Arrays类中的asList()方法

在做java.io操作文件的时候，有一需求是：将给定目录路径下的所有文件、文件夹罗列出来。

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这样就不可避免用到File类中的listFiles()方法，一开始想到的是用File类实例化一个对象，构造方法中给出初始的目录路径；然后通过.listFiles()方法返回路径下的所有资源，用一个File[]数组接收一下，之后遍历数组打印出来就可以了。话不多说上代码……

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我们先看一下文档和源码中对方法的描述吧：

文档&源码

listFiles_methods.jpg

这里一共有3个，两个重载，我们只看最上面的那个。

 /**
     * 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,&nbsp;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;
    }

大致就是说，该方法返回一个抽象路径名数组，用来表示该路径名对应目录下的文件。

先来感受一下吧！

import java.io.*;
import java.util.*;

public class CopyFile {
    public static void main(String[] args) {
        File file = new File("c:/");
        File[] dirs = file.listFiles();

        for(File f: dirs) {
            System.out.println(f.getAbsoluteFile());
        }
    }
}

运行结果

"C:\Program Files\Java\jdk1.8.0_172\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2018.1.5\lib\idea_rt.jar=56512:C:\Program Files\JetBrains\IntelliJ IDEA 2018.1.5\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_172\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\rt.jar;C:\Users\Administrator\IdeaProjects\Practice20180728\out\production\Practice20180728" copy_file_demo.CopyFile
c:\$360Section
c:\$RECYCLE.BIN
c:\360Downloads
c:\360SANDBOX
c:\360WiFi
c:\BaiduNetdiskDownload
c:\Boot
c:\bootmgr
c:\BOOTSECT.BAK
c:\Documents and Settings
c:\Drivers
c:\GreenSoftWare
c:\JAVA_Learning
c:\java基础_代码笔记
c:\pagefile.sys
c:\Program Files
c:\Program Files (x86)
c:\ProgramData
c:\SUDIJ
c:\System Volume Information
c:\Users
c:\VPM
c:\Windows

Process finished with exit code 0

基本上c盘所有的文件和文件夹都显示出来了。

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现在我又有个新的想法了，我想把这个返回的数组转换成List，该怎么做呢？至于为什么想转换成List，可能因为我想用迭代器遍历？或者仅仅是因为好奇？你管我呀！反正总有用到的时候。

查来查去，最终把目光锁定在了Arrays.asList()这个方法上，老规矩，先看文档，再看源码：

文档&源码

Arrays.asList().jpg

/**
     * Returns a fixed-size list backed by the specified array.  (Changes to
     * the returned list "write through" to the array.)  This method acts
     * as bridge between array-based and collection-based APIs, in
     * combination with {@link Collection#toArray}.  The returned list is
     * serializable and implements {@link RandomAccess}.
     *
     * <p>This method also provides a convenient way to create a fixed-size
     * list initialized to contain several elements:
     * <pre>
     *     List&lt;String&gt; stooges = Arrays.asList("Larry", "Moe", "Curly");
     * </pre>
     *
     * @param <T> the class of the objects in the array
     * @param a the array by which the list will be backed
     * @return a list view of the specified array
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    public static <T> List<T> asList(T... a) {
        return new ArrayList<>(a);
    }

这是一个静态方法，基本上是和collection.toArray()互补使用的，这两个方法作为数组和集合类的桥梁，使用频率还是蛮高的。

贴代码：

package copy_file_demo;

import java.io.*;
        import java.util.*;

public class CopyFile {
    public static void main(String[] args) {
        File file = new File("c:/");
//        File[] dirs = file.listFiles();
        List<File> arrDirs = Arrays.asList(file.listFiles());

        Iterator iter = arrDirs.iterator();

        while(iter.hasNext()) {
            System.out.println(iter.next());
        }

//        for(File f: dirs) {
//            System.out.println(f.getAbsoluteFile());
//        }
    }
}

运行结果

"C:\Program Files\Java\jdk1.8.0_172\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2018.1.5\lib\idea_rt.jar=56708:C:\Program Files\JetBrains\IntelliJ IDEA 2018.1.5\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_172\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\rt.jar;C:\Users\Administrator\IdeaProjects\Practice20180728\out\production\Practice20180728" copy_file_demo.CopyFile
c:\$360Section
c:\$RECYCLE.BIN
c:\360Downloads
c:\360SANDBOX
c:\360WiFi
c:\BaiduNetdiskDownload
c:\Boot
c:\bootmgr
c:\BOOTSECT.BAK
c:\Documents and Settings
c:\Drivers
c:\GreenSoftWare
c:\JAVA_Learning
c:\java基础_代码笔记
c:\pagefile.sys
c:\Program Files
c:\Program Files (x86)
c:\ProgramData
c:\SUDIJ
c:\System Volume Information
c:\Users
c:\VPM
c:\Windows

Process finished with exit code 0

哇哈哈哈哈哈~完美! perfect！
不过要注意的是，List接收返回值后向下转型编译期间不会报错，一运行就会报错，我估计是因为本来返回值就是ArrayList类型，只不过用List接口引用了一下，把自己转为同类肯定会报错。这只是猜测，我也不知道具体原因是什么，这里做个记号，也恳请大神们不吝赐教，小的实在太菜。

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同样有一个值得注意的问题，代码说话：

import java.util.*;

public class Main {

    public static void main(String[] args) {
        Integer[] nums = new Integer[]{1, 2, 3, 4, 5, 6};
        List arr = Arrays.asList(nums);

        Iterator iter = arr.iterator();
        while(iter.hasNext()) {
            System.out.println(iter.next());
        }
//        System.out.println(arr);
    }
}

这里做了个实验，把Arrays.asList()的入参换成成员是Number类型的数组（注意：collection接口中不能放基本数据类型，所以尝试把int、double等数组放进去根本就是徒劳，不过可以转换成对应的包装类型后放进去。）一切相安无事：

"C:\Program Files\Java\jdk1.8.0_172\bin\java.exe" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2018.1.5\lib\idea_rt.jar=57150:C:\Program Files\JetBrains\IntelliJ IDEA 2018.1.5\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_172\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_172\jre\lib\rt.jar;C:\Users\Administrator\IdeaProjects\test\out\production\test" Main
1
2
3
4
5
6

Process finished with exit code 0

可是一旦把List改为ArrayList，就会立马报错，这个还和前面的不一样，前面的例子在改用ArrayList接收的时候并没有报错。总之，懂得还是太少，路漫漫其修远兮。