在程序中定义一字符串”hello,how are you!”，分别用字节流和字符流实现将该字符串保存到d:\t2_1.txt, d:\t2_2.txt中，程序分别保存为Test2_1.java和Test2_2.java。

好的，这是一份使用Java语言实现的代码示例，可以实现您的需求：

import java.io.*;

public class Test2_1 {
    public static void main(String[] args) {
        String str = "hello,how are you!";
        
        // 使用字节流保存字符串到d:\t2_1.txt
        try (FileOutputStream fos = new FileOutputStream("d:\\t2_1.txt")) {
            byte[] bytes = str.getBytes();
            fos.write(bytes);
            System.out.println("保存到d:\\t2_1.txt成功！");
        } catch (IOException e) {
            System.out.println("保存到d:\\t2_1.txt失败！" + e.getMessage());
        }
        
        // 使用字符流保存字符串到d:\t2_2.txt
        try (FileWriter fw = new FileWriter("d:\\t2_2.txt")) {
            fw.write(str);
            System.out.println("保存到d:\\t2_2.txt成功！");
        } catch (IOException e) {
            System.out.println("保存到d:\\t2_2.txt失败！" + e.getMessage());
        }
    }
}

import java.io.*;

public class Test2_2 {
    public static void main(String[] args) {
        // 从d:\t2_1.txt读取字节流并保存为字符串
        try (FileInputStream fis = new FileInputStream("d:\\t2_1.txt")) {
            byte[] bytes = new byte[fis.available()];
            fis.read(bytes);
            String str = new String(bytes);
            System.out.println("从d:\\t2_1.txt读取的字符串为：" + str);
        } catch (IOException e) {
            System.out.println("从d:\\t2_1.txt读取字符串失败！" + e.getMessage());
        }
        
        // 从d:\t2_2.txt读取字符流并保存为字符串
        try (FileReader fr = new FileReader("d:\\t2_2.txt")) {
            char[] chars = new char[1024];
            StringBuilder sb = new StringBuilder();
            int len;
            while ((len = fr.read(chars)) != -1) {
                sb.append(chars, 0, len);
            }
            String str = sb.toString();
            System.out.println("从d:\\t2_2.txt读取的字符串为：" + str);
        } catch (IOException e) {
            System.out.println("从d:\\t2_2.txt读取字符串失败！" + e.getMessage());
        }
    }
}