本文我们学习Guava Maps对java Map封装和升级,实现强大、简洁应用方式。
首先让我们看看Guava不用new创建HashMap:
Map<String, String> aNewMap = Maps.newHashMap();
创建不可变Map:
@Test
public void whenCreatingImmutableMap_thenCorrect() {
Map salary = ImmutableMap. builder()
.put("John", 1000)
.put("Jane", 1500)
.put("Adam", 2000)
.put("Tom", 2000)
.build();
assertEquals(1000, salary.get("John").intValue());
assertEquals(2000, salary.get("Tom").intValue());
}
salary初始化后,不能在更改,调用put方法会报java.lang.UnsupportedOperationException异常。
本节我们看下创建SortMap,下面示例我们使用Guava相应builder创建一个有序map:
@Test
public void whenUsingSortedMap_thenKeysAreSorted() {
ImmutableSortedMap salary = new ImmutableSortedMap
.Builder(Ordering.natural())
.put("John", 1000)
.put("Jane", 1500)
.put("Adam", 2000)
.put("Tom", 2000)
.build();
assertEquals("Adam", salary.firstKey());
assertEquals(2000, salary.lastEntry().getValue().intValue());
}
我们看到salary中记录按照字母进行排序。
本节我们讨论如何使用BiMap,BiMap也可以反向把值映射到键,只要确保值唯一。
下面示例中,我们创建BiMap,并使用其inverse()方法:
@Test
public void whenCreateBiMap_thenCreated() {
BiMap words = HashBiMap.create();
words.put("First", 1);
words.put("Second", 2);
words.put("Third", 3);
assertEquals(2, words.get("Second").intValue());
assertEquals("Third", words.inverse().get(3));
}
BiMap是双向映射,但要确保值和键都唯一。
本节我们使用Multimap ,对每个键关联多个值:
@Test
public void whenCreateMultimap_thenCreated() {
Multimap multimap = ArrayListMultimap.create();
multimap.put("fruit", "apple");
multimap.put("fruit", "banana");
multimap.put("pet", "cat");
multimap.put("pet", "dog");
assertThat(multimap.get("fruit"), containsInAnyOrder("apple", "banana"));
assertThat(multimap.get("pet"), containsInAnyOrder("cat", "dog"));
}
Multimap的get方法返回 java.util.Collection,上述测试代码除了junit,还需要增加:
testCompile group: 'org.hamcrest', name: 'hamcrest-all', version: '1.3'
当需要多余一个键索引值时,需要Table。下面示例中,我们使用Table存储城市之间距离:
@Test
public void whenCreatingTable_thenCorrect() {
Table distance = HashBasedTable.create();
distance.put("London", "Paris", 340);
distance.put("New York", "Los Angeles", 3940);
distance.put("London", "New York", 5576);
assertEquals(3940, distance.get("New York", "Los Angeles").intValue());
assertThat(distance.columnKeySet(),
containsInAnyOrder("Paris", "New York", "Los Angeles"));
assertThat(distance.rowKeySet(), containsInAnyOrder("London", "New York"));
}
Table
@Test
public void whenTransposingTable_thenCorrect() {
Table<String,String,Integer> distance = HashBasedTable.create();
distance.put("London", "Paris", 340);
distance.put("New York", "Los Angeles", 3940);
distance.put("London", "New York", 5576);
Table<String, String, Integer> transposed = Tables.transpose(distance);
assertThat(transposed.rowKeySet(),
containsInAnyOrder("Paris", "New York", "Los Angeles"));
assertThat(transposed.columnKeySet(), containsInAnyOrder("London", "New York"));
}
本节介绍ClassToInstanceMap,把类作为键映射至对象:
@Test
public void whenCreatingClassToInstanceMap_thenCorrect() {
ClassToInstanceMap numbers = MutableClassToInstanceMap.create();
numbers.putInstance(Integer.class, 1);
numbers.putInstance(Double.class, 1.5);
assertEquals(1, numbers.get(Integer.class));
assertEquals(1.5, numbers.get(Double.class));
}
Integer,Double都继承子Number,ClassToInstanceMap让不同的子类作为key。
通过Multimap对List进行分组,下面示例中是使用Multimaps.index方法依据list元素的长度进行分组。
@Test
public void whenGroupingListsUsingMultimap_thenGrouped() {
List<String> names = Lists.newArrayList("John", "Adam", "Tom");
Function<String,Integer> func = new Function<String,Integer>(){
public Integer apply(String input) {
return input.length();
}
};
Multimap<Integer, String> groups = Multimaps.index(names, func);
assertThat(groups.get(3), containsInAnyOrder("Tom"));
assertThat(groups.get(4), containsInAnyOrder("John", "Adam"));
}
本文介绍Guava库中使用Map的有用示例,从简单的新建HashMap到复杂Map相关应用,有些功能是jdk没有提供,实际中却非常有用的。