Guava中针对集合的 filter和过滤功能

在guava库中，自带了过滤器(filter)的功能，可以用来对collection 进行过滤，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); } }

在这个例子中，给出一个list，过滤出含有字母a的元素 
此外，可以使用Collections2.filter() 去进行过滤 

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); } }

再来看下predicates判断语言， 
com.google.common.base. Predicate : 根据输入值得到 true 或者 false 
拿Collections2中有2个函数式编程的接口：filter , transform ,例如 ：在Collection<Integer>中过滤大于某数的内容：

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类型转换为String , 并添加test作为后缀字符

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); } }

需要说明的是每次调用返回都是新的对象，同时操作过程不是线程安全的。 

将多个prdicate进行组合 

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; 
再看下如何将集合中的空元素删除： 

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); } }

检查一个collection中的所有元素是否符合某个条件： 

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); } }