Guava中Predicate的常見用法

1.  Predicate基本用法

guava提供了許多利用Functions和Predicates來操作Collections的工具，一般在 Iterables, Lists, Sets, Maps, Multimaps中用到。

Predicate最基本的用法就是對Collection進行過濾，guava中很多集合的filter方法都是用Predicate來實現過濾的。

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沒有提供filter方法；

過濾后的集合一般通過Lists.newArrayList(Collections2.filter(list, predicate))拿到。

 

2. Predicate接口

Predicate接口提供了一個泛型方法apply,在使用時根據需求實現

Predicate繼承了Object的equals方法，並提供了多個實現，主要是為了提供一個通用的方法，用於Object為Predicate類型時。

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時guava中與Predicate配套使用的工具類，返回Predicate實例。

下面是一個例子

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  返回為true 的保留， 返回為false的過濾掉
 * Predicates.and(predicate1, predicate2)  predicate1 與 predicate2 返回都為true的保留
 * Predicates.or(predicate1, predicate2)   predicate1 與 predicate2 有一個返回true 則保留
 */
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不為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不為 26 以及 userName 是 chang 的User
        Predicate<User> predicate1_and_predicate2 = Predicates.and(predicate1, predicate2);

        //保留age不為26 或 userName 是 chang的User
        Predicate<User> predicate1_or_predicate2 = Predicates.or(predicate1, predicate2);

        //與predicate1條件相反
        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


    }

}

 

更多關於guava中Predicates與Functions的用法參見

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