Variable and Method Scope in Java

Explains variable and method scope in the Java programming language. Includes an interactive example of Java class, instance, and local variables and class and instance methods.

Prerequisites

Understand Java primitive types and variables
Understand Java methods

Objectives

Find and declare local, instance, and class variables
Find and declare instance and class methods
Call instance and class methods
Know when to use local, instance, and class variables
Know when to use instance and class methods
Declare private, (default), protected, and public attributes and methods
Know when to use private, (default), protected, and public attributes and methods

Purpose of Scope

Used correctly, variable scope enables a program to reuse and conserve memory.
Correct variable scope makes code more understandable and less error prone.
Correct method scope allows a method to be called more simply.

Variable Scope

The scope of a variable determines what owns (contains) the variable and how long the variable will exist (hold a value).
Three categories of variable scope in Java: local, instance, and class.
The declaration of a variable determines its scope.
A variable declaration is the variable’s type and name together. For example:
```
int i;
float gpa;
Pupil p1;
String name;
double[] array;
```

	Local	Instance (non static)	Class (static)
Where to Declare	Inside a method	Inside a class, outside all methods, but without `static`	Inside a class, outside all methods, with `static`
Owner	Method where it is declared	Each object (instance of the class) owns a copy	Class where it is declared
Lifetime	Only as long as the method is executing	As long as its containing object exists	As long as its containing class is loaded
Where Usable	Only inside the method where it is declared	In all instance methods of the class	In all methods of the class

Example

Pupil

−schoolName : String

−name : String

−gpa : float

+toString() : String

{class variable}

{instance variable}

public class Pupil {
    private static String schoolName;  // class variable
    private String name;  // instance variable
    private float gpa;    // instance variable

    public String toString() {
        // s is a local variable
        String s = "Name: " + this.name;
        return s;
    }
}

Method Scope

Two categories of method scope in Java: instance and class.
The header of a method determines its scope.
- Without static, scope is instance.
- With static, scope is class.

	Instance (non static)	Class (static)
How to Declare	Without `static`: `public returnType methodName(…) {`	With `static`: `public static returnType methodName(…) {`
How to Call	Using object name: `objectName.methodName(…);`	Using class name: `ClassName.methodName(…);`
Variables That Can Be Used Inside	local, instance, and class	local and class but not instance
`this` Keyword	`this` keyword can be used inside an instance method	`this` keyword cannot be used inside a class method

Example

Pupil

−schoolName : String

−name : String

−gpa : float

+getSchoolName() : String

+toString() : String

{class variable}

{instance variable}

{class method}

{instance method}

public class Pupil {
    private static String schoolName = "Western University";
    private String name;
    private float gpa;

    public static String getSchoolName() {  // class method
        return schoolName;
    }

    public String toString() {  // instance method
        String s = "Name: " + this.name;
        return s;
    }
}

Variable and Method Scope

Variable Scope	Method Scope
Variable Scope	Instance (non static)	Class (static)
Local	A single local variable can be used in one method only.
Instance (non static)	A single instance variable can be used in all instance methods.	Instance variables, including the keyword `this`, cannot be used in class methods.
Class (static)	A single class variable can be used in all methods.

Demonstration

Move your mouse pointer over a variable declaration, and the browser will highlight that variable’s scope in yellow.
Move your mouse pointer over a method header and your browser will highlight in orange the variables that method can use.

Pupil

−schoolName : String

−name : String

−gpa : float

+getSchoolName() : String

+Pupil(name : String)

+toString() : String

{class variable}

{instance variable}

{class method}

{constructor (instance method)}

{instance method}

public class Pupil {
    // class variable
    private static String schoolName = "Western University";

    private String name;  // instance variable
    private float gpa;  // instance variable

    public static String getSchoolName() {  // class method
        return schoolName;
    }

    public Pupil(String name) {  // constructor (instance method)
        this.name = name;
        this.gpa = 0;
    }

    public String toString() {  // instance method
        String s = "School Name: " + schoolName +
            "  Name: " + this.name + "  GPA: " + this.gpa;
        return s;
    }
}