Chapter 5:Understanding Variable Scope and Class Construction

Understanding Variable Scope and
Class Construction

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Understanding Variable Scope
• Exam Objective 4.2 Given an algorithm as pseudo-code,
determine the correct scope for a variable used in the algorithm
and develop code to declare variables in any of the following
scopes: instance variable, method parameter, and local variable.

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Variables and Initialization
Member variable A member variable of a class is created when an
instance is created, and it is destroyed when the object is destroyed.
Subject to accessibility rules and the need for a reference to the object,
member variables are accessible as long as the enclosing object exists.
Automatic variable An automatic variable of a method is created on entry
to the method and exists only during execution of the method, and
therefore it is accessible only during the execution of that method. (You’ll see
an exception to this rule when you look at inner classes, but don’t worry
about that for now.)
Class variable A class variable (also known as a static variable) is created
when the class is loaded and is destroyed when the class is unloaded.
There is only one copy of a class variable, and it exists regardless of the
number of instances of the class, even if the class is never instantiated.
Static variables are initialized at class load time
Ben Abdallah Helmi Architect en
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All member variables that are not explicitly assigned a value upon declaration are
automatically assigned an initial value. The initialization value for member
variables depends on the member variable’s type.
A member value may be initialized in its own declaration line:
1. class HasVariables {
2. int x = 20;
3. static int y = 30;
When this technique is used, nonstatic instance variables are initialized just before
the class constructor is executed; here x would be set to 20 just before invocation
of any HasVariables constructor.

Static variables are initialized at class load time; here y would be set to 30 when the
HasVariables class is loaded.

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Automatic variables (also known as method local variables are not initialized by the system; every automatic variable must be explicitly
initialized before being used. For example, this method will not compile:

1. public int wrong() {

2. int i;

3. return i+5;

4. }
The compiler error at line 3 is, “Variable i may not have been initialized.” This error often appears when initialization of an automatic variable
occurs at a lower level of curly braces than the use of that variable. For example, the following method returns the fourth root of a
positive number:
1. public double fourthRoot(double d) { 2. double result;
3. if (d >= 0) {
4. result = Math.sqrt(Math.sqrt(d));
5. } 6. return result; 7. }
Here the result is initialized on line 4, but the initialization takes place within the curly braces of lines 3 and 5. The compiler will flag line 6,
complaining that “Variable result may not have been initialized.” A common solution is to initialize result to some reasonable default as
soon as it is declared:
1. public double fourthRoot(double d) {

2. double result = 0.0; // Initialize
3. if (d >= 0) {
4. result = Math.sqrt(Math.sqrt(d));
5. } 6. return result; 7. }

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5. Consider the following line of code:
int[] x = new int[25];
After execution, which statements are true? (Choose all that
apply.)
A. x[24] is 0
B. x[24] is undefined
C. x[25] is 0
D. x[0] is null
E. x.length is 25
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5. A, E. The array has 25 elements, indexed from 0
through 24. All elements are initialized to 0.

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Argument Passing: By Reference or by
Value
1. public void bumper(int bumpMe) {
2. bumpMe += 15;
3. }
Line 2 modifies a copy of the parameter passed by the
caller. For example
1. int xx = 12345;
2. bumper(xx);
3. System.out.println(“Now xx is “ + xx);
line 3 will report that xx is still 12345.
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When Java code appears to store objects in variables or pass
objects into method calls, the object references are stored or
passed.

1. Button btn;
2. btn = new Button(“Pink“);
3. replacer(btn);
4. System.out.println(btn.getLabel());
5.
6. public void replacer(Button replaceMe) {
7. replaceMe = new Button(“Blue“);
8. }
Line 2 constructs a button and stores a reference to that button in
btn. In line 3, a copy of the reference is passed into the replacer()
method.
the string printed out is “Pink”.
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1. Button btn;
2. btn = new Button(“Pink“);
3. changer(btn);
4. System.out.println(btn.getLabel());
5.
6. public void changer(Button changeMe) {
7. changeMe.setLabel(“Blue“);
8. }
the value printed out by line 4 is “Blue”.
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Arrays are objects, meaning that programs deal with
references to arrays, not with arrays themselves. What
gets passed into a method is a copy of a reference to an
array. It is therefore possible for a called method to
modify the contents of a caller’s array.

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6.Consider the following application:
class Q6 {
public static void main(String args[]) {
Holder h = new Holder();
h.held = 100;
h.bump(h);
System.out.println(h.held);
}
}
class Holder {
public int held;
public void bump(Holder theHolder) {
theHolder.held++; }
}
}
What value is printed out at line 6?
A. 0
B. 1
C. 100
D. 101

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6. D. A holder is constructed on line 3. A reference to
that holder is passed into method bump() on line 5.
Within the method call, the holder’s held variable is
bumped from 100 to 101.

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7. Consider the following application:
1. class Q7 {
2. public static void main(String args[]) {
3. double d = 12.3;
4. Decrementer dec = new Decrementer();
5. dec.decrement(d);
6. System.out.println(d);
7. }
8. }
9.
10. class Decrementer {
11. public void decrement(double decMe) {
12. decMe = decMe - 1.0;

13. }
14. }
Review Questions 31
What value is printed out at line 6?
A. 0.0
B. 1.0

C. 12.3
D. 11.3

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7. C. The decrement() method is passed a copy of the
argument d; the copy gets decremented, but the
original is untouched.

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20. Which of the following are true? (Choose all that
apply.)
A. Primitives are passed by reference.
B. Primitives are passed by value.
C. References are passed by reference.
D. References are passed by value.

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20. B, D. In Java, all arguments are passed by value.

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Constructing Methods
• Exam Objective 4.4 Given an algorithm with multiple inputs
and an output, develop method code that implements the
algorithm using method parameters, a return type, and the
return statement, and recognize the effects when object
references and primitives are passed into methods that
modify them.

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• The SCJA exam will likely have a question asking the
difference between passing variables by reference and
value. The question will not directly ask you the difference.
• It will present some code and ask for the output. In the group
of answers to select from, there will be an answer that will be
correct if you assume the arguments are passed by value,
and another answer that will be correct if the arguments
were passed by reference.
• It is easy to get this type of question incorrect if passing
variables by reference and value are poorly understood.
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Declaring a Return Type
• A return statement must be the keyword return followed
by a variable or a literal of the declared return type. Once
the return statement is executed, the method is finished.

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Two-Minute Drill

• A variable’s scope defines what parts of the code have
access to that variable.
• An instance variable is declared in the class, not inside of
any method. It is in scope for the entire method and remains
in memory for as long as the instance of the class it was
declared in remains in memory.
• Method parameters are declared in the method declaration;
they are in scope for the entire method.
• Local variables may be declared anywhere in code. They
remain in scope as long as the execution of code does not
leave the block they were declared in.
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• Code that invokes a method may pass arguments to it for
input.
• Methods receive arguments as method parameters.
• Primitives are passed by value.
• Objects are passed by reference.
• Methods may return one variable or none at all. It can be
a primitive or an object.
• A method must declare the data type of any variable it
returns.
• If a method does not return any data, it must use void as
its return type.

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Chapter 5:Understanding Variable Scope and Class Construction

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Chapter 5:Understanding Variable Scope and Class Construction