WC.8: SCOPE AND ACCESS

OBJECTIVE

I can:

explain where variables can be used in the program code

The scope of a variable is defined as where a variable is accessible or can be used.

The scope is determined by where you declare the variable when you write your programs.

When you declare a variable, look for the closest enclosing curly braces { } - this is its scope.

Java has three levels of scope that correspond to different types of variables:

1) Class Level Scope for instance variables inside a class.

2) Method Level Scope for local variables (including parameter variables) inside a method.

3) Block Level Scope for loop variables and other local variables defined inside of blocks of code with { }.

The following image shows these three levels of scope:

LOCAL VARIABLES

Local variables are variables that are declared inside a method, usually at the top of the method.

These variables can only be used within the method and do not exist outside of the method.

Parameter variables are also considered local variables that only exist for that method.

It is good practice to declare any variables that are used by just one method as local variables in that method.

Instance variables at class scope are shared by all the methods in the class and can be marked as public or private with respect to their access outside of the class.

They have Class scope regardless of whether they are public or private.

Another way to look at scope is that a variable's scope is where it lives and exists.

You cannot used the variable in code outside of its scope because it doesn't exist!

In other words, the following program has variables that would cause a lot of "cannot find symbol" errors:

So, I made sure to make sure that all of the variables were in scope to fix the program:

If there is a local variable with the same name as an instance variable, the variable name will refer to the local variable instead of the instance variable, as seen below.

In the next lesson, we will see that we can distinguish between the local variable and the instance variable using the keyword this to refer to this object's instance variables.

SUMMARY

Scope is defined as where a variable is accessible or can be used.
Local variables can be declared in the body of constructors and methods. These variables may only be used within the constructor or method and cannot be declared to be public or private.
When there is a local variable with the same name as an instance variable, the variable name will refer to the local variable instead of the instance variable.
Formal parameters and variables declared in a method or constructor can only be used within that method or constructor.

EVIDENCE

1) Complete the following Google Form. This form must be 100% correct and includes released AP practice questions. To stop working and return later, hit submit! You can "edit your response" and continue where you left off.

WC.8 Evidence

2) Debugging activity. Debug the following program that has scope violations. You may need to add methods or use methods that are in the class Fraction appropriately. Then, add comments that label the variable declarations as class, method, or block scope.

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WC.8: SCOPE AND ACCESS

OBJECTIVE

The scope of a variable is defined as where a variable is accessible or can be used.

The scope is determined by where you declare the variable when you write your programs.

When you declare a variable, look for the closest enclosing curly braces { } - this is its scope.

Java has three levels of scope that correspond to different types of variables:

1) Class Level Scope for instance variables inside a class.

2) Method Level Scope for local variables (including parameter variables) inside a method.

3) Block Level Scope for loop variables and other local variables defined inside of blocks of code with { }.

The following image shows these three levels of scope:

LOCAL VARIABLES

Local variables are variables that are declared inside a method, usually at the top of the method.

These variables can only be used within the method and do not exist outside of the method.

Parameter variables are also considered local variables that only exist for that method.

It is good practice to declare any variables that are used by just one method as local variables in that method.

Instance variables at class scope are shared by all the methods in the class and can be marked as public or private with respect to their access outside of the class.

They have Class scope regardless of whether they are public or private.

Another way to look at scope is that a variable's scope is where it lives and exists.

You cannot used the variable in code outside of its scope because it doesn't exist!

In other words, the following program has variables that would cause a lot of "cannot find symbol" errors:

So, I made sure to make sure that all of the variables were in scope to fix the program:

If there is a local variable with the same name as an instance variable, the variable name will refer to the local variable instead of the instance variable, as seen below.

In the next lesson, we will see that we can distinguish between the local variable and the instance variable using the keyword this to refer to this object's instance variables.

SUMMARY

Scope is defined as where a variable is accessible or can be used.

Local variables can be declared in the body of constructors and methods. These variables may only be used within the constructor or method and cannot be declared to be public or private.

When there is a local variable with the same name as an instance variable, the variable name will refer to the local variable instead of the instance variable.

Formal parameters and variables declared in a method or constructor can only be used within that method or constructor.

EVIDENCE

1) Complete the following Google Form. This form must be 100% correct and includes released AP practice questions. To stop working and return later, hit submit! You can "edit your response" and continue where you left off.

WC.8 Evidence

2) Debugging activity. Debug the following program that has scope violations. You may need to add methods or use methods that are in the class Fraction appropriately. Then, add comments that label the variable declarations as class, method, or block scope.