如何用编程语言实现以下字符串操作：计算字符串长度、拼接两个字符串、进行字符串比较以及查找子串是否存在？请提供相应的代码示例。

当然可以。下面是几种常见编程语言中关于字符串操作的简单例子：

1. **计算字符串长度**:
- Python:

     str = "Hello World"
     length = len(str)
     print("Length of the string:", length)

- Java:

     String str = "Hello World";
     int length = str.length();
     System.out.println("Length of the string: " + length);

2. **拼接两个字符串**:
- Python:

     str1 = "Hello"
     str2 = "World"
     combined_str = str1 + " " + str2;
     System.out.println(combined_str);  # Output: Hello World

- Java:

     String str1 = "Hello";
     String str2 = "World";
     String combined_str = str1 + " " + str2;
     System.out.println(combined_str);  // Output: Hello World

3. **字符串比较**:
- Python:

     str1 = "Hello"
     str2 = "World"
     if str1 == str2:
         print("Strings are equal")
     else:
         print("Strings are not equal")

- Java:

     String str1 = "Hello";
     String str2 = "World";
     if (str1.equals(str2)) {
         System.out.println("Strings are equal");
     } else {
         System.out.println("Strings are not equal");
     }

4. **查找子串是否存在**:
- Python:

     main_str = "Hello World"
     sub_str = "World"
     if sub_str in main_str:
         print("Substring found")
     else:
         print("Substring not found")

- Java:

     String mainStr = "Hello World";
     String subStr = "World";
     boolean found = mainStr.contains(subStr);
     if (found) {
         System.out.println("Substring found");
     } else {
         System.out.println("Substring not found");
     }