关于FilenameFilter的一个简单例子，但总是有空指针异常，哪位大侠帮忙看看吧

Question

public class Demo {

public static void main(String[] args) {
    File dir=new File("D:\\test");

    JavaTest filter=new JavaTest("java");

    File[] files=dir.listFiles(filter);

    for(File a:files){
        System.out.println(a.getName());
    }

}

}
class JavaTest implements FilenameFilter{
private String str;
public JavaTest(String str){
str="."+str;
}
public boolean accept(File dir, String name) {
return name.endsWith(str);
}

}

Answer 1

Constructor please use the object's parameter.

Could fix it as following:
public JavaTest(String str){
     this.str="."+str;
}