Regex: Pattern to find Java class in .java file

Question

I want to find a class' name when reading through a .java file. I'm returning no match right now using this regular expression:

\\s*[public][private]\\s*class\\s*(\\w*)\\s*\\{

Here is my code so far:

import java.io.*;
import java.util.*;
import java.util.regex.*;


public class HW4Solution {

    public static void main(String [] args){
        //Prompt user for path to file.
        File file = null;
        Scanner pathScan = new Scanner(System.in);
        while (file == null || !file.exists())
        {
            System.out.print("Enter valid file path: ");
            file = new File(pathScan.next());
        }
        pathScan.close();
        System.out.println("File: " + file.getPath() + " found.");

        //Read file line by line into buffered reader
        StringBuffer componentString = new StringBuffer(100);
        String currentLine;
        BufferedReader bufferedReader = null;
        try {
            bufferedReader = new BufferedReader(new FileReader(file.getPath()));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }

        //TODO: Find class declarations
        //TODO: Find superclasses
        //TODO: Find instance variable declarations
        //TODO: Find method signatures
        //TODO: Find access modifier
        //TODO: Find return type
        //TODO: Find method name

        //Creating patterns to look for!
        //Class declarations
        Pattern classDeclarationPattern = Pattern.compile("\\s*[public][private]\\s*class\\s*(\\w*)\\s*\\{");
        try {
            while((currentLine = bufferedReader.readLine()) != null){
                Matcher classDeclarationMatcher = classDeclarationPattern.matcher(currentLine);
                if(classDeclarationMatcher.group(1) != null){
                    componentString.append("Found class declaration: " + classDeclarationMatcher.group(3) + "\n");
                    /*if(classDeclarationMatcher.group(5) != null){
                        componentString.append("\tsuperclass: " + classDeclarationMatcher.group(5) + "\n");
                    }*/
                    System.out.println(classDeclarationMatcher.group());
                }
            }
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        finally{
            try{
                if (bufferedReader !=null) {
                    bufferedReader.close();
                }
            }
            catch(IOException e){
                e.printStackTrace();
            }
        }

        System.out.println(componentString.toString());

    }
}

I eventually want to be able to determine if a class declaration has a superclass and get that too but for now I'm having enough trouble (which I shouldn't) getting the name of the class.

you never call find() on the matcher. That means you are not looking for results. — jlordo
– jlordo, Commented Feb 14, 2013 at 22:38
@iTech huh? [class] is definitely not what he is looking for — Miserable Variable
– Miserable Variable, Commented Feb 14, 2013 at 22:38
@kmaz13 you realize this is essentially a heuristic method and will have false positives (and probably false negatives also)? — Miserable Variable
– Miserable Variable, Commented Feb 14, 2013 at 22:40
@MiserableVariable string-quoted class headers are such false positives, and generic classes would need to be covered as well. — John Dvorak
– John Dvorak, Commented Feb 14, 2013 at 22:41
If you need this to work reliably, you'd be more likely to succeed by parsing the source using the Eclipse JDT API, for example (regular expressions? now you have two problems, etc) — hertzsprung
– hertzsprung, Commented Feb 14, 2013 at 22:51

Rohit Jain · Accepted Answer · 2013-02-14 22:44:34Z

[public] is not matching what you think it should. It is a character class, which matches either of those characters in public. You should use pipe to match alternate words.

You can use following regex, extended to consider super classes or interface: -

Pattern.compile("\\s*(public|private)\\s+class\\s+(\\w+)\\s+((extends\\s+\\w+)|(implements\\s+\\w+( ,\\w+)*))?\\s*\\{");

Note that, you should use \\s+ with + quantifier when matching space between public|private and the class keyword. Because you expect at least 1 space there. \\s* will match 0 space, which is not correct, since publicclass is not valid.

Apart from that, there can be some more options to consider here. Like static inner classes, or generic classes, that will require minor modification in there, which you can do on your own. I've just given a little insight on to how to approach.

Also, one more important thing. Before you can get the result from your group, you should call Matcher#find() method to do the actual matching.

Miraj50 · Accepted Answer · 2018-12-20 14:00:17Z

1

RegExp for class declaration:

private String classRegExp = "(((|public|final|abstract|private|static|protected)(\\s+))?(class)(\\s+)(\\w+)(<.*>)?(\\s+extends\\s+\\w+)?(<.*>)?(\\s+implements\\s+)?(.*)?(<.*>)?(\\s*))\\{$";

for interface declaration:

private String interfaceRegExp = "(((|public|final|abstract|private|static|protected)(\\s+))?interface(\\s+)(\\w+)(<.*>)?(\\s+extends\\s+\\w+)?(<.*>)?(\\s+implements\\s+)?(.*)?(<.*>)?(\\s*))\\{$";

for enum declaration :

private String enumRegExp = "((((public|final|private|static|protected)(\\s+))?enum(\\s+)(\\w+)?(\\s+implements\\s+\\w+)?(.*)?\\s*))\\{$";

edited Dec 20, 2018 at 14:00

Miraj50

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answered Dec 20, 2018 at 13:13

ATN54

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