When an object is assigned to another object, a copy of the reference to the object is made, not a copy of the actual object.
Shape myCopy = myObject;
myCopy myObject
Copying Objects
In this example, myCopy is an alias for myObject. Both myObject and myCopy refer to the same object.
myCopy myObject
Passing Objects as Parameters
In the example involving primitive data types, when an int type is passed as a parameter, a copy, known as the formal parameter is used by the called method. When an object is passed to a method, the formal parameter used is an alias, which is a copy of the reference to an object.
myObject
Passing Objects as Parameters
If the following method is called :
foo.someMethod(myObject);A copy or alias of myObject is created and used as the formal parameter.
myObject
Passing Objects as Parameters
If the following method is called :
foo.someMethod(myObject);A copy or alias of myObject is created and used as the formal parameter.
myObject
The formal parameter
Passing Objects as Parameters
If the following method is called :
foo.someMethod(myObject);A copy or alias of myObject is created and used as the formal parameter.
myObject
The formal parameter
Inside a method, if the formal parameter calls class methods that modify the state of the object, the object will remain in that state when the program flow returns to the instruction following the method call.
Passing Objects as Parameters
myObjectGiven the following code fragments:
public class Foo {
public void changeObject(Shape myObject) {
myObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
Passing Objects as Parameters
myObjectGiven the following code fragments:
public class Foo {
public void changeObject(Shape myObject) {
myObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
When the changeShape method is called, a formal parameter is created.
Passing Objects as Parameters
myObject
The formal parameter
Given the following code fragments:
public class Foo {
public void changeObject(Shape passedObject) {
passedObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
When the changeShape method is called, a formal parameter is created.
This formal parameter is referred to as passedObject in the changeObject method.
Passing Objects as Parameters
myObject
The formal parameter
Given the following code fragments:
public class Foo {
public void changeObject(Shape passedObject) {
passedObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
When passedObject.changeShape() is called, the formal parameter, passedObject, calls the object’s method changeShape().
Passing Objects as Parameters
myObject
The formal parameter
Given the following code fragments:
public class Foo {
public void changeObject(Shape passedObject) {
passedObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
This causes the object to change its state (in this case, its shape).
Passing Objects as Parameters
myObject
The formal parameter
Given the following code fragments:
public class Foo {
public void changeObject(Shape passedObject) {
passedObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
When the changeObject method ends, the formal parameter ceases to exist.
Passing Objects as Parameters
myObjectGiven the following code fragments:
public class Foo {
public void changeObject(Shape passedObject) {
passedObject.changeShape();
}
. . .
}
. . .
Foo changer = new Foo();
myObject = new Shape();
changer.changeObject(myObject);
When the changeObject method ends, the formal parameter ceases to exist.
BUT, the object still exists in its changed state.
Changing object formal parameters
If an object formal parameter can change an object’s state by calling the object’s methods, what happens when the value of the formal parameter is changed?
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
The code from the previous example is changed slightly.
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
If myObject is passed to the changeObject method,
myObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
If myObject is passed to the changeObject method,
a formal parameter passedObject is created.
myObjectpassedObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
Another object, newObject is created.
myObjectpassedObject newObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
Another object, newObject is created.
The value of passedObject is changed to newObject.
myObjectpassedObject newObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
The formal parameter passedObject, calls the method changeShape().
myObjectpassedObject newObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
This causes the object to change its state (in this case, its shape).
myObjectpassedObject newObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
When the changeObject method ends, the formal parameter passedObject ceases to exist.
myObjectpassedObject newObject
Changing object formal parameters
public class Foo {
public void changeObject(Shape passedObject) {
Shape newObject = new Shape();
passedObject = newObject;
passedObject.changeShape();
}
. . .
}
When the changeObject method ends, the formal parameter passedObject ceases to exist.
myObject remains unchanged.
myObjectnewObject
Summary
Java passes all parameters to a method by value. This means that the current value of the actual
parameter is copied into the formal parameter in the method header.
Since a reference to an object actually contains the address of the object, the name of the object can been viewed as a “pointer” to the actual object.
Summary
When an object is passed to a method, the formal parameter used is an alias, which is a copy of the reference to an object.
An object’s formal parameter can change an object’s state by calling the class’ methods, but, If the value of the formal parameter is changed, the
actual object remains unchanged.
