Google Guava 库用法整理

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);
}
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
", 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
", 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
", from.get(i), to.get(i) ? " " : " NOT ");
}
// changes in the "from" list are reflected in the "to" list
System.out.printf("
now replace hannah with megan...

");
from.set(2, "megan");
for (int i = 0; i < from.size(); i++) {
    System.out.printf("%s is%sa palindrome
", from.get(i), to.get(i) ? " " : " NOT ");
}