【Guava的用法】1. base

Joiner

Guava中Joiner的用法:

int [] numbers = {   1 ,   2 ,   3 ,   4 ,   5  };  
String numbersAsString = Joiner.on( ";" ).join( Ints.asList(numbers));  

 

另一种写法:

String numbersAsStringDirectly = Ints.join( ";" , numbers);  

 

Joiner的用法:
以前这样写:

public    class  ShoppingList {  
    private  List <Item > items = ...;  
  
  ...  
  
    public  String toString() {  
    StringBuilder stringBuilder =   new  StringBuilder();  
      for  (Iterator <Item > s = items.iterator(); s.hasNext(); ) {  
      stringBuilder.append(s.next());  
        if  (s.hasNext()) {  
        stringBuilder.append( " and " );  
      }  
    }  
      return  stringBuilder.toString();  
  }  
}

 

现在这样写:

public    class  ShoppingList {  
   private  List <Item > items = ...;  
  
 ...  
  
   public  String toString() {  
     return   Join.join( " and " , items);  
 }  
}

 

Splitter

Guava中Splitter的用法:

Iterable split = Splitter.on( "," ).split(numbsAsString);  

 

对于这样的字符串进行切分:

String testString =   "foo , what,,,more," ;  
Iterable <String > split = Splitter.on( "," ).omitEmptyStrings().trimResults().split(testString);  

 

 

Preconditions：验证与条件检查

原来的写法:

if  (count < =   0 ) {                                                                                           
      throw    new  IllegalArgumentException( "must be positive: "   + count);         
}    

      

           
Guava的写法(Jakarta Commons中有类似的方法):

Preconditions.checkArgument(count >   0 ,   "must be positive: %s" , count);  

 

一个更酷的用法:

public  PostExample( final  String title,   final  Date date,   final  String author) {  
      this .title = Preconditions.checkNotNull(title);  
      this .date = checkNotNull(date);  
      this .author = checkNotNull(author);  
}

  

Objects

• euqals

public    boolean  equals(Object o) {  
    if  (o   instanceof  Order) {  
    Order that = (Order)o;  
  
      return   Objects.equal(address, that.address)  
         && Objects.equal(targetArrivalDate, that.targetArrivalDate)  
         && Objects.equal(lineItems, that.lineItems);  
  }   else  {  
      return   false ;  
  }  

}  
  

 

    public static boolean equal(Object a, Object b)

    {

        return a == b || a != null && a.equals(b);

    }

• hashCode

 

public  int hashCode() {  
     return  Objects.hashCode(address, targetArrivalDate, lineItems);  
}

    public static transient int hashCode(Object objects[])

    {

        return Arrays.hashCode(objects);

    }

 

• toString

import com.google.common.base.Objects;
 
public class Point {
     public Integer x;
     public Integer y;
     public Point(Integer x, Integer y) {
         this.x = x;
         this.y = y;
    }
 
    @Override
     public String toString() {

        // 等同于 String.format("Point{x=%d, y=%d}", x, y);
         return Objects.toStringHelper( this)
                      .add( "x", x)
                      .add( "y", y)
                      .toString();
    }
    @Override
     public boolean equals(Object that) {
         if(that instanceof Point) {
            Point p = (Point) that;
             return Objects.equal(x, p.x) && Objects.equal(y, p.y);
        }
         return false;
    }
   @Override
     public int hashCode() {
         return Objects.hashCode(x, y);
    }

 

 

Strings

关于Strings的一些用法( http://blog.ralscha.ch/?p=888):

assertEquals( "test" , Strings.emptyToNull( "test" ));  
assertEquals( " " , Strings.emptyToNull( " " ));  
assertNull(Strings.emptyToNull( "" ));  
assertNull(Strings.emptyToNull( null ));  
   
assertFalse( Strings.isNullOrEmpty( "test" ));  
assertFalse(Strings.isNullOrEmpty( " " ));  
assertTrue(Strings.isNullOrEmpty( "" ));  
assertTrue(Strings.isNullOrEmpty( null ));  
   
assertEquals( "test" , Strings.nullToEmpty( "test" ));  
assertEquals( " " , Strings.nullToEmpty( " " ));  
assertEquals( "" , Strings.nullToEmpty( "" ));  
assertEquals( "" , Strings.nullToEmpty( null ));  
   
assertEquals( "Ralph_____" , Strings.padEnd( "Ralph" ,   10 ,   '_' ));  
assertEquals( "Bob_______" , Strings.padEnd( "Bob" ,   10 ,   '_' ));  
   
assertEquals( "_____Ralph" , Strings.padStart( "Ralph" ,   10 ,   '_' ));  
assertEquals( "_______Bob" , Strings.padStart( "Bob" ,   10 ,   '_' ));  
  
assertEquals( "xyxyxyxyxy" , Strings.repeat( "xy" ,   5 ));  

 

 

Throwables

(将检查异常转换成未检查异常):

 

Before:

public void doSomething() throws IOException, SQLException {
      try {
         someMethodThatCouldThrowAnything();
     } catch (IKnowWhatToDoWithThisException e) {
         handle(e);
     } catch (SQLException e) {
         log(e);
          throw e;
     } catch (IOException e) {
         log(e);
          throw e;
     } catch (Throwable t) {
         log(t);
          throw new RuntimeException(t);
     } 
}

 

 

After:

public void doSomething() throws IOException, SQLException {
      try {
         someMethodThatCouldThrowAnything();
     } catch (IKnowWhatToDoWithThisException e) {
         handle(e);
     } catch (Throwable t) {
         log(t);
          Throwables.propagateIfInstanceOf(t, IOException. class );
         Throwables.propagateIfInstanceOf(t, SQLException. class );
          throw Throwables.propagate(t);
     } 
}  

Functions用于转换集合

(闭包功能)

 

Function <String, Integer > strlen =   new  Function <String, Integer >() {  
      public  Integer apply(String from) {  
        Preconditions.checkNotNull(from);  
         return  from.length();  
    }  
};  

List <String > from = Lists.newArrayList( "abc" ,   "defg" ,   "hijkl" );  
List <Integer > to = Lists.transform(from, strlen);  
for  ( int  i =   0 ; i < from.size(); i ++) {  
    System.out.printf( "%s has length %d\n" , from.get(i), to.get(i));  
}

  

不过这种转换是在访问元素的时候才进行, 下面的例子可以说明:

 

Function <String, Boolean > isPalindrome =   new  Function <String, Boolean >() {  
      public  Boolean apply(String from) {  
        Preconditions.checkNotNull(from);  
          return    new  StringBuilder(from).reverse().toString().equals(from);  
    }  
};  

List <String > from = Lists.newArrayList( "rotor" ,   "radar" ,   "hannah" ,   "level" ,   "botox" );  
List <Boolean > to = Lists. transform(from, isPalindrome);  
for  ( int  i =   0 ; i < from.size(); i ++) {  
    System.out.printf( "%s is%sa palindrome\n" , from.get(i), to.get(i) ?   " "   :   " NOT " );  
}   

// changes in the "from" list are reflected in the "to" list   
System.out.printf( "\nnow replace hannah with megan...\n\n" );  
from.set( 2 ,  "megan" );  
for  ( int  i =   0 ; i < from.size(); i ++) {  
    System.out.printf( "%s is%sa palindrome\n" , from.get(i), to.get(i) ?   " "   :   " NOT " );  
}

  

rotor is a palindrome

radar is a palindrome

hannah is a palindrome

level is a palindrome

botox is NOT a palindrome

 

now replace hannah with megan...

 

rotor is a palindrome

radar is a palindrome

megan is NOT a palindrome

level is a palindrome

botox is NOT a palindrome

 

Predicate用于过滤

public interface Predicate {

    public abstract boolean apply(Object obj);

    public abstract boolean equals(Object obj);

}

 

com.google.common.collect. Sets.filter(Set, Predicate)

 

private static class LengthLessThanPredicate implements Predicate <String > {
         private final int length;
         private LengthLessThanPredicate( final int length) {
             this.length = length;
        }
         public boolean apply( final String s) {
             return s.length() < length;
        }
    }

 

Set result = Sets.filter(mySet, new LengthLessThanPredicate( 5));

 

CharMatcher?

从字符串中得到、去掉所有数字:

assertEquals( "89983" , CharMatcher.DIGIT.retainFrom( "some text 89983 and more" ))  
assertEquals( "some text  and more" , CharMatcher.DIGIT.removeFrom( "some text 89983 and more" ))