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Guava中Predicate的常见用法
1. Predicate基本用法
guava提供了许多利用Functions和Predicates来操作Collections的工具,一般在 Iterables, Lists, Sets, Maps, Multimaps中用到。
Predicate最基本的用法就是对Collection进行过滤,guava中很多集合的filter方法都是用Predicate来实现过滤的。
Collection type | Filter method | ||
Iterable | Iterables.filter(Iterable, Predicate) | FluentIterable.filter(Predicate) | |
Iterator | Iterators.filter(Iterator, Predicate) | ||
Collection | Collections2.filter(Collection, Predicate) | ||
Set | Sets.filter(Set, Predicate) | ||
SortedSet | Sets.filter(SortedSet, Predicate) | ||
Map | Maps.filterKeys(Map, Predicate) | Maps.filterValues(Map, Predicate) | Maps.filterEntries(Map, Predicate) |
SortedMap | Maps.filterKeys(SortedMap, Predicate) | Maps.filterValues(SortedMap, Predicate) | Maps.filterEntries(SortedMap, Predicate) |
Multimap | Multimaps.filterKeys(Multimap, Predicate) | Multimaps.filterValues(Multimap, Predicate) | Multimaps.filterEntries(Multimap, Predicate) |
注意:
Lists没有提供filter方法;
过滤后的集合一般通过Lists.newArrayList(Collections2.filter(list, predicate))拿到。
2. Predicate接口
Predicate接口提供了一个泛型方法apply,在使用时根据需求实现
Predicate继承了Object的equals方法,并提供了多个实现,主要是为了提供一个通用的方法,用于Object为Predicate类型时。
package com.google.common.base;import com.google.common.annotations.GwtCompatible;import javax.annotation.Nullable;@GwtCompatiblepublic interface Predicate<T> { boolean apply(@Nullable T input); @Override boolean equals(@Nullable Object object);}
3. Predicates的常用方法
Predicates时guava中与Predicate配套使用的工具类,返回Predicate实例。
下面是一个例子
package link.mengya;/** * Created by chang on 16/2/19. */public class User { private String userName; private int age; public User(String userName, int age) { this.userName = userName; this.age = age; } public String getUserName() { return userName; } public int getAge() { return age; } public void setUserName(String userName) { this.userName = userName; } public void setAge(int age) { this.age = age; }}
package link.mengya.utils;import com.google.common.base.Predicate;import com.google.common.base.Predicates;import com.google.common.collect.Iterables;import com.google.common.collect.Iterators;import com.google.common.collect.Lists;import link.mengya.User;import java.util.ArrayList;import java.util.List;import java.util.Objects;/** * Created by chang on 16/2/19. *//** * Predicate 返回为true 的保留, 返回为false的过滤掉 * Predicates.and(predicate1, predicate2) predicate1 与 predicate2 返回都为true的保留 * Predicates.or(predicate1, predicate2) predicate1 与 predicate2 有一个返回true 则保留 */public class PredicateTest { public static void main(String[] args){ List<User> users = new ArrayList<User>(); users.add(new User("chang",24)); users.add(new User("chen",26)); users.add(new User("sun",24)); //保留age不为26的User Predicate<User> predicate1 = new Predicate<User>() { public boolean apply(User user) { if(user.getAge() != 26){ return true; } return false; } }; //保留userName 是 chang 的user Predicate<User> predicate2 = new Predicate<User>() { public boolean apply(User user) { return Objects.equals(user.getUserName(),"chang"); } }; //保留age不为 26 以及 userName 是 chang 的User Predicate<User> predicate1_and_predicate2 = Predicates.and(predicate1, predicate2); //保留age不为26 或 userName 是 chang的User Predicate<User> predicate1_or_predicate2 = Predicates.or(predicate1, predicate2); //与predicate1条件相反 Predicate<User> notpredicate1 = Predicates.not(predicate1); //List<User> filteredUsers = Lists.newArrayList(Iterators.filter(users.iterator(), predicate1)); List<User> filteredUsers1 = Lists.newArrayList(Iterables.filter(users,predicate1)); List<User> filteredUsers2 = Lists.newArrayList(Iterables.filter(users,predicate2)); List<User> filteredUsers1and2 = Lists.newArrayList(Iterables.filter(users,predicate1_and_predicate2)); List<User> filteredUsers1or2 = Lists.newArrayList(Iterables.filter(users,predicate1_or_predicate2)); List<User> filteredUsersNot1 = Lists.newArrayList(Iterables.filter(users,notpredicate1)); System.out.println("result size for filteredUsers1: " + filteredUsers1.size()); //2-> chang sun System.out.println("result size for filteredUsers2: " + filteredUsers2.size()); //1-> chang System.out.println("result size for filteredUsers1and2: " + filteredUsers1and2.size()); //1-> chang System.out.println("result size for filteredUsers1or2: " + filteredUsers1or2.size()); //2-> chang sun System.out.println("result size for filteredUsersNot1: " + filteredUsersNot1.size()); //1-> chen }}
更多关于guava中Predicates与Functions的用法参见
guava-libraries的wiki: https://code.google.com/p/guava-libraries/wiki/FunctionalExplained
guava github上的wiki:https://github.com/google/guava/wiki/FunctionalExplained#predicates
Guava中Predicate的常见用法