Abstraction

1// Abstract class providing abstraction
2public abstract class DatabaseConnection {
3    protected String connectionString;
4    
5    // Template method - defines the workflow
6    public final void executeQuery(String query) {
7        connect();
8        validate(query);
9        runQuery(query);
10        disconnect();
11    }
12    
13    // Abstract methods - implementation hidden
14    protected abstract void connect();
15    protected abstract void runQuery(String query);
16    protected abstract void disconnect();
17    
18    // Concrete method with default implementation
19    protected void validate(String query) {
20        if (query == null || query.isEmpty()) {
21            throw new IllegalArgumentException("Query cannot be empty");
22        }
23    }
24}
25
26// Concrete implementation for MySQL
27public class MySQLConnection extends DatabaseConnection {
28    @Override
29    protected void connect() {
30        System.out.println("Connecting to MySQL database...");
31        // MySQL specific connection logic
32    }
33    
34    @Override
35    protected void runQuery(String query) {
36        System.out.println("Executing MySQL query: " + query);
37        // MySQL specific query execution
38    }
39    
40    @Override
41    protected void disconnect() {
42        System.out.println("Disconnecting from MySQL...");
43    }
44}
45
46// Concrete implementation for PostgreSQL
47public class PostgreSQLConnection extends DatabaseConnection {
48    @Override
49    protected void connect() {
50        System.out.println("Connecting to PostgreSQL database...");
51    }
52    
53    @Override
54    protected void runQuery(String query) {
55        System.out.println("Executing PostgreSQL query: " + query);
56    }
57    
58    @Override
59    protected void disconnect() {
60        System.out.println("Disconnecting from PostgreSQL...");
61    }
62}
63
64// Client code - works with abstraction
65public class Application {
66    public static void main(String[] args) {
67        // Client doesn't need to know connection details
68        DatabaseConnection db = new MySQLConnection();
69        db.executeQuery("SELECT * FROM users");
70        
71        // Easy to switch databases
72        db = new PostgreSQLConnection();
73        db.executeQuery("SELECT * FROM users");
74    }
75}