Scope and Lifetime of a Variable
The scope of a variable is the region of the program where the variable can be accessed. The scope of a variable is determined by where it is declared.
A local variable is declared within a method or a block. It can only be accessed within the method or block in which it is declared. For example, the following code declares a local variable named x within the main() method:
Java
public static void main(String[] args) {
int x = 10;
System.out.println(x);
}
The x variable is only visible within the main() method. It cannot be accessed from outside the method.
An instance variable is declared within a class but outside of any method or block. It can be accessed from all methods and blocks in the class. For example, the following code declares an instance variable named name in the Student class:
Java
public class Student {
String name;
public Student(String name) {
this.name = name;
}
public void setName(String name) {
this.name = name;
}
public String getName() {
return this.name;
}
}
The name variable is visible to all methods in the Student class. It can be accessed from any method in the class.
A static variable is declared with the static keyword. It is shared by all instances of the class. For example, the following code declares a static variable named counter in the Counter class:
Java
public class Counter {
static int counter = 0;
public void increment() {
counter++;
}
public static int getCounter() {
return counter;
}
}
The counter variable is shared by all instances of the Counter class. Each instance of the class has its own copy of the counter variable, but all copies of the variable point to the same memory location.
The lifetime of a variable is the period of time for which a variable is allocated memory. The lifetime of a variable is determined by its scope and the type of variable.
A local variable is created when the method or block in which it is declared is entered and destroyed when the method or block is exited.
An instance variable is created when an object of the class in which it is declared is created and destroyed when the object is garbage collected.
A static variable is created when the class in which it is declared is loaded into memory and destroyed when the class is unloaded from memory.
Here are some other important points to keep in mind about the scope and lifetime of variables in Java:
The scope of a variable cannot be changed after the variable is declared.
The lifetime of a variable cannot be changed after the variable is declared.
A variable can only be accessed within its scope.
A variable cannot be accessed after its lifetime has ended.
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