Guava Collection/List transform 陷阱

首先看段代码吧.

package demo.collection.transform;

import java.util.List;

import com.google.common.collect.Lists;

import demo.util.vo.Person;

public class GuavaList {

	public static void main(String[] args) throws Exception {
		List<String> names = Lists.newArrayList("Andy", "Bob");
		List<Person> persons = Lists.transform(names, name -> new Person(name, 12));
		System.out.println(persons);
		// 设置所有age到20
		for (Person person : persons) {
			person.setAge(20);
		}
		System.out.println(persons);
	}

}

Person 有两个属性, Name和Age.

问题来了, 在for循环里把每个person的age改完后, 会打印出什么来呢? 是结果A还是B?

结果A

[Person [name=Andy, age=12], Person [name=Bob, age=12]]
[Person [name=Andy, age=20], Person [name=Bob, age=20]]

结果B

[Person [name=Andy, age=12], Person [name=Bob, age=12]]
[Person [name=Andy, age=12], Person [name=Bob, age=12]]

正确是结果B. 具体原因看下源码就知道了, transform之后会返回一个TransformingSequentialList. 这个list会让每次被遍历的时候, 都会调用transform的function. 而导致每次生成新的Person.

同时介绍下其他的两种transform

JDK 8 Stream Transform
Apache Transform

JDK 8 Stream Transform 可以产生结果A

package demo.collection.transform;

import java.util.List;
import java.util.stream.Collectors;

import com.google.common.collect.Lists;

import demo.util.vo.Person;

public class JdkStream {

	public static void main(String[] args) throws Exception {
		List<String> names = Lists.newArrayList("Andy", "Bob");
		List<Person> persons = names.stream().map(name -> new Person(name, 12)).collect(Collectors.toList());
		System.out.println(persons);
		// 设置所有age到20
		for (Person person : persons) {
			person.setAge(20);
		}
		System.out.println(persons);
	}

}

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结果

[Person [name=Andy, age=12], Person [name=Bob, age=12]]
[Person [name=Andy, age=20], Person [name=Bob, age=20]]

Apache Transform 可以产生结果A, 但是是改变了原List的内容.

package demo.collection.transform;

import java.util.List;

import org.apache.commons.collections.CollectionUtils;

import com.google.common.collect.Lists;

import demo.util.vo.Person;

public class ApacheTransform {

	public static void main(String[] args) throws Exception {
		List<Object> objects = Lists.newArrayList("Andy", "Bob");
		CollectionUtils.transform(objects, name -> new Person((String) name, 12));

		System.out.println(objects);
		// 设置所有age到20
		for (Object person : objects) {
			((Person) person).setAge(20);
		}
		System.out.println(objects);
	}

}

结果

[Person [name=Andy, age=12], Person [name=Bob, age=12]]
[Person [name=Andy, age=20], Person [name=Bob, age=20]]

可以看到之前List里面是String, transform之后List内容变成了Person