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\n", i, freq);
- }
输出结果:
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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