Hashmap get() 方法返回 null 但与 null 比较时打印错误

Question

There is a hashmap

HashMap<String,ArrayList<String>> hm = new HashMap<>();

It has these values

entertain - have, 

some - one, 

very - extremely, actually, really, super, 

auto - car, 

lunch - meal, 

wagon - car, 

truck - car, 

Problem statement

System.out.println("test " + hm.get("delicious")); //prints null


System.out.println("test " + hm.get("delicious") == null); //prints false

why is the second print statement printing false, should it not print true?

Answer 1

The expression passed to the println method is evaluated from left to right. First the String "test " is concatenated to hm.get("delicious") , and only then the output is compared to null .

Therefore:

System.out.println("test " + hm.get("delicious") == null);

is equivalent to:

System.out.println(("test " + hm.get("delicious")) == null);

"test " +hm.get("delicious") is definitely not null (it's actually equal to the String "test null"), so false is printed.

System.out.println("test " + (hm.get("delicious")==null));

will print "test true".

Answer 2

Before we try to find a solution to this question, we should understand that what exactly is null in memory? Or What is the null value in Java?

First of all, null is not a valid object instance, so there is no memory allocated for it. It is simply a value that indicates that the object reference is not currently referring to an object.

Let's come to the actual question. the following code is internally used by java to compare.

public static boolean compare(String str1, String str2) {
    return (str1 == null ? str2 == null : str1.equals(str2));
}

I hope, now you can understand why your program says false .