常用的java工具类

StringUtils
这是象面包和奶油一样必须的通用语言库，这个实用工具类包括一些很早以前在String中未包含的重要方法。

StringUtils.isEmpty(null) && StringUtils.isEmpty(""); // true  

StringUtils.isBlank("   \n\t");                       // true  

StringUtils.substringAfterLast("foo.bar.baz", ".");   // "baz"  

StringUtils.substringBeforeLast("foo.bar.baz", ".");  // "foo.bar"  

StringUtils.split("foo.bar.baz", '.');                // { "foo", "bar", "baz" }  

StringUtils.split("foo,  bar,baz", ", ");             // { "foo", "bar", "baz" }  

StringUtils.leftPad("1", 3, '0');                     // "001"  

IOUtils and FileUtils

在一种当你需要手动操作多个文件罕见情况下必须具备的工具，这两个工具很相似(FileUtils操作文件，IOUtils操作InputStream和Reader classes)，和捆绑常用IO.

File file1;

File file2;

InputStream inputStream;

OutputStream outputStream;



// copy one file into another

FileUtils.copyFile(file1, file2);

IOUtils.copy(inputStream, outputStream);



// read a file into a String

String s1 = FileUtils.readFileToString(file1);

String s2 = IOUtils.toString(inputStream);



// read a file into a list of Strings, one item per line




List<String> l1 = FileUtils.readLines(file1);

List<String> l2 = IOUtils.readLines(inputStream);



// put this in your finally() clause after manipulating streams

IOUtils.closeQuietly(inputStream);



// return the list of xml and text files in the specified folder and any subfolders

Collection<File> c1 = FileUtils.listFiles(file1, { "xml", "txt" }, true);



// copy one folder and its contents into another

FileUtils.copyDirectoryToDirectory(file1, file2);



// delete one folder and its contents

FileUtils.deleteDirectory(file1);

1.文件内容拷贝：

import java.io.File;

import java.io.FileWriter;

import java.io.IOException;

import java.io.InputStream;

import java.io.Writer;

 

import org.apache.commons.io.FileUtils;

import org.apache.commons.io.IOUtils;

 

 

publicclass FileCopyExample {

    publicstaticvoid main(String[] args) {

       try {

           File src = new File( "test.dat" );

           File dest = new File( "test.dat.bak" );

           

           FileUtils.copyFile( src, dest );

       } catch( IOException ioe ) {

           System.out.println( "Problem copying file." );

       }

       

       try {

           File src = new File( "test.dat" );

           File dir = new File( "./temp" );

 

           FileUtils.copyFileToDirectory( src, dir );

       } catch( IOException ioe ) {

           System.out.println( "Problem copying file to dir.");

       }

       

       try {

           String string = "Blah blah blah";

           File dest = new File( "test.tmp" );

           

           FileUtils.writeStringToFile( dest, string, "ISO-8859-1" );

       } catch( IOException ioe ) {

           System.out.println( "Error writing out a String." );        

       }

       

       

       try {

           Writer writer = new FileWriter( "test.dat" );

           InputStream inputStream =FileCopyExample.class. 

              getClass().getResourceAsStream("/io/web.xml");

           IOUtils.copy( inputStream, writer );

           writer.close();

           inputStream.close();

       } catch (IOException e) {

           System.out.println( "Error copying data" );

       }

 

try {

           File src = new File( "test.txt" );

           OutputStream output = new FileOutputStream(src);

           InputStream inputStream =FileCopyExample.class. 

              getClass().getResourceAsStream("/io/web.xml");

           IOUtils.copy( inputStream, output );

//         writer.close();

           inputStream.close();

           output.close();

       } catch (IOException e) {

           System.out.println( "Error copying data" );

       }

    }

 

}

 

2.文件删除：

File file = new File( ("io/project.properties") );

       String display = FileUtils.byteCountToDisplaySize( file.length() );

       System.out.println( "project.xml is " + display );

       FileUtils.forceDelete(file) 

 

3.读取取文本中的每一行：

import java.io.File;

import java.io.IOException;

import java.util.List;

 

import org.apache.commons.io.FileSystemUtils;

import org.apache.commons.io.FileUtils;

 

 

public class Test {

 

       /**

        * @param args

        * @throws IOException 

        */

       public static void main(String[] args) throws IOException {

              // TODO Auto-generated method stub

              File file = new File((Test.class.getClass().getResource("/io/web.xml")).getFile());

               List lines = FileUtils.readLines(file, "UTF-8");

               for(int i=0;i<lines.size();i++)

               System.out.println(lines.get(i));

       }

 

}   

Google collections

这是我所知道的最好的扩展实现包，其中一些被社区叫嚣着要加入JDK：

// create an ArrayList with three arguments

List<String> list = Lists.newArrayList("foo", "bar", "baz");



// notice that there is no generics or class cast,

// and still this line does not generate a warning.

Set<String> s = Sets.newConcurrentHashSet();



// intersect and union are basic features of a Set, if you ask me

Set<String> s = Sets.intersect(s1, s2);



// Example  of multiple values in a Map

ListMultimap<String, Validator> validators = new ArrayListMultimap<String, Validator>();

validators.put("save", new RequiredValidator());

validators.put("save", new StringValidator());

validators.put("delete", new NumberValidator());



validators.get("save"); // { RequiredValidator, StringValidator }

validators.get("foo");  // empty List (not null)

validators.values();    // { RequiredValidator, StringValidator, NumberValidator }

java.util.concurrent

不是每个人都需要这么重的java.util.concurrent，但是很好用:

// a map that may be modified (by the same or different thread) while being iterated

Map<String, Something> repository = new ConcurrentHashMap<String, Something>();



// same with lists. This one is only available with Java 6

List<Something> list = new CopyOnWriteArrayList<Something>();