Classes and Objects - Beginning Java 8 Fundamentals - page 236

#1: a = 19, b = 37
#2: x = 19, y = 37
#3: x = 19, y = 223
#4: a = 19, b = 37
Let's discuss the parameter passing mechanism for reference type parameters. Java lets you use the pass by
reference value and pass by constant reference value mechanisms to pass reference type parameters to a method.
When a parameter is passed by reference value, the reference stored in the actual parameter is copied to the formal
parameter. When the method starts executing, both the actual parameter and the formal parameter refer to the same
object in memory. If the actual parameter has a null reference, the formal parameter will contain the null reference.
You can assign a reference to another object to the formal parameter inside the method's body. In this case, the formal
parameter starts referencing the new object in memory and the actual parameter still references the object it was
referencing before the method call. Listing 6-24 demonstrates the pass by reference mechanism in Java. It creates a
Car object inside the main() method and stores the reference of the Car object in myCar reference variable.
// Create a Car object and assign its reference to myCar
Car myCar = new Car();
It modifies the model, year, and price of the newly created Car object using myCar reference variable.
// Change model, year and price of Car object using myCar
myCar.model = "Civic LX";
myCar.year = 1999;
myCar.price = 16000.0;
The message labeled #1 in the output shows the state of the Car object. The myCar reference variable is passed to
the test() method using the following call:
PassByReferenceValueTest.test(myCar);
Since the type of the formal parameter xyCar in the test() method is Car , which is a reference type, Java
uses the pass by reference value mechanism to pass the value of the myCar actual parameter to the xyCar formal
parameter. When the test(myCar) method is called, Java copies the reference of the Car object stored in the myCar
reference variable to the xyCar reference variable. When the execution enters the test() method's body, myCar and
xyCar reference the same object in memory. At this time, there is only one Car object in memory and not two. It is
very important to understand that the test(myCar) method call did not make a copy of the Car object referenced by
the myCar reference variable. Rather, it made a copy of the reference (memory address) of the Car object referenced
by the myCar reference variable, which is the actual parameter, and copied that reference to the xyCar reference
variable, which is the formal parameter. The fact that both myCar and xyCar reference the same object in memory
is indicated by the message labeled #2 in the output, which is printed using the xyCar formal parameter inside the
test() method.
Now you create a new Car object and assign its reference to the xyCar formal parameter inside the test()
method.
// Let's make xyCar refer to a new Car object
xyCar = new Car();