Arrays and ArrayLists - Java: How to Program Early Objects - page 266

Unlike some other languages, Java does not allow you to choose pass-by-value or pass-
by-reference— all arguments are passed by value . A method call can pass two types of values
to a method—copies of primitive values (e.g., values of type int and double ) and copies
of references to objects. Objects themselves cannot be passed to methods. When a method
modifies a primitive-type parameter, changes to the parameter have no effect on the orig-
inal argument value in the calling method. For example, when line 30 in main of Fig. 7.13
passes array[3] to method modifyElement , the statement in line 45 that doubles the
value of parameter element has no effect on the value of array[3] in main . This is also
true for reference-type parameters. If you modify a reference-type parameter so that it
refers to another object, only the parameter refers to the new object—the reference stored
in the caller's variable still refers to the original object.
Although an object's reference is passed by value, a method can still interact with the
referenced object by calling its public methods using the copy of the object's reference.
Since the reference stored in the parameter is a copy of the reference that was passed as an
argument, the parameter in the called method and the argument in the calling method
refer to the same object in memory. For example, in Fig. 7.13, both parameter array2 in
method modifyArray and variable array in main refer to the same array object in memory.
Any changes made using the parameter array2 are carried out on the object that array
references in the calling method. In Fig. 7.13, the changes made in modifyArray using
array2 affect the contents of the array object referenced by array in main . Thus, with a
reference to an object, the called method can manipulate the caller's object directly.
Performance Tip 7.1
Passing references to arrays, instead of the array objects themselves, makes sense for perfor-
mance reasons. Because everything in Java is passed by value, if array objects were passed,
a copy of each element would be passed. For large arrays, this would waste time and con-
sume considerable storage for the copies of the elements.
7.10 Case Study: Class GradeBook Using an Array to
Store Grades
We now present the first part of our case study on developing a GradeBook class that in-
structors can use to maintain students' grades on an exam and display a grade report that
includes the grades, class average, lowest grade, highest grade and a grade distribution bar
chart. The version of class GradeBook presented in this section stores the grades for one
exam in a one-dimensional array. In Section 7.12, we present a version of class GradeBook
that uses a two-dimensional array to store students' grades for several exams.
Storing Student Grades in an Array in Class GradeBook
Class GradeBook (Fig. 7.14) uses an array of int s to store several students' grades on a sin-
gle exam. Array grades is declared as an instance variable (line 7), so each GradeBook ob-
ject maintains its own set of grades. The constructor (lines 10-14) has two parameters—
the name of the course and an array of grades. When an application (e.g., class GradeBook-
Test in Fig. 7.15) creates a GradeBook object, the application passes an existing int array
to the constructor, which assigns the array's reference to instance variable grades (line 13).
The grades array's size is determined by the length instance variable of the constructor's