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;

@GwtCompatible
public 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