详细描述如何掌握核心Java库-优快云博客

1. java.lang 包

java.lang 包是自动导入的，包含了Java编程的基础类，如 String, Math, System, Thread 等。

字符串示例
public class StringExample {
    public static void main(String[] args) {
        String greeting = "Hello, World!";
        System.out.println(greeting.toUpperCase()); // 输出: HELLO, WORLD!
        System.out.println(greeting.toLowerCase()); // 输出: hello, world!
        System.out.println(greeting.contains("World")); // 输出: true
    }
}

2. java.util 包

java.util 包提供了包含集合框架、日期时间工具、实用工具类等。

集合框架示例
import java.util.ArrayList;
import java.util.List;

public class CollectionExample {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>();
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Cherry");

        for (String fruit : fruits) {
            System.out.println(fruit);
        }
    }
}

3. java.io 包

java.io 包提供了文件输入/输出操作的类。

文件读写示例
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class FileExample {
    public static void main(String[] args) {
        try (BufferedWriter writer = new BufferedWriter(new FileWriter("example.txt"))) {
            writer.write("Hello, File!");
        } catch (IOException e) {
            e.printStackTrace();
        }

        try (BufferedReader reader = new BufferedReader(new FileReader("example.txt"))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

4. java.net 包

java.net 包提供了用于网络通信的类。

网络请求示例
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class NetworkExample {
    public static void main(String[] args) {
        try {
            URL url = new URL("http://www.example.com");
            HttpURLConnection connection = (HttpURLConnection) url.openConnection();
            connection.setRequestMethod("GET");

            BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
            reader.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

5. java.time 包

java.time 包是Java 8引入的新日期和时间API。

日期时间示例
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.format.DateTimeFormatter;

public class DateTimeExample {
    public static void main(String[] args) {
        LocalDate today = LocalDate.now();
        System.out.println("Today's date: " + today);

        LocalDateTime now = LocalDateTime.now();
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
        String formattedNow = now.format(formatter);
        System.out.println("Current time: " + formattedNow);
    }
}