这是什么意思？ 地图何时为空？

Question

If I do

System.out.println(nameOfaHashMap);

when that map is "empty", the system says " {} ".

In this case, is that map empty or null?

How can I write if I would:

if (nameOfanHashMap is != null)
{
    System.out.println(nameOfaHashMap);  
}
else System.out.println("I'm sorry, this map is empty!");

---

Thanks so much to everyone and sorry for my English e for my Java :-) You reply me so fast, also thanks really to everyone.

I wrote this code, that is ok:

if (disponibilita.isEmpty())
{
   System.out.println("Sorry, ..");
}
else 
   System.out.println(disponibilita);

Answer 1

If it's null , then it isn't even a map; if it's a map, only then it gets the chance of being empty.

An empty map's toString will commonly return {} , whereas String.valueOf(null) returns the string "null" , and this is what is printed.

If you want to print "sorry, this map is empty" when you have a map, but it is empty, then you need

if (map.isEmpty()) System.out.println("sorry, this map is empty");

Answer 2

As said in the docs :

The string representation consists of a list of key-value mappings in the order returned by the map's entrySet view's iterator, enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value.

That means the map is empty, you can test by calling isEmpty() . If it were null, it would simply print out null .

Answer 3

You mean, like this?

if (nameOfanHashMap == null) {
    System.out.println("I'm sorry, this map is null!");
} else if (nameOfanHashMap.isEmpty()) {
    System.out.println("I'm sorry, this map is empty!");
} else {
    System.out.println(nameOfanHashMap);
} 

Notice that a Map that gets printed as {} is empty , but non- null . A null map will get printed as null .

Answer 4

If a map is null it is printed as null just like any other object.

If it is any but null it must be something, and { } means it is empty.

Answer 5

When you pass an Object to System.out.println, its method toString is called.

Now, if the map has been instantiated but it is empty, the

toString()

method returns

{}

otherwise if it is null, the toString returns

null

Finally, the check can be done with:

if (nameOfanHashMap == null) {
    System.out.println("I'm sorry, this map is null!");
} else if (nameOfanHashMap.isEmpty()) {
    System.out.println("I'm sorry, this map is empty!");
} else {
    System.out.println(nameOfanHashMap);
}