檢查金鑰是否存在
Map<String, String> num = new HashMap<>();
num.put("one", "first");
if (num.containsKey("one")) {
System.out.println(num.get("one")); // => first
}
地圖可以包含空值
對於地圖,人們必須不要混淆包含鍵和有價值。例如,HashMap
s 可以包含 null,這意味著以下是完全正常的行為:
Map<String, String> map = new HashMap<>();
map.put("one", null);
if (map.containsKey("one")) {
System.out.println("This prints !"); // This line is reached
}
if (map.get("one") != null) {
System.out.println("This is never reached !"); // This line is never reached
}
更正式的,不能保證 map.contains(key) <=>
map.get(key)!=null