static Keyword

1// Static Variable (Class Variable)
2public class Counter {
3    // Static variable - shared by ALL instances
4    private static int count = 0;
5    
6    // Instance variable - unique to each object
7    private String name;
8    
9    public Counter(String name) {
10        this.name = name;
11        count++;  // Increment shared counter
12    }
13    
14    public static int getCount() {
15        return count;
16    }
17}
18
19// Usage
20Counter c1 = new Counter("First");
21Counter c2 = new Counter("Second");
22Counter c3 = new Counter("Third");
23
24System.out.println(Counter.getCount());  // 3 (shared count)
25
26// Static variable use cases
27public class MathConstants {
28    public static final double PI = 3.14159;
29    public static final double E = 2.71828;
30}
31
32double area = MathConstants.PI * radius * radius;

1// Static Methods
2public class MathUtils {
3    // Static method - no object needed
4    public static int add(int a, int b) {
5        return a + b;
6    }
7    
8    public static int max(int a, int b) {
9        return a > b ? a : b;
10    }
11    
12    public static boolean isEven(int n) {
13        return n % 2 == 0;
14    }
15}
16
17// Call without creating object
18int sum = MathUtils.add(5, 3);      // 8
19int bigger = MathUtils.max(10, 7); // 10
20
21// Built-in static methods
22int abs = Math.abs(-10);           // 10
23double sqrt = Math.sqrt(16);       // 4.0
24String str = String.valueOf(42);   // "42"
25int parsed = Integer.parseInt("42"); // 42
26
27// Static method restrictions
28public class Example {
29    private int instanceVar = 10;
30    private static int staticVar = 20;
31    
32    public static void staticMethod() {
33        // System.out.println(instanceVar);  // ERROR!
34        System.out.println(staticVar);       // OK
35        
36        // Access instance through object
37        Example obj = new Example();
38        System.out.println(obj.instanceVar); // OK
39    }
40}

1// Static Block - Initialization when class loads
2public class DatabaseConfig {
3    private static String url;
4    private static String username;
5    private static Connection connection;
6    
7    // Static block - runs once when class is loaded
8    static {
9        System.out.println("Loading config...");
10        url = "jdbc:mysql://localhost:3306/mydb";
11        username = "admin";
12        
13        try {
14            connection = DriverManager.getConnection(url, username, "password");
15        } catch (SQLException e) {
16            throw new RuntimeException("Failed to connect", e);
17        }
18    }
19    
20    public static Connection getConnection() {
21        return connection;
22    }
23}
24
25// Multiple static blocks execute in order
26public class MultiStatic {
27    static int value;
28    
29    static {
30        System.out.println("First block");
31        value = 10;
32    }
33    
34    static {
35        System.out.println("Second block");
36        value = value * 2;  // 20
37    }
38}

1// Singleton Pattern - Common use of static
2public class Singleton {
3    // Static instance
4    private static Singleton instance;
5    
6    // Private constructor
7    private Singleton() {}
8    
9    // Static factory method
10    public static Singleton getInstance() {
11        if (instance == null) {
12            instance = new Singleton();
13        }
14        return instance;
15    }
16}
17
18// Thread-safe singleton (eager initialization)
19public class EagerSingleton {
20    private static final EagerSingleton INSTANCE = new EagerSingleton();
21    
22    private EagerSingleton() {}
23    
24    public static EagerSingleton getInstance() {
25        return INSTANCE;
26    }
27}
28
29// Static import - use static members without class name
30import static java.lang.Math.*;
31import static java.lang.System.out;
32
33public class StaticImportDemo {
34    public static void main(String[] args) {
35        out.println(sqrt(16));  // Instead of System.out, Math.sqrt
36        out.println(PI);        // Instead of Math.PI
37        out.println(max(5, 3)); // Instead of Math.max
38    }
39}