Guava中針對集合的 filter和過濾功能

在guava庫中，自帶了過濾器(filter)的功能，可以用來對collection 進行過濾，先看例子： 

import com.google.common.base.Predicates; import com.google.common.collect.Iterables; import com.google.common.collect.Lists; import java.util.List; public class Test { public static void main(String[] args) { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Iterable<String> result = Iterables.filter(names, Predicates.containsPattern("a")); // [Jane, Adam]
 System.out.println(result); } }

在這個例子中，給出一個list，過濾出含有字母a的元素 
此外，可以使用Collections2.filter() 去進行過濾 

import com.google.common.base.Predicates; import com.google.common.collect.Collections2; import com.google.common.collect.Lists; import java.util.Collection; import java.util.List; public class Test { public static void main(String[] args) { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<String> result = Collections2.filter(names, Predicates.containsPattern("a")); // [Jane, Adam]
 System.out.println(result); } }

再來看下predicates判斷語言， 
com.google.common.base. Predicate : 根據輸入值得到 true 或者 false 
拿Collections2中有2個函數式編程的接口：filter , transform ,例如 ：在Collection<Integer>中過濾大於某數的內容：

import com.google.common.base.Predicate; import com.google.common.collect.Collections2; import com.google.common.collect.Lists; import java.util.Collection; import java.util.List; public class Test { public static void main(String[] args) { List<Integer> collections = Lists.newArrayList(1, 2, 3, 4); Collection<Integer> filter = Collections2.filter( collections, new Predicate<Integer>() { @Override public boolean apply(Integer input) { return input >= 3; } }); // [3, 4]
 System.out.println(filter); } }

把Lis<Integer>中的Integer類型轉換為String , 並添加test作為后綴字符

import com.google.common.base.Function; import com.google.common.collect.Lists; import java.util.List; public class Test { public static void main(String[] args) { List<Integer> list = Lists.newArrayList(1, 2, 3, 4); List<String> transform = Lists.transform(list, new Function<Integer, String>() { @Override public String apply(Integer input) { return input + "_test"; } }); // [1_test, 2_test, 3_test, 4_test]
 System.out.println(transform); } }

需要說明的是每次調用返回都是新的對象，同時操作過程不是線程安全的。 

將多個prdicate進行組合 

import com.google.common.base.Predicates; import com.google.common.collect.Collections2; import com.google.common.collect.Lists; import java.util.Collection; import java.util.List; public class Test { public static void main(String[] args) { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); Collection<String> result = Collections2.filter(names, Predicates.or(Predicates.containsPattern("J"), Predicates.not(Predicates.containsPattern("a")))); // [John, Jane, Tom]
 System.out.println(result); } }

上面的例子中找出包含J字母或不包含a的元素; 
再看下如何將集合中的空元素刪除： 

import com.google.common.base.Predicates; import com.google.common.collect.Collections2; import com.google.common.collect.Lists; import java.util.Collection; import java.util.List; public class Test { public static void main(String[] args) { List<String> names = Lists.newArrayList("John", null, "Jane", null, "Adam", "Tom"); Collection<String> result = Collections2.filter(names, Predicates.notNull()); // [John, Jane, Adam, Tom]
 System.out.println(result); } }

檢查一個collection中的所有元素是否符合某個條件： 

import com.google.common.base.Predicates; import com.google.common.collect.Iterables; import com.google.common.collect.Lists; import java.util.List; public class Test { public static void main(String[] args) { List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom"); boolean result = Iterables.all(names, Predicates.containsPattern("n|m")); // true
 System.out.println(result); result = Iterables.all(names, Predicates.containsPattern("a")); // false
 System.out.println(result); } }