查找文件空指针异常-各位大神帮忙看一下 谢谢

[老男人1543]

错的地方我都标记出来了，各位帮帮看一眼，这个是因为什么导致的，还请详细的说一下，再次言谢~~！~~
public class Demo {
public static void main(String[] args) {
File file = new File("E:\\");
selectFile(file);//空指针异常
}

public static void selectFile(File file){
File [] files = file.listFiles();
List<File> list = new ArrayList<File>();
for(File f :files){//空指针异常
if(f.isFile()){
list.add(f);
}else if(f.isDirectory()){
selectFile(f);//空指针异常
}
}
for(File fileList:list){
if(".rar".endsWith(fileList.getName())){
System.out.println(fileList.getName());
}
}
}
}

[奋斗的佘先生丶]

file.listFiles(); 返回值是有可能为空的。 /** * Returns an array of abstract pathnames denoting the files in the * directory denoted by this abstract pathname. * * <p> If this abstract pathname does not denote a directory, then this * method returns {@code null}. Otherwise an array of {@code File} objects * is returned, one for each file or directory in the directory. Pathnames * denoting the directory itself and the directory's parent directory are * not included in the result. Each resulting abstract pathname is * constructed from this abstract pathname using the {@link #File(File, * String) File(File, String)} constructor. Therefore if this * pathname is absolute then each resulting pathname is absolute; if this * pathname is relative then each resulting pathname will be relative to * the same directory. * * <p> There is no guarantee that the name strings in the resulting array * will appear in any specific order; they are not, in particular, * guaranteed to appear in alphabetical order. * * <p> Note that the {@link java.nio.file.Files} class defines the {@link * java.nio.file.Files#newDirectoryStream(Path) newDirectoryStream} method * to open a directory and iterate over the names of the files in the * directory. This may use less resources when working with very large * directories. * * @return An array of abstract pathnames denoting the files and * directories in the directory denoted by this abstract pathname. * The array will be empty if the directory is empty. Returns * {@code null} if this abstract pathname does not denote a * directory, or if an I/O error occurs. * * @throws SecurityException * If a security manager exists and its {@link * SecurityManager#checkRead(String)} method denies read access to * the directory * * @since 1.2 */ public File[] listFiles() { String[] ss = list(); if (ss == null) return null; int n = ss.length; File[] fs = new File[n]; for (int i = 0; i < n; i++) { fs[i] = new File(ss[i], this); } return fs; }

[老男人1543]

引用 1 楼 syy19940213 的回复:

file.listFiles(); 返回值是有可能为空的。 /** * Returns an array of abstract pathnames denoting the files in the * directory denoted by this abstract pathname. * * <p> If this abstract pathname does not denote a directory, then this * method returns {@code null}. Otherwise an array of {@code File} objects * is returned, one for each file or directory in the directory. Pathnames * denoting the directory itself and the directory's parent directory are * not included in the result. Each resulting abstract pathname is * constructed from this abstract pathname using the {@link #File(File, * String) File(File, String)} constructor. Therefore if this * pathname is absolute then each resulting pathname is absolute; if this * pathname is relative then each resulting pathname will be relative to * the same directory. * * <p> There is no guarantee that the name strings in the resulting array * will appear in any specific order; they are not, in particular, * guaranteed to appear in alphabetical order. * * <p> Note that the {@link java.nio.file.Files} class defines the {@link * java.nio.file.Files#newDirectoryStream(Path) newDirectoryStream} method * to open a directory and iterate over the names of the files in the * directory. This may use less resources when working with very large * directories. * * @return An array of abstract pathnames denoting the files and * directories in the directory denoted by this abstract pathname. * The array will be empty if the directory is empty. Returns * {@code null} if this abstract pathname does not denote a * directory, or if an I/O error occurs. * * @throws SecurityException * If a security manager exists and its {@link * SecurityManager#checkRead(String)} method denies read access to * the directory * * @since 1.2 */ public File[] listFiles() { String[] ss = list(); if (ss == null) return null; int n = ss.length; File[] fs = new File[n]; for (int i = 0; i < n; i++) { fs[i] = new File(ss[i], this); } return fs; }

我把他的目录在添加一层，就没空指针了放到E://就会报空指针，为什么呢？

[Be_nurturing]

你这个没有异常啊，我试过了，没有异常啊

[自由自在_Yu]

public class Test1 {
	public static void main(String[] args) {
		File file = new File("F:/");
		selectFile(file);// 空指针异常
	}

	public static void selectFile(File file) {
		File[] files = file.listFiles();
		List<File> list = new ArrayList<File>();
		for (File f : files) {// 空指针异常
			if(!(f.toString().contains("$RECYCLE.BIN"))){
				//每个磁盘都会有$RECYCLE.BIN这个回收站文件夹，这个文件夹读取不到，所以要排除这个文件夹
				if (f.isFile()) {
					list.add(f);
				} else if (f.isDirectory()) {
					selectFile(f);// 空指针异常
				}
			}			
		}
		for (File fileList : list) {
			if (fileList.getName().endsWith(".rar")) {	
				//这里endsWith写反了
				System.out.println(fileList.getName());
			}
		}
	}
}

希望我回答的是对的

[ichavin]

因为你这里面有一个递归，然而file.listFiles()这个方法是可能范围null的，当用foreach对null进行遍历的时候会报错

[奋斗的佘先生丶]

引用 2 楼 oldman1543 的回复:

[quote=引用 1 楼 syy19940213 的回复:] file.listFiles(); 返回值是有可能为空的。 /** * Returns an array of abstract pathnames denoting the files in the * directory denoted by this abstract pathname. * * <p> If this abstract pathname does not denote a directory, then this * method returns {@code null}. Otherwise an array of {@code File} objects * is returned, one for each file or directory in the directory. Pathnames * denoting the directory itself and the directory's parent directory are * not included in the result. Each resulting abstract pathname is * constructed from this abstract pathname using the {@link #File(File, * String) File(File, String)} constructor. Therefore if this * pathname is absolute then each resulting pathname is absolute; if this * pathname is relative then each resulting pathname will be relative to * the same directory. * * <p> There is no guarantee that the name strings in the resulting array * will appear in any specific order; they are not, in particular, * guaranteed to appear in alphabetical order. * * <p> Note that the {@link java.nio.file.Files} class defines the {@link * java.nio.file.Files#newDirectoryStream(Path) newDirectoryStream} method * to open a directory and iterate over the names of the files in the * directory. This may use less resources when working with very large * directories. * * @return An array of abstract pathnames denoting the files and * directories in the directory denoted by this abstract pathname. * The array will be empty if the directory is empty. Returns * {@code null} if this abstract pathname does not denote a * directory, or if an I/O error occurs. * * @throws SecurityException * If a security manager exists and its {@link * SecurityManager#checkRead(String)} method denies read access to * the directory * * @since 1.2 */ public File[] listFiles() { String[] ss = list(); if (ss == null) return null; int n = ss.length; File[] fs = new File[n]; for (int i = 0; i < n; i++) { fs[i] = new File(ss[i], this); } return fs; }

我把他的目录在添加一层，就没空指针了放到E://就会报空指针，为什么呢？[/quote]检查你的目录结构，你的E盘有可能再在某个目录的时候返回为空了

[zss980909]

是因为你没有权限去遍历 返回了null 所以才会空指针异常

[zss980909]

你可以直接用ishidden判断是否为隐藏文件 是就忽略 然后就不会报错了