用Java减去2个字符串等于null

Question

In C, if I subtract a string from the same string , it will result in NULL . So, if str1 = str2 , str1 = str2 - str1 = NULL . How do I achieve the same null operation/result in Java?

Thank you.

Answer 1

You can't subtract strings in C or Java.

In C, you can subtract two addresses of strings. If they're the same address, you'll get zero. If they're not the same address, you'll get a meaningful result only if they're in the same block of allocated memory.

You can get a nonzero result from the subtraction of the addresses of two equal strings stored in different memory locations. You can test C string equality with strcmp or strncmp() . (The latter should be preferred.)

In Java, if you want to determine if two strings are the same object , you can use the equality operator. For example, str1 == str2 will return true if the two are the same object.

If you want to determine if the two strings are equal , you can use the equals() method. For example, str1.equals( str2 ) will return true if the two strings are equal. If either might be null, you can use instead Objects.equals( str1, str2 ) .

Answer 2

There is not a way of doing that in java, because you cannot use "-" operator on objects. Only if your objective is put null when both objects are equals, you can achieve the null result as follows:

    String str1 = "hola";
    String str2 = "hola";
    String str3 = "";

    if(str1.equals(str2)){
        str3 = null;
    }

Answer 3

Actually, no. Subtracting same pointers should not necessarily result in NULL . What it will surely be is zero, of type ptrdiff_t . NULL being zero is an assumption not guaranteed to be valid.

But this doesn't work in Java at all since it doesn't have pointers (not as primitive types) and, subsequently, pointer arithmetic; strings are known by their references. You can't do too much with Java's references apart from dereferencing them (and a reference is guaranteed to succeed unless it's null ), assigning and comparing them for equality. That's all you need.

Answer 4

In C your string variables are pointers, which are numeric, and you can perform subtraction on them.

In Java your string variables are references to String objects and attempting subtraction is a type error. Specifically - is limited to use on specific primitive numeric types: byte , short , int , long , float , and double .

To determine if two String variables reference the same object in memory you would use str1 == str2 (double-equals) which determines reference equality. To determine if two String variables have the same value (but may be distinct references) you would use str1.equals(str2) assuming str1 != null .