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 Example Using 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 Example Using 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. value parameter 25 Output :

An Example Using 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. value parameter 25 Output :

An Example Using 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. value parameter 25 Output : Initial parameter value is 25

An Example Using 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. value parameter 25 Output : Initial parameter value is 25

An Example Using 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. value formal parameter 25 parameter 25 Output : Initial parameter value is 25

An Example Using 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. value formal parameter 100 parameter 25 Output : Initial parameter value is 25

An Example Using 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. value formal parameter 100 parameter 25 Output : Initial parameter value is 25 Inside method changeValue, parameter value changed to 100

An Example Using 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. value formal parameter 100 parameter 25 Output : Initial parameter value is 25 Inside method changeValue, parameter value changed to 100

An Example Using 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. value parameter 25 Output : Initial parameter value is 25 Inside method changeValue, parameter value changed to 100

An Example Using 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. value parameter 25 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 Shape myCopy = myObject; 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; myObject myCopy

Copying Objects In this example, myCopy is an alias for myObject. Both myObject and myCopy refer to the same object. myObject myCopy

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 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. The formal parameter

Passing Objects as Parameters Given the following code fragments: public class Foo { public void changeObject(Shape myObject) { myObject.changeShape(); } . . . Foo changer = new Foo(); myObject = new Shape(); changer.changeObject(myObject); myObject

Passing Objects as Parameters Given the following code fragments: public class Foo { public void changeObject(Shape myObject) { myObject.changeShape(); } . . . Foo changer = new Foo(); myObject = new Shape(); changer.changeObject(myObject); myObject When the changeShape method is called, a formal parameter is created.

Passing Objects as Parameters 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); myObject The formal parameter 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 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); myObject The formal parameter When passedObject.changeShape() is called, the formal parameter, passedObject, calls the object’s method changeShape().

Passing Objects as Parameters 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); myObject The formal parameter This causes the object to change its state (in this case, its shape).

Passing Objects as Parameters 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); myObject The formal parameter When the changeObject method ends, the formal parameter ceases to exist.

Passing Objects as Parameters 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); 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. myObject passedObject

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. myObject passedObject 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. myObject passedObject 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(). myObject passedObject 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). myObject passedObject 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 passedObject 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. myObject newObject

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.