Scope and Access: Revision Notes (Computer Science A 3.8)

These notes recap Topic 3.8. For worked examples, read the study guide.

Recap

A local variable is declared in the header or body of a block and can be used only in that block.
Constructors and methods are blocks, so their parameters are local variables. They cannot be declared public or private.
Instance and class variables are in scope throughout the class. public / private decide access from other classes.
If a local variable or parameter has the same name as an instance variable, the name refers to the local one inside that constructor or method.
name = name; in a constructor assigns the parameter to itself and leaves the instance variable at its default.

Key relationships

Variable kind	Declared	Visible	Access modifier?
Instance variable	In the class, outside methods	Whole class; others by access	Yes
Class variable	In the class with `static`	Whole class; others by access	Yes
Parameter	In a method or constructor header	That method or constructor	No
Local variable	In a method, constructor or inner block	That block, after its declaration	No
Loop variable	In a `for` header	The loop only	No

Worked reminder

For class Calc with private int x = 2; and calc(int y): calc(1) has z = 3 and skips the if, returning 3; calc(4) has z = 6, enters the if, makes w = 12 and returns 12. For Pet(String name, int age) { name = name; age = age; } the object prints null 0. For Scorer with total 7, a method whose parameter is also named total returns 25 and leaves the object’s total at 7.

Reading scope in the exam

Underline each declaration and draw a bracket from it to the closing brace of its block. A use of the name outside the bracket is a compile error. When a parameter and an instance variable share a name, cross out the instance variable inside that method: the name now means the parameter, and a change to it never reaches the object.

Compile errors to recognise

Message	Typical cause
`cannot find symbol` (variable)	Used outside its block, in another method, or never declared
(no error, wrong output)	A parameter or local variable hides an instance variable

Mistakes to avoid

Using a loop variable after the loop.
Declaring a variable inside an if and using it after the if.
Naming a constructor parameter the same as an instance variable and writing x = x;.
Re-declaring an instance variable inside a method when you meant to change it (int total = ... instead of total = ...).
Believing a local variable can be public or private.

Quick self-check

Where can a variable declared in a for header be used? Answer: only in that loop.
A method has int count = 0; and the class also has an instance variable count. Which does count++ change? Answer: the local variable.
What does new Pet("Rex", 4).describe() print for the buggy constructor with name = name;? Answer: null 0.
Can a parameter be declared private? Answer: No.