用Java写的遍历目录下面文件，总是出错

Java code

public class traverse {    public static void tra(File path){        if(path == null){            return;        }        if(path.isDirectory()){            String[] files = path.list();            for(int i = 0; i < files.length;i++){                tra(new File(path,files[i]));            }        }else{            if(path.getAbsolutePath().endsWith(".exe"))                System.out.println(path);        }    }    public static void main(String[] args) {        try{        File file = new File("D:\\");        tra(file);        }catch(Exception e){            e.printStackTrace();        }    }}

老是抛出异常
java.lang.NullPointerException；
请问这是为什么啊

[解决办法]

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Java code
public class traverse {
public static void tra(File path){
if(path == null){
return;
}
if(path.isDirectory()){
String[] files = path.list……