检查字符串是否以特定字母开头或结尾-Java

Question

I have a little problem checking if a string have a letter in it. Right now, i have a server/client situation where the server sends back a command to the client.

That command can be:

RM20 B
RM20 L
RM20 I

RM20 A "User input"

RM20 C

How do i check the letter if its a B, L, I or A or C??

Right now i made this code:

if ((fromServer.startsWith("RM20 A"))) {

else if ((fromServer.startsWith("RM20 C")))

else {

}

does this work? or will the if statement and else if statement be the same? i can't use ends with, because the RM20 A have an input in the end which can be alot.

is there a way to check whats in the middle. when you get, lets says

"RM20 A 123213124" as a String?

Answer 1

Assuming your format is something like:

 <RM Code> <Letter> <Numbers> <Anything else>

Then you could do:

 String[] values = input.split(" ");

 if(values[1].equals("A")) {
     // Code for A.
 }

Answer 2

I would only use the startsWith() test for the "A" command input, since that is the only one that has an arbitrary ending. The other commands have exact values, so you should use an equals() test for those. This way, you can reliably catch errors (eg, "RM20 B xyz").

static final String A_COMMAND_PREFIX = "RM20 A ";

.
.
.

if (fromServer.startsWith(A_COMMAND_PREFIX)) {
  // Process user input
  final String userInput = fromServer.substring(A_COMMAND_PREFIX.length());

} else if (fromServer.equals("RM20 B")) {
  // Process B command
} else if (fromServer.equals("RM20 L")) {
  // Process L command
} else if (fromServer.equals("RM20 I")) {
  // Process I command
} else if (fromServer.equals("RM20 C")) {
  // Process C command
} else {
  // Handle unknown command
}

I assume you have already determined that fromServer is not null.

Answer 3

Use regex to capture different groups. The String regex expression captures any word character up until a space. Then it captures any letter az or AZ. Then, if there's a space and some other input, it captures that and uses it.

import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class test {
    public static void main(String ar[]) throws java.io.IOException {       

        String regex = "(\\w*) ([a-zA-Z])( (.*))?";

        String str1 = "RM20 A";
        String str2 = "RM20 B";
        String str3 = "RM20 C this is a user message.";
        Pattern pattern = Pattern.compile(regex);

        Matcher match1 = pattern.matcher(str1);
        Matcher match2 = pattern.matcher(str2);
        Matcher match3 = pattern.matcher(str3);

        String a;
        while (match1.find()) {          
            a = "Group 1: " + match1.group(1) + "\nGroup 2: " + match1.group(2);  
            System.out.println(a);        
        }  

        System.out.println();

        while (match2.find()) {          
            a = "Group 1: " + match2.group(1) + "\nGroup 2: " + match2.group(2);  
            System.out.println(a);
        }   

        System.out.println();

        while (match3.find()) {          
            a = "Group 1: " + match3.group(1) + "\nGroup 2: " + match3.group(2) + "\nGroup 4: " + match3.group(4);  
            System.out.println(a);       
        }
    }
}

---

>>> Group 1: RM20
>>> Group 2: A

>>> Group 1: RM20
>>> Group 2: B

>>> Group 1: RM20
>>> Group 2: C
>>> Group 4: this is a user message.

---

You could also capture the letters/number in the input code:

    String regex = "([a-zA-Z]*)(\\d*) ([a-zA-Z])( (.*))?";

    String str1 = "RM20 A message thingy";
    Pattern pattern = Pattern.compile(regex);

    Matcher match1 = pattern.matcher(str1);
    while (match1.find()) {          
        System.out.println(match1.group(1));
        System.out.println(match1.group(2));
        System.out.println(match1.group(3));
        System.out.println(match1.group(5));
    }

---

>>> RM
>>> 20
>>> A
>>> message thingy