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。
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.
*//**
System.out.println("result size for filteredUsersNot1: " + filteredUsersNot1.size()); //1-> chen
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));</span><span style="color: #008000;">//</span><span style="color: #008000;">保留age不为26的User</span> Predicate<User> predicate1 = <span style="color: #0000ff;">new</span> Predicate<User><span style="color: #000000;">() { </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">boolean</span><span style="color: #000000;"> apply(User user) { </span><span style="color: #0000ff;">if</span>(user.getAge() != 26<span style="color: #000000;">){ </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">true</span><span style="color: #000000;">; } </span><span style="color: #0000ff;">return</span> <span style="color: #0000ff;">false</span><span style="color: #000000;">; } }; </span><span style="color: #008000;">//</span><span style="color: #008000;">保留userName 是 chang 的user</span> Predicate<User> predicate2 = <span style="color: #0000ff;">new</span> Predicate<User><span style="color: #000000;">() { </span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">boolean</span><span style="color: #000000;"> apply(User user) { </span><span style="color: #0000ff;">return</span> Objects.equals(user.getUserName(),"chang"<span style="color: #000000;">); } }; </span><span style="color: #008000;">//</span><span style="color: #008000;">保留age不为 26 以及 userName 是 chang 的User</span> Predicate<User> predicate1_and_predicate2 =<span style="color: #000000;"> Predicates.and(predicate1, predicate2); </span><span style="color: #008000;">//</span><span style="color: #008000;">保留age不为26 或 userName 是 chang的User</span> Predicate<User> predicate1_or_predicate2 =<span style="color: #000000;"> Predicates.or(predicate1, predicate2); </span><span style="color: #008000;">//</span><span style="color: #008000;">与predicate1条件相反</span> Predicate<User> notpredicate1 =<span style="color: #000000;"> Predicates.not(predicate1); </span><span style="color: #008000;">//</span><span style="color: #008000;">List<User> filteredUsers = Lists.newArrayList(Iterators.filter(users.iterator(), predicate1));</span> List<User> filteredUsers1 =<span style="color: #000000;"> Lists.newArrayList(Iterables.filter(users,predicate1)); List</span><User> filteredUsers2 =<span style="color: #000000;"> Lists.newArrayList(Iterables.filter(users,predicate2)); List</span><User> filteredUsers1and2 =<span style="color: #000000;"> Lists.newArrayList(Iterables.filter(users,predicate1_and_predicate2)); List</span><User> filteredUsers1or2 =<span style="color: #000000;"> Lists.newArrayList(Iterables.filter(users,predicate1_or_predicate2)); List</span><User> filteredUsersNot1 =<span style="color: #000000;"> Lists.newArrayList(Iterables.filter(users,notpredicate1)); System.out.println(</span>"result size for filteredUsers1: " + filteredUsers1.size()); <span style="color: #008000;">//</span><span style="color: #008000;">2-> chang sun</span> System.out.println("result size for filteredUsers2: " + filteredUsers2.size()); <span style="color: #008000;">//</span><span style="color: #008000;">1-> chang</span> System.out.println("result size for filteredUsers1and2: " + filteredUsers1and2.size()); <span style="color: #008000;">//</span><span style="color: #008000;">1-> chang</span> System.out.println("result size for filteredUsers1or2: " + filteredUsers1or2.size()); <span style="color: #008000;">//</span><span style="color: #008000;">2-> chang sun</span>}}
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
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