Java null String equals result

Question

Please help me how does the string.equals in java work with null value? Is there some problem with exceptions? Three cases:

boolean result1,result2, result3;         


        

Answer 1

Indeed, you cannot use the dot operator on a null variable to call a non static method.

Despite this, all depends on overriding the equals() method of the Object class. In the case of the String class, is:

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = count;
        if (n == anotherString.count) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = offset;
            int j = anotherString.offset;
            while (n-- != 0) {
                if (v1[i++] != v2[j++])
                    return false;
            }
            return true;
        }
    }
    return false;
}

If you pass null as parameter, both "if" will fail, returning false;

An alternative for your case is to build a method for your requirements:

public static boolean myEquals(String s1, String s2){
    if(s1 == null)
        return s2 == null;
    return s1.equals(s2);
}

Answer 2

You will get a NullPointerException in case 1 and case 3.

You cannot call any methods (like equals()) on a null object.

Answer 3

Use Objects.equals() to compare strings, or any other objects if you're using JDK 7 or later. It will handle nulls without throwing exceptions. See more here: how-do-i-compare-strings-in-java

Answer 4

Our most common use-case of this type of thing is when we have a database field that contains "Y" or "N" to represent a Boolean (it's an old system, don't ask).

Thus, we do this:

if ("Y".equals(stringObjectThatMayBeNull) ? result : otherResult);

Instead of this:

if (stringObjectThatMayBeNull.equals("Y") ? result : otherResult);

... which avoids a NullPointerException when executing the .equals method.

Answer 5

That piece of code will throw a NullPointerException whenever string1 is null and you invoke equals on it, as is the case when a method is implicitly invoked on any null object.

To check if a string is null, use == rather than equals.

Although result1 and result3 will not be set due to NullPointerExceptions, result2 would be false (if you ran it outside the context of the other results).

Answer 6

We cannot use dot operator with null since doing so will give NullPointerException. Therefore we can take advantage of try..catch block in our program. This is a very crude way of solving your problem, but you will get desired output.

try {
   result = string1.equals(string2);
} catch (NullPointerException ex) {
   result = string2 == null; //This code will be executed only when string 1 is null
}