IDEA相对路径问题

public class MainSystem {
    public static void main(String[] args) throws IOException {
        ArrayList<String> array= new ArrayList<String>();
        array.add("hello");
        array.add("world");
        array.add("java");
        BufferedWriter bw=new BufferedWriter(new FileWriter("test2/xx.txt"));
        for(String s:array){
            bw.write(s);
            bw.newLine();
            bw.flush();
        }
        bw.close();
    }
}

如以上所示，在写相对路径时可以直接写当前文件所在的包名加要创建的文件名。
如果写成

BufferedWriter bw=new BufferedWriter(new FileWriter("HelloWorld/test2/xx.txt"));

则会报错

Exception in thread "main" java.io.FileNotFoundException: HelloWorld\test2\xx.txt (系统找不到指定的路径。)
	at java.base/java.io.FileOutputStream.open0(Native Method)
	at java.base/java.io.FileOutputStream.open(FileOutputStream.java:298)
	at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:237)
	at java.base/java.io.FileOutputStream.<init>(FileOutputStream.java:126)
	at java.base/java.io.FileWriter.<init>(FileWriter.java:66)
	at com.lwh.test.MainSystem.main(MainSystem.java:12)