Java学习之常用工具类

小编来为大家介绍几款比较常用的Java开发工具：

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>();

　　以上就是关于“比较常用的Java工具类”的介绍