本文涵盖的JDK8中引进的Map方法有：getOrDefault(Object,V)，putIfAbsent(K,V),remove(Object,Object)，replace(K,V)，和 replace(K,V,V)。
import java.util.HashMap;
import java.util.Map;
public class Jdk8Map {
private final static Map<String, String> statesAndCapitals;
static {
statesAndCapitals = new HashMap<>();
statesAndCapitals.put("Alaska", "Anchorage");
statesAndCapitals.put("California", "Sacramento");
statesAndCapitals.put("Colorado", "Denver");
statesAndCapitals.put("Florida", "Tallahassee");
statesAndCapitals.put("Nevada", "Las Vegas");
statesAndCapitals.put("New Mexico", "Sante Fe");
statesAndCapitals.put("Utah", "Salt Lake City");
statesAndCapitals.put("Wyoming", "Cheyenne");
}
public static void main(String[] args) {
/**
* getOrDefault(Object,V)
* 允许调用者在代码语句中规定获得在map中符合提供的键的值，否则在没有找到提供的键的匹配项的时候返回一个“默认值”。
* 它不仅仅在“键”没找到的时候返回提供的默认值，而且还会将键和默认值增加到调用的map中。
*/
String capitalNevada = statesAndCapitals.getOrDefault(
"Nevada", "Unknown");
System.out.println(capitalNevada);
String capitalWisconsin = statesAndCapitals.getOrDefault(
"Wisconsin", "Unknown");
System.out.println(capitalWisconsin);
/**
* putIfAbsent(v,v)
* 当map中不存在指定的键时，把<key,value>添加到map中
*/
String capitalWyoming =
statesAndCapitals.putIfAbsent("Wyoming", "Albany");
String capitalNewYork =
statesAndCapitals.putIfAbsent("New York", "Albany");
System.out.println(statesAndCapitals);
/**
* remove(Object,Object)超越了长期有效的Map.remove(Object)
* 方法，只有在提供的键和值都匹配的时候才会删除该map项
*/
boolean removedCalifornia =
statesAndCapitals.remove("California", "Sacramento");
boolean removedUtah = statesAndCapitals.remove("Utah", "Sacramento");
System.out.println(removedCalifornia);
System.out.println(removedUtah);
/**
* replace(K,V)
* 有在指定的键已经存在并且有与之相关的映射值时才会将指定的键映射到指定的值（新值）
*/
String replacedAlaska = statesAndCapitals.replace("Alaska",
"Anchorage");
System.out.println(replacedAlaska);
/**
* replace(K,V,V)
* 当那个方法（上一个replace方法）只是涵盖指定的键在映射中有任意一个有效的值的替换处理，而这个“replace”方法
* 接受一个额外的（第三个）参数，只有在指定的键和值都匹配的情况下才会替换。
*/
Boolean replacedNevada = statesAndCapitals.replace("Nevada", "Juneau",
"USA");
System.out.println(replacedNevada);
replacedNevada = statesAndCapitals.replace("Nevada", "Las Vegas",
"USA");
System.out.println(replacedNevada);
}
}
运行结果：
Las Vegas
Unknown
{New York=Albany, New Mexico=Sante Fe, Wyoming=Cheyenne, Colorado=Denver, Alaska=Anchorage, California=Sacramento, Florida=Tallahassee, Utah=Salt Lake City, Nevada=Las Vegas}
true
false
Anchorage
false
true
