字符串处理之Splitter和Joiner

在google guava包中提供了用于字符串连接（Joiner）和字符串分割（Splitter）的类

Joiner

主要方法有：

/**
*返回一个在连续元素之间自动放置分隔符（separator）的joiner
*Returns a joiner which automatically places {@code separator} between consecutive   
*elements.
*/
public static Joiner on(String separator) {
    return new Joiner(separator);
}

//该方法同上
public static Joiner on(char separator) {
    return new Joiner(String.valueOf(separator));
}

//构造方法用private修饰，不允许外部类直接用new创建
private Joiner(String separator) {
   //separator不能为null，否则将抛出空指针异常
   this.separator = checkNotNull(separator);   
}
private Joiner(Joiner prototype) {
   this.separator = prototype.separator;
}

/**
  *返回一个用分隔符将Iterable中每一个参数进行分离后的结果，例子见后面
  *Returns a string containing the string  representation of each of {@code parts}, using             
  *the previously configured separator between each.
  */
public final String join(Iterable<?> parts) {
   return join(parts.iterator());
}

同样的，对于数组有：
public final String join(Object[] parts) {
   return join(Arrays.asList(parts));
}

还提供了skipNulls()方法和useForNull()方法对空元素进行处理

example：

import com.google.common.base.Joiner;
public class Demo {
   public static void main(String args[]) {
     Joiner joiner=Joiner.on(";");
     String result=joiner.join("whisper","godyan","yan");
     System.out.println(result);
  }
}

输出：

whisper;godyan;yan

默认情况下，元素不能包含null，否则将抛出空指针异常，例如

joiner.join("whisper","godyan",null,"yan")

当参数中的元素包含null时，可通过skipNulls()和useForNull()方法将null排除
example:

import com.google.common.base.Joiner;
public class Demo {
  public static void main(String args[]) {
    Joiner joiner=Joiner.on(";").skipNulls();
    //或Joiner joiner=Joiner.on(";").useForNull();
    String result=joiner.join("whisper","godyan",null,"yan");
    System.out.println(result);
  }
}

输出：

whisper;godyan;yan

Splitter

主要方法有：

//返回使用给定的单字符分隔符的拆分器
/**
 *Returns a splitter that uses the given single-character separator. For example,
 *{@code Splitter.on(',').split("foo,,bar")} returns an iterable containing
 *{@code ["foo", "", "bar"]}.
 *@param separator the character to recognize as a separator
 *@return a splitter, with default settings, that recognizes that separator
 */
public static Splitter on(char separator) {
   return on(CharMatcher.is(separator));
}

//返回一个将字符串分割为给定长度部分的分割器
public static Splitter fixedLength(final int length) {
   checkArgument(length > 0, "The length may not be less than 1");
   return new Splitter(
    new Strategy() {
      @Override
      public SplittingIterator iterator(final Splitter splitter, CharSequence toSplit) {
        return new SplittingIterator(splitter, toSplit) {
          @Override
          public int separatorStart(int start) {
            int nextChunkStart = start + length;
            return (nextChunkStart < toSplit.length() ? nextChunkStart : -1);
          }
          @Override
          public int separatorEnd(int separatorPosition) {
            return separatorPosition;
          }
        };
      }
    });
}

//指定将字符串分割为limit部分
public Splitter limit(int limit) {
   checkArgument(limit > 0, "must be greater than zero: %s", limit);
   return new Splitter(strategy, omitEmptyStrings, trimmer, limit);
}

//调用该方法将自动移除元素首尾的空格
public Splitter trimResults() {
   return trimResults(CharMatcher.whitespace());
}

//与上述方法不同的是，该方法可以将每个元素中所有头部或者所有尾部指定字符移除，具体见例子
public Splitter trimResults(CharMatcher trimmer) {
   checkNotNull(trimmer);
   return new Splitter(strategy, omitEmptyStrings,  trimmer, limit);
}

//返回一个用指定分隔符对参数（sequence）进行分割后生成的Iterable对象
/**
 *Splits {@code sequence} into string components and makes them available through
 *an {@link Iterator}, which may be lazily evaluated. If you want an eagerly computed 
 *{@link List}, use {@link #splitToList(CharSequence)}.
 *@param sequence the sequence of characters to split
 *@return an iteration over the segments split from the parameter.
 */
public Iterable<String> split(final CharSequence sequence) {
   checkNotNull(sequence);
   return new Iterable<String>() {
     @Override
     public Iterator<String> iterator() {
        return splittingIterator(sequence);
    }
    @Override
    public String toString() {
      return Joiner.on(", ")
        .appendTo(new StringBuilder().append('['), this)
        .append(']')
        .toString();
    }
  };
}

//使用该方法可以自动省略空字符串
public Splitter omitEmptyStrings() {
   return new Splitter(strategy, true, trimmer, limit);
}

example:
import com.google.common.base.Splitter;
public class Demo {
public static void main(String args[]) {
Splitter splitter=Splitter.on(",");
Iterable<String> itr=splitter.split("whisper,godyan,yan");
for(String str:itr) {
System.out.print(str+"\t");
}
}
}
输出：

whisper godyan  yan

example:

import com.google.common.base.Splitter;

public class Demo {
  public static void main(String args[]) {
    Splitter splitter=Splitter.on(",");
    Iterable<String> itr=splitter.split("whisper,,,godyan,yan");
     for(String str:itr) {
        System.out.print(str+"\t");
     }
  }
}

输出：

whisper         godyan  yan 

其中whisper和godyan之间包含两个空字符串
example:

import com.google.common.base.Splitter;
public class Demo {
   public static void main(String args[]) {
     Splitter splitter=Splitter.on(",")..trimResults();
    //,和yan之间的空格将自动移除
    //Splitter splitter=Splitter.on(",")..trimResults(CharMatcher.is('_')).split("_a ,_b_ ,c__")将a中头部的_移除，b中头部_移除，c中尾部__移除，所以分割后的元素为a,b_,c，注意b后面的_没有移除
    Iterable<String> itr=splitter.split("whisper,,,godyan,                          yan");
    for(String str:itr) {
        System.out.print(str+"\t");
    }
  }
}

注意：
如果要去掉null元素，这种写法是错的，skipNull()方法将不起作用

Joiner joiner = Joiner.on(',');
joiner.skipNulls(); 
return joiner.join("wrong", null, "wrong");}

同样的，像Splitter这种方法也是错误的，trimResults()方法也不起作用

Splitter splitter = Splitter.on('/');
splitter.trimResults();
return splitter.split("wrong / wrong / wrong");}