Google Guava 库用法整理

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推荐:[Google Guava笔记](一)annotations

[       最近在看Google Guava库的源码,为什么会看这个库的源码呢?不做解释。         这个项目包含了一些Google的核心Java1.5+类库,这些类库已经� ...]

参考:
http://codemunchies.com/2009/10/beautiful-code-with-google-collections-guava-and-static-imports-part-1/ (2,3,4)
http://blog.publicobject.com

更多用法参考 http://ajoo.iteye.com/category/119082

以前这么用:
Map<String, Map<Long, List<String>>> map = new HashMap<String, Map<Long,List<String>>>();

现在这么用(JDK7将实现该功能):
Map<String, Map<Long, List<String>>> map = Maps.newHashMap();


针对不可变集合:
以前这么用:
List<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
list.add("c");
list.add("d");

现在Guava这么用:
ImmutableList<String> of = ImmutableList.of("a", "b", "c", "d");
ImmutableMap<String,String> map = ImmutableMap.of("key1", "value1", "key2", "value2");


文本文件读取现在Guava这么用
File file = new File(getClass().getResource("/test.txt").getFile());
List<String> lines = null;
try {
lines = Files.readLines(file, Charsets.UTF_8);
} catch (IOException e) {
e.printStackTrace();
}


基本类型比较, 现在Guava这么用:
int compare = Ints.compare(a, b);


Guava中CharMatcher的用法:
assertEquals("89983", CharMatcher.DIGIT.retainFrom("some text 89983 and more"))
assertEquals("some text  and more", CharMatcher.DIGIT.removeFrom("some text 89983 and more"))


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);


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);


Ints中一些用法:
int[] array = { 1, 2, 3, 4, 5 };
int a = 4;
boolean contains = Ints.contains(array, a);
int indexOf = Ints.indexOf(array, a);
int max = Ints.max(array);
int min = Ints.min(array);
int[] concat = Ints.concat(array, array2);


集合
set的交集, 并集, 差集的用法( http://publicobject.com/2008/08/coding-in-small-with-google-collections.html)
HashSet setA = newHashSet(1, 2, 3, 4, 5);
HashSet setB = newHashSet(4, 5, 6, 7, 8);
 
SetView union = Sets.union(setA, setB);
System.out.println("union:");
for (Integer integer : union)
    System.out.println(integer);       
 
SetView difference = Sets.difference(setA, setB);
System.out.println("difference:");
for (Integer integer : difference)
    System.out.println(integer);      
 
SetView intersection = Sets.intersection(setA, setB);
System.out.println("intersection:");
for (Integer integer : intersection)
    System.out.println(integer);


针对Map的用法:
MapDifference differenceMap = Maps.difference(mapA, mapB);

differenceMap.areEqual();
Map entriesDiffering = differenceMap.entriesDiffering();
Map entriesOnlyOnLeft = differenceMap.entriesOnlyOnLeft();
Map entriesOnlyOnRight = differenceMap.entriesOnlyOnRight();
Map entriesInCommon = differenceMap.entriesInCommon();


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 = checkNotNull(title);
    this.date = checkNotNull(date);
    this.author = checkNotNull(author);
}


如果一个key对应多个value的Map, 你会怎么处理? 如果还在使用Map<K, List<V>>的话, 你就out了
使用MultiMap吧:
Multimap<Person, BlogPost> multimap = ArrayListMultimap.create();


Multimap的另一个使用场景:
比如有一个文章数据的map:
List<Map<String, String>> listOfMaps = mapOf("type", "blog", "id", "292", "author", "john");

如果要按照type分组生成一个List
Multimap<String, Map<String, String>> partitionedMap = Multimaps.index(   
                listOfMaps,                                                                                       
                new Function<Map<String, String>, String>() {                                 
                    public String apply(final Map<String, String> from) {                    
                        return from.get("type");                                                             
                    }                                                                                                   
                }); 
                                                                                                    
针对集合中只有一个元素的情况:
Iterables.getOnlyElement();
这个主要是用来替换Set.iterator.next()或 List.get(0), 而且在测试中使用非常方便, 如果出现0个或者2+则直接抛出异常

比较的最大最小值:
Comparators.max
Comparators.min

equals和hashcode的用法:
  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 int hashCode() {
    return Objects.hashCode(address, targetArrivalDate, lineItems);
  }


ImmutableList.copyOf的用法:
以前这么用:
  public Directions(Address from, Address to, List<Step> steps) {
    this.from = from;
    this.to = to;
    this.steps = Collections.unmodifiableList(new ArrayList<Step>(steps));
  }

现在这么用:
  public Directions(Address from, Address to, List<Step> steps) {
    this.from = from;
    this.to = to;
    this.steps = ImmutableList.of(steps);
  }


Iterables.concat()的用法:
以前这么用:
  public boolean orderContains(Product product) {
    List<LineItem> allLineItems = new ArrayList<LineItem>();
    allLineItems.addAll(getPurchasedItems());
    allLineItems.addAll(getFreeItems());

    for (LineItem lineItem : allLineItems) {
      if (lineItem.getProduct() == product) {
        return true;
      }
    }

    return false;
  }

现在这么用:
  public boolean orderContains(Product product) {
    for (LineItem lineItem : Iterables.concat(getPurchasedItems(), getFreeItems())) {
      if (lineItem.getProduct() == product) {
        return true;
      }
    }

    return false;
  }


Constraints.constrainedList: 给List操作注入约束逻辑, 比如添加不合法元素直接报错.
以前这么写:
  private final List<LineItem> purchases = new ArrayList<LineItem>();

  /**
   * Don't modify this! Instead, call {@link #addPurchase(LineItem)} to add
   * new purchases to this order.
   */
  public List<LineItem> getPurchases() {
    return Collections.unmodifiableList(purchases);
  }

  public void addPurchase(LineItem purchase) {
    Preconditions.checkState(catalog.isOffered(getAddress(), purchase.getProduct()));
    Preconditions.checkState(purchase.getCharge().getUnits() > 0);
    purchases.add(purchase);
  }
现在这么写:
  private final List<LineItem> purchases = Constraints.constrainedList(
      new ArrayList<LineItem>(),
      new Constraint<LineItem>() {
        public void checkElement(LineItem element) {
          Preconditions.checkState(catalog.isOffered(getAddress(), element.getProduct()));
          Preconditions.checkState(element.getCharge().getUnits() > 0);
        }
      });

  /**
   * Returns the modifiable list of purchases in this order.
   */
  public List<LineItem> getPurchases() {
    return purchases;
  }


不允许插入空值的Set(Constraints的用法):
        Set<String> set = Sets.newHashSet();
        Set<String> constrainedSet = Constraints.constrainedSet(set, Constraints.notNull());
        constrainedSet.add("A");
        constrainedSet.add(null); // NullPointerException here


Multimap的用法(允许多值的map):
以前这么写:
  Map<Salesperson, List<Sale>> map = new Hashmap<SalesPerson, List<Sale>>();

  public void makeSale(Salesperson salesPerson, Sale sale) {
    List<Sale> sales = map.get(salesPerson);
    if (sales == null) {
      sales = new ArrayList<Sale>();
      map.put(salesPerson, sales);
    }
    sales.add(sale);
  }

现在这么写:
  Multimap<Salesperson, Sale> multimap 
      = new ArrayListMultimap<Salesperson,Sale>();

  public void makeSale(Salesperson salesPerson, Sale sale) {
    multimap.put(salesperson, sale);
  }

以前这么写:
  public Sale getBiggestSale() {
    Sale biggestSale = null;
    for (List<Sale> sales : map.values()) {
      Sale biggestSaleForSalesman
          = Collections.max(sales, SALE_COST_COMPARATOR);
      if (biggestSale == null
          || biggestSaleForSalesman.getCharge() > biggestSale().getCharge()) {
        biggestSale = biggestSaleForSalesman;
      }
    }
    return biggestSale;
  }

现在这么写(需要将map转换成multimap):
  public Sale getBiggestSale() {
    return Collections.max(multimap.values(), SALE_COST_COMPARATOR);
  }


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);
 }
}


Comparators.fromFunction的用法:
以前这样写:
 public Comparator<Product> createRetailPriceComparator(
     final CurrencyConverter currencyConverter) {
   return new Comparator<Product>() {
     public int compare(Product a, Product b) {
       return getRetailPriceInUsd(a).compareTo(getRetailPriceInUsd(b));
     }
     public Money getRetailPriceInUsd(Product product) {
       Money retailPrice = product.getRetailPrice();
       return retailPrice.getCurrency() == CurrencyCode.USD
           ? retailPrice
           : currencyConverter.convert(retailPrice, CurrencyCode.USD);
     }
   };
 }

现在这样写(感觉也没省多少):
 public Comparator<Product> createRetailPriceComparator(
     final CurrencyConverter currencyConverter) {
   return Comparators.fromFunction(new Function<Product,Money>() {
     /** returns the retail price in USD */
     public Money apply(Product product) {
       Money retailPrice = product.getRetailPrice();
       return retailPrice.getCurrency() == CurrencyCode.USD
           ? retailPrice
           : currencyConverter.convert(retailPrice, CurrencyCode.USD);
     }
   });
 }


BiMap(双向map)的用法:
以前的用法:
 private static final Map<Integer, String> NUMBER_TO_NAME;
 private static final Map<String, Integer> NAME_TO_NUMBER;
 
 static {
   NUMBER_TO_NAME = Maps.newHashMap();
   NUMBER_TO_NAME.put(1, "Hydrogen");
   NUMBER_TO_NAME.put(2, "Helium");
   NUMBER_TO_NAME.put(3, "Lithium");
   
   /* reverse the map programatically so the actual mapping is not repeated */
   NAME_TO_NUMBER = Maps.newHashMap();
   for (Integer number : NUMBER_TO_NAME.keySet()) {
     NAME_TO_NUMBER.put(NUMBER_TO_NAME.get(number), number);
   }
 }

 public static int getElementNumber(String elementName) {
   return NUMBER_TO_NAME.get(elementName);
 }

 public static string getElementName(int elementNumber) {
   return NAME_TO_NUMBER.get(elementNumber);
 }

现在的用法:
 private static final BiMap<Integer,String> NUMBER_TO_NAME_BIMAP;
 
 static {
   NUMBER_TO_NAME_BIMAP = Maps.newHashBiMap();
   NUMBER_TO_NAME_BIMAP.put(1, "Hydrogen");
   NUMBER_TO_NAME_BIMAP.put(2, "Helium");
   NUMBER_TO_NAME_BIMAP.put(3, "Lithium");
 }

 public static int getElementNumber(String elementName) {
   return NUMBER_TO_NAME_BIMAP.inverse().get(elementName);
 }

 public static string getElementName(int elementNumber) {
   return NUMBER_TO_NAME_BIMAP.get(elementNumber);
 }

换一种写法:
 private static final BiMap<Integer,String> NUMBER_TO_NAME_BIMAP
   = new ImmutableBiMapBuilder<Integer,String>()
       .put(1, "Hydrogen")
       .put(2, "Helium")
       .put(3, "Lithium")
       .getBiMap();


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的用法(将检查异常转换成未检查异常):
package com.ociweb.jnb.apr2010;

import com.google.common.base.Throwables;

import java.io.InputStream;
import java.net.URL;

public class ExerciseThrowables {
    public static void main(String[] args) {
        try {
            URL url = new URL("http://ociweb.com");
            final InputStream in = url.openStream();
            // read from the input stream
            in.close();
        } catch (Throwable t) {
            throw Throwables.propagate(t);
        }
    }
}


Multimap用法整理(http://jnb.ociweb.com/jnb/jnbApr2008.html):
用来统计多值出现的频率:
        Multimap<Integer, String> siblings = Multimaps.newHashMultimap();
        siblings.put(0, "Kenneth");
        siblings.put(1, "Joe");
        siblings.put(2, "John");
        siblings.put(3, "Jerry");
        siblings.put(3, "Jay");
        siblings.put(5, "Janet");

        for (int i = 0; i < 6; i++) {
            int freq = siblings.get(i).size();
            System.out.printf("%d siblings frequency %d\n", i, freq);
        }

输出结果:
引用         0 siblings frequency 1
        1 siblings frequency 1
        2 siblings frequency 1
        3 siblings frequency 2
        4 siblings frequency 0
        5 siblings frequency 1

Functions(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 ");
        }
    }



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