文件搜索功能

Question

我找到了一段 Android(Java) 的代码，它在 sd-card 中搜索给定的文件名（字符串） 如果该文件存在于 sd 卡中，它可以正常工作，但如果没有具有该名称的文件，它将引发空指针异常。 任何人都可以帮助我吗？还是给我另一种选择？ 这是代码：

public File findFile(File dir, String name) {
    File[] children = dir.listFiles();

    for(File child : children) {//the exception is thrown here!
        if(child.isDirectory()) {
           File found = findFile(child, name);
           if(found != null) return found;
        } else {
            if(name.equals(child.getName())) return child;
        }
    }

    return null;
}

这是 logcat 结果：

threadid=1: thread exiting with uncaught exception (group=0x400205a0)
FATAL EXCEPTION: main
java.lang.NullPointerException
at ir.zinutech.ssn.Settings.findFile(Settings.java:93)//>> which is "for(File child : children) { "
at ir.zinutech.ssn.Settings.findFile(Settings.java:95)//>> which is "File found = findFile(child, name);//the exception is thrown here!"
at ir.zinutech.ssn.Settings.onClick(Settings.java:69)
at android.view.View.performClick(View.java:2532)
at android.view.View$PerformClick.run(View.java:9293)`end`

Answer 1

我认为您可以稍微修改一下代码。因为不是所有的递归情况都返回一个值/对象。

package vinhnt.example;

import java.io.File;

public class Finder {
    public static void main(String... args) {
        File result = findFileInDirectory(new File("C:/"), "Finder.class");
        if (result == null) {
            System.out.println("File not found");
        }else {
            System.out.println(result.getAbsolutePath());
        }

    }
    public static File findFileInDirectory(File dir, String name) {
        File[] children = dir.listFiles();
        if (children==null) return null;

        for(File child : children) {
            if(child.isDirectory()) {
               File result = findFileInDirectory(child, name);
               if (result!=null) return result;
            } else {
                String fileName = child.getName();
                if(name.equals(fileName)) {
                    System.out.println(fileName);
                    return child;
                }
            }
        }
        return null;
    }
}

Answer 2

感谢@njzk2 和@VinhNT 帮助我找到了解决方法，问题正如@VinhNT 所说，无法访问某些文件夹，所以我修改了代码使其不会陷入这样的困境案例，这是我的处理方式：

File sourceFile = findFile(Environment.getExternalStorageDirectory(),"x.apk");
            if (sourceFile==null){
                Toast.makeText(getApplicationContext(), 
                        "Couldn't find x.apk file in your SD card",
                        Toast.LENGTH_LONG).show();
                break;
            }

在搜索功能中：

public File findFile(File dir, String name) {
    File[] children = dir.listFiles();
    try{
        for(File child : children) {
            if(child.isDirectory()) {
               File found = findFile(child, name);
               if(found != null) return found;
            } else {
                if(name.equals(child.getName())) return child;
            }
        }
    }catch(Exception e){
        //ignore here because we have no access to this folder
        return null;
    }

    return null;
}

希望它能帮助像我一样被卡住的其他人。