Database.java

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class Database {

    private Map<String, Table> tables;

    public Database() {
        this.tables = new HashMap<>();
    }

    public Table createTable(String tableName,
                             List<String> fieldNames,
                             List<String> fieldTypes,
                             List<String> primaryKeys) {
        return createTable(tableName, fieldNames, fieldTypes, primaryKeys, new ArrayList<>());
    }

    public Table createTable(String tableName,
                             List<String> fieldNames,
                             List<String> fieldTypes,
                             List<String> primaryKeys,
                             List<String> secondaryKeys) {
        if (tables.containsKey(tableName)) {
            return null;
        }
        Table table = new Table(tableName, fieldNames, fieldTypes, primaryKeys, secondaryKeys);
        tables.put(tableName, table);
        return table;
    }

    public boolean hasTable(String tableName) {
        return tables.containsKey(tableName);
    }

    public Table selectTable(String tableName) {
        return tables.get(tableName);
    }

    public boolean deleteTable(String tableName) {
        if (!tables.containsKey(tableName)) {
            return false;
        }
        tables.remove(tableName);
        return true;
    }

    public Set<String> getTableNames() {
        return new HashSet<>(tables.keySet());
    }

    public void listTables() {
        if (tables.isEmpty()) {
            System.out.println("No tables in database.");
        } else {
            System.out.println("Tables: " + tables.keySet());
        }
    }
}