Understanding Static Variables in Java with Examples

답변:

In Java, a static variable is a class-level variable that is shared among all instances of that class. This means that there is only one copy of the static variable, regardless of how many objects of the class are created. Static variables are declared using the static keyword.

Characteristics of Static Variables:

Shared Among Instances: All instances of the class share the same static variable.
Memory Allocation: Static variables are allocated memory only once when the class is loaded.
Access: Static variables can be accessed directly using the class name, without needing to create an instance of the class.
Lifecycle: The lifecycle of a static variable is tied to the class itself, not to any specific instance.

Example of Static Variables

Here’s a simple example to illustrate the concept of static variables:

class Counter {
    // Static variable to keep track of the number of instances
    static int count = 0;

    // Constructor
    Counter() {
        count++; // Increment the count each time a new instance is created
    }

    // Static method to get the current count
    static int getCount() {
        return count;
    }
}

public class Main {
    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        Counter c3 = new Counter();

        // Accessing the static variable using the class name
        System.out.println("Total instances created: " + Counter.getCount());
    }
}

Explanation of the Example:

The Counter class has a static variable count that keeps track of how many instances of the class have been created.
Each time a new Counter object is instantiated, the constructor increments the count variable.
The static method getCount() returns the current value of count.
In the Main class, three instances of Counter are created, and the total number of instances is printed using the static method.

Diagram Representation

Here’s a simple diagram to represent the concept of static variables:

+-------------------+
|      Counter      |
+-------------------+
| - count: int      |  <--- Static Variable (Shared)
+-------------------+
| + Counter()       |  <--- Constructor
| + getCount(): int |  <--- Static Method
+-------------------+

Instances:
+---------+     +---------+     +---------+
|   c1    |     |   c2    |     |   c3    |
+---------+     +---------+     +---------+
|  (refers to the same static variable)  |
+-----------------------------------------+

Key Points: