Java Scope
Java Scope
In Java, variables are only accessible inside the region where they are created. This is called scope.
Method Scope
Variables declared directly inside a method are available anywhere in the method following the line of code in which they were declared:
Example
public class Main {
public static void main(String[] args) {
// Code here CANNOT use x
int x = 100;
// Code here CAN use x
System.out.println(x);
}
}Block Scope
A block of code refers to all of the code between curly braces { }.
Variables declared inside a block of code are only accessible by the code between the curly braces, and only after the line in which the variable was declared:
Example
public class Main {
public static void main(String[] args) {
// Code here CANNOT use x
{ // This is a block
// Code here CANNOT use x
int x = 100;
// Code here CAN use x
System.out.println(x);
} // The block ends here
// Code here CANNOT use x
}
}A block of code can stand alone, or be part of an if, while, or for statement.
In a for loop, the variable declared in the loop header (like int i = 0) only exists inside the loop.
Loop Scope
Variables declared inside a for loop only exist inside the loop:
Example
public class Main {
public static void main(String[] args) {
for (int i = 0; i < 5; i++) {
System.out.println(i); // i is accessible here
}
// i is NOT accessible here
}
}- The
forloop has its own block ({ ... }). - The variable
ideclared in the loop header (int i = 0) is only accessible inside that loop block. - Once the loop ends,
iis destroyed, so you can't use it outside.
Why this matters
Loop variables are not available outside the loop.
You can safely reuse the same variable name (i, j, etc.) in different loops in the same method:
Example
public class Main {
public static void main(String[] args) {
for (int i = 0; i < 3; i++) {
System.out.println("Loop 1: " + i);
}
for (int i = 0; i < 2; i++) {
System.out.println("Loop 2: " + i);
}
}
}Class Scope
Variables declared inside a class but outside any method have class scope (also called fields). These variables can be accessed by all methods in the class:
Example
public class Main {
int x = 5; // Class variable
public static void main(String[] args) {
Main myObj = new Main();
System.out.println(myObj.x); // Accessible here
}
}Note: You will learn more about classes and objects in the Java OOP chapters.