System.getenv() 返回 \\

Question

I'm trying to make a path to a place on the computer with the System.getenv function and it returns a \\ in the path not a / which is what I need. I have tried with the replaceAll method but it returns a error:

Exception in thread "main" java.util.regex.PatternSyntaxException: Unexpected internal error near index 1
\
 ^
    at java.util.regex.Pattern.error(Unknown Source)
    at java.util.regex.Pattern.compile(Unknown Source)
    at java.util.regex.Pattern.<init>(Unknown Source)
    at java.util.regex.Pattern.compile(Unknown Source)
    at java.lang.String.replaceAll(Unknown Source)
    at Launcher.start(Launcher.java:75)
    at Launcher.Download(Launcher.java:55)
    at Launcher.<init>(Launcher.java:31)
    at Launcher.main(Launcher.java:17)

the line of code is:

InputStream OS = Runtime.getRuntime().exec(new String[]{"java",System.getenv("APPDATA").replaceAll("\\", "/")+"/MS2-torsteinv/MS2-bin/no/torsteinv/MarsSettlement2/Client/Client.class"}).getErrorStream();

Answer 1

You need to double the backslash:

.replaceAll("\\\\", "/")

The canonical regex is indeed \\\\ , but in Java regexes are in strings, and in Java strings, a literal backslash needs to be escaped with another backslash. Hence \\\\ becomes "\\\\\\\\" .

Answer 2

In Java regular expressions you have to escape the backslash and in java string again. That makes in total four backslashes.

replaceAll("\\\\", "/")

Answer 3

it returns a \\ in the path not a / wich is what i need.

The platform default sure is what you need.

import java.io.File;

class FormPath {
    public static void main(String[] args) {
        String relPath = "/MS2-torsteinv/MS2-bin/no/" +
            "torsteinv/MarsSettlement2/Client/Client.class";
        String[] parts = relPath.split("/");
        File f = new File(System.getenv("APPDATA"));
        System.out.println(f + " exists: " + f.exists());

        for (String part : parts) {
            // use the File constructor that will insert the correct separator
            f = new File(f,part);
        }
        System.out.println(f + " exists: " + f.exists());
    }
}

Output

C:\Users\Andrew\AppData\Roaming exists: true
C:\Users\Andrew\AppData\Roaming\MS2-torsteinv\MS2-bin\no\torsteinv\MarsSettlement2\Client\Client.class exists: false
Press any key to continue . . .