Guava学习之Map

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Guava 中文是石榴的意思，该项目是 Google 的一个开源项目，包含许多 Google 核心的 Java 常用库。

目前主要包含：

com.google.common.annotations

com.google.common.base

com.google.common.collect

com.google.common.io

com.google.common.net

com.google.common.primitives

com.google.common.util.concurrent
在线API doc：http://tool.oschina.net/apidocs/apidoc?api=guava

现有如下Person类：

public class Person {    private String name;    private int age;    public Person(String name, int age) {        this.name = name;        this.age = age;    }    public String getName() {        return name;    }    public int getAge() {        return age;    }    @Override    public String toString() {        return "Person{" +                "name='" + name + '\'' +                ", age=" + age +                '}';    }}

1 将List转化为Map

1.1 转化后具有唯一Key

public class TestMap {    public static void main(String args[]) {        List<Person> persons = Arrays.asList(                new Person("zhang", 15),                new Person("wang", 16),                new Person("lee", 18)        );        /**         * 转换后的Map具有唯一键         */        Map<String, Person> map = Maps.uniqueIndex(persons, new Function<Person, String>() {            @Override            public String apply(Person person) {                return person.getName();            }        });    }}

1.2 转化后的Key不唯一

public class TestMap {    public static void main(String args[]) {        List<Person> persons = Lists.newArrayList(                new Person("zhang", 15),                new Person("zhang", 16),                new Person("lee", 18)        );        /**         * 转换后的Map有重复键         */        Multimap<String, Person> multiMap = Multimaps.index(persons, new Function<Person, String>() {            public String apply(Person person) {                return person.getName();            }        });    }}

2 将Set转化为Map

public class TestMap {    public static void main(String args[]) {        Set<Person> persons = Sets.newHashSet(                new Person("zhang", 15),                new Person("zhang", 16),                new Person("lee", 18)        );        /**         * 以Set的值为Key，计算出一个Value         */        Map<Person, String> map = Maps.asMap(persons, new Function<Person, String>() {            public String apply(Person person) {                return person.getName();            }        });    }}

3 转换Map的值

public class TestMap {    public static void main(String args[]) {        List<Person> persons = Lists.newArrayList(                new Person("zhang", 15),                new Person("wang", 15),                new Person("lee", 18)        );        Map<String, Person> map = Maps.uniqueIndex(persons, new Function<Person, String>() {            public String apply(Person person) {                return person.getName();            }        });        /**         * 使用Key和Value作为输入，计算出一个新的Value         */        Map<String, Integer> map2 = Maps.transformEntries(map, new Maps.EntryTransformer<String, Person, Integer>() {            @Override            public Integer transformEntry(String s, Person person) {                return person.getAge();            }        });        /**         * 使用Function计算出一个新的Value         */        Map<String, Double> map3 = Maps.transformValues(map, new Function<Person, Double>() {            @Override            public Double apply(Person person) {                return (double)person.getAge();            }        });    }}

4 筛选Map中符合条件的映射

public class TestMap {    public static void main(String args[]) {        List<Person> persons = Lists.newArrayList(                new Person("zhang", 15),                new Person("wang", 15),                new Person("lee", 18)        );        Map<String, Person> map = Maps.uniqueIndex(persons, new Function<Person, String>() {            public String apply(Person person) {                return person.getName();            }        });        map = Maps.filterEntries(map, new Predicate<Map.Entry<String, Person>>() {            @Override            public boolean apply(Map.Entry<String, Person> stringPersonEntry) {                return stringPersonEntry.getKey() != null && stringPersonEntry.getValue() != null;            }        });        map = Maps.filterKeys(map, new Predicate<String>() {            @Override            public boolean apply(String s) {                return s != null && s.length() > 2;            }        });        map = Maps.filterValues(map, new Predicate<Person>() {            @Override            public boolean apply(Person person) {                return person != null && person.getAge() > 15;            }        });        for (Map.Entry<String, Person> entry : map.entrySet()) {            System.out.println(entry.getKey() + " " + entry.getValue());        }    }}

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