Guava简化集合操作
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简化工作
可以简化集合的创建和初始化;
类别原来的写法guava的写法集合创建Map<String, Map<String, String>> map = new HashMap<String, Map<String,String>>();
List<List<Map<String, String>>> list = new ArrayList<List<Map<String,String>>>();
Map<String, Map<String, String>> map = Maps.newHashMap();
List<List<Map<String, String>>> list = Lists.newArrayList();
//1,简化集合的创建
List<Person> personList= Lists.newLinkedList();
Set<Person> personSet= Sets.newHashSet();
Map<String,Person> personMap= Maps.newHashMap();
Integer[] intArrays= ObjectArrays.newArray(Integer.class,10);
Set<String> set = new HashSet<String>();
set.add("one");
set.add("two");
set.add("three");
Set<String> set = Sets.newHashSet("one","two","three");
List<String> list = Lists.newArrayList("one","two","three");
Map<String, String> map = ImmutableMap.of("ON","TRUE","OFF","FALSE");
//2,简化集合的初始化
List<Person> personList2= Lists.newArrayList(new Person(1, 1, "a", "46546", 1, 20),
new Person(2, 1, "a", "46546", 1, 20));
Set<Person> personSet2= Sets.newHashSet(new Person(1,1,"a","46546",1,20),
new Person(2,1,"a","46546",1,20));
Map<String,Person> personMap2= ImmutableMap.of("hello",new Person(1,1,"a","46546",1,20),"fuck",new Person(2,1,"a","46546",1,20));
3.2 不变性
很大一部分是google集合提供了不变性,不变对比可变:
- 数据不可改变
- 线程安全
- 不需要同步逻辑
- 可以被自由的共享
- 容易设计和实现
- 内存和时间高效
不变对比不可修改
google的不变被确保真正不可改变,而不可修改实际上还是可以修改数据;如下所示:
Set<Integer> data = new HashSet<Integer>();
data.addAll(Arrays.asList(10, 20, 30, 40, 50, 60, 70, 80));
Set<Integer> fixedData = Collections.unmodifiableSet(data); // fixedData - [50, 70, 80, 20, 40, 10, 60, 30]
data.add(90); // fixedData - [50, 70, 80, 20, 40, 10, 90, 60, 30]
如何创建不可变的集合:
ImmutableSet<Integer> numbers = ImmutableSet.of(10, 20, 30, 40, 50);
使用copyOf方法
ImmutableSet<Integer> another = mmutableSet.copyOf(numbers);
使用Builder方法
ImmutableSet<Integer> numbers2 = ImmutableSet.<Integer>builder().addAll(numbers) .add(60) .add(70).add(80).build();
//3,创建不可变的集合
ImmutableList<Person> personImmutableList=
ImmutableList.of(new Person(1, 1, "a", "46546", 1, 20), new Person(2, 1, "a", "46546", 1, 20));
ImmutableSet<Person> personImmutableSet=ImmutableSet.copyOf(personSet2);
ImmutableMap<String,Person> personImmutableMap=ImmutableMap.<String,Person>builder()
.put("hell",new Person(1,1,"a","46546",1,20)).putAll(personMap2) .build();
3.3 新的集合类型
The Guava API provides very useful new collection types that work very nicely with existing java collections.
guava API 提供了有用的新的集合类型,协同已经存在的java集合工作的很好。
分别是 MultiMap, MultiSet, Table, BiMap, ClassToInstanceMap
种类写的例子MultiMap一种key可以重复的map,子类有ListMultimap和SetMultimap,对应的通过key分别得到list和set
Multimap<String, Person> customersByType =ArrayListMultimap.create();customersByType.put("abc", new Person(1, 1, "a", "46546", 1, 20));
customersByType.put("abc", new Person(1, 1, "a", "46546", 1, 30));
customersByType.put("abc", new Person(1, 1, "a", "46546", 1, 40));
customersByType.put("abc", new Person(1, 1, "a", "46546", 1, 50));
customersByType.put("abcd", new Person(1, 1, "a", "46546", 1, 50));
customersByType.put("abcde", new Person(1, 1, "a", "46546", 1, 50));
for(Person person:customersByType.get("abc"))
{
System.out.println(person.getAge());
}
不是集合,可以增加重复的元素,并且可以统计出重复元素的个数,例子如下:
private static void testMulitiSet() {
Multiset<Integer> multiSet = HashMultiset.create();
multiSet.add(10);
multiSet.add(30);
multiSet.add(30);
multiSet.add(40);
System.out.println( multiSet.count(30)); // 2
System.out.println( multiSet.size()); //4
}
相当于有两个key的map,不多解释
private static void testTable() {
Table<Integer,Integer,Person> personTable=HashBasedTable.create();
personTable.put(1,20,new Person(1, 1, "a", "46546", 1, 20));
personTable.put(0,30,new Person(2, 1, "ab", "46546", 0, 30));
personTable.put(0,25,new Person(3, 1, "abc", "46546", 0, 25));
personTable.put(1,50,new Person(4, 1, "aef", "46546", 1, 50));
personTable.put(0,27,new Person(5, 1, "ade", "46546",0, 27));
personTable.put(1,29,new Person(6, 1, "acc", "46546", 1, 29));
personTable.put(0,33,new Person(7, 1, "add", "46546",0, 33));
personTable.put(1,66,new Person(8, 1, "afadsf", "46546", 1, 66));
//1,得到行集合
Map<Integer,Person> rowMap= personTable.row(0);
int maxAge= Collections.max(rowMap.keySet());
}
BiMap是一个一一映射,可以通过key得到value,也可以通过value得到key;private static void testBitMap() {
//双向map
BiMap<Integer,String> biMap=HashBiMap.create();
biMap.put(1,"hello");
biMap.put(2,"helloa");
biMap.put(3,"world");
biMap.put(4,"worldb");
biMap.put(5,"my");
biMap.put(6,"myc");
int value= biMap.inverse().get("my");
System.out.println("my --"+value);
}
ClassToInstanceMapClassToInstanceMap<Number> numberDefaults = MutableClassToInstanceMap.create();
numberDefaults.putInstance(Integer.class, Integer.valueOf(0));
private static void testClass() {
ClassToInstanceMap<Person> classToInstanceMap =MutableClassToInstanceMap.create();
Person person= new Person(1,20,"abc","46464",1,100);
classToInstanceMap.putInstance(Person.class,person);
// System.out.println("string:"+classToInstanceMap.getInstance(String.class));
// System.out.println("integer:" + classToInstanceMap.getInstance(Integer.class));
Person person1=classToInstanceMap.getInstance(Person.class);
}
3.4 谓词和筛选
谓词(Predicate)是用来筛选集合的;
谓词是一个简单的接口,只有一个方法返回布尔值,但是他是一个很令人惊讶的集合方法,当你结合collections2.filter方法使用,这个筛选方法返回原来的集合中满足这个谓词接口的元素;
举个例子来说:筛选出集合中的女人
public static void main(String[] args)
{
Optional<ImmutableMultiset<Person>> optional=Optional.fromNullable(testPredict());
if(optional.isPresent())
{
for(Person p:optional.get())
{
System.out.println("女人:"+p);
}
}
System.out.println(optional.isPresent());
}
public static ImmutableMultiset<Person> testPredict()
{
List<Person> personList=Lists.newArrayList(new Person(1, 1, "a", "46546", 1, 20),
new Person(2, 1, "ab", "46546", 0, 30),
new Person(3, 1, "abc", "46546", 0, 25),
new Person(4, 1, "aef", "46546", 1, 50),
new Person(5, 1, "ade", "46546",0, 27),
new Person(6, 1, "acc", "46546", 1, 29),
new Person(7, 1, "add", "46546",0, 33));
return ImmutableMultiset.copyOf(Collections2.filter(personList,new Predicate<Person>() {
@Override
public boolean apply( Person input) {
return input.getSex()==0;
}
}));
}
Predicates含有一些内置的筛选方法,比如说 in ,and ,not等,根据实际情况选择使用。
3.5 功能和转换
转换一个集合为另外一个集合;
实例如下:
public static void main(String[] args)
{
Optional<ImmutableMultiset<String>> optional=Optional.fromNullable(testTransform());
if(optional.isPresent())
{
for(String p:optional.get())
{
System.out.println("名字:"+p);
}
}
System.out.println(optional.isPresent());
}
public static ImmutableMultiset<String> testTransform()
{
List<Person> personList=Lists.newArrayList(new Person(1, 1, "a", "46546", 1, 20),
new Person(2, 1, "ab", "46546", 0, 30),
new Person(3, 1, "abc", "46546", 0, 25),
new Person(4, 1, "aef", "46546", 1, 50),
new Person(5, 1, "ade", "46546",0, 27),
new Person(6, 1, "acc", "46546", 1, 29),
new Person(7, 1, "add", "46546",0, 33));
return ImmutableMultiset.copyOf(Lists.transform(personList,new Function<Person, String>() {
@Override
public String apply( Person input) {
return input.getName();
}
}));
}
3.6 排序
是guava一份非常灵活的比较类,可以被用来操作,扩展,当作比较器,排序提供了集合排序的很多控制;
实例如下:
Lists.newArrayList(30, 20, 60, 80, 10);
Ordering.natural().sortedCopy(numbers); //10,20,30,60,80
Ordering.natural().reverse().sortedCopy(numbers); //80,60,30,20,10
Ordering.natural().min(numbers); //10
Ordering.natural().max(numbers); //80
Lists.newArrayList(30, 20, 60, 80, null, 10);
Ordering.natural().nullsLast().sortedCopy(numbers); //10, 20,30,60,80,null
Ordering.natural().nullsFirst().sortedCopy(numbers); //null,10,20,30,60,80
public static void testOrdering()
{
List<Person> personList=Lists.newArrayList(
new Person(3, 1, "abc", "46546", 0, 25),
new Person(2, 1, "ab", "46546", 0, 30),
new Person(5, 1, "ade", "46546",0, 27),
new Person(1, 1, "a", "46546", 1, 20),
new Person(6, 1, "acc", "46546", 1, 29),
new Person(4, 1, "aef", "46546", 1, 50),
new Person(7, 1, "add", "46546",0, 33)
);
Ordering<Person> byAge=new Ordering<Person>() {
@Override
public int compare( Person left, Person right) {
return right.getAge()-left.getAge();
}
};
for(Person p: byAge.immutableSortedCopy(personList))
{
System.out.println(p);
}
}
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