How to Check If My String Is Equal to Null

Check whether a String is not Null and not Empty

What about isEmpty() ?

if(str != null && !str.isEmpty())

Be sure to use the parts of && in this order, because java will not proceed to evaluate the second part if the first part of && fails, thus ensuring you will not get a null pointer exception from str.isEmpty() if str is null.

Beware, it's only available since Java SE 1.6. You have to check str.length() == 0 on previous versions.

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To ignore whitespace as well:

if(str != null && !str.trim().isEmpty())

(since Java 11 str.trim().isEmpty() can be reduced to str.isBlank() which will also test for other Unicode white spaces)

Wrapped in a handy function:

public static boolean empty( final String s ) {
  // Null-safe, short-circuit evaluation.
  return s == null || s.trim().isEmpty();
}

Becomes:

if( !empty( str ) )

How to check if String is null

An object can't be null - the value of an expression can be null. It's worth making the difference clear in your mind. The value of s isn't an object - it's a reference, which is either null or refers to an object.

And yes, you should just use

if (s == null)

Note that this will still use the overloaded == operator defined in string, but that will do the right thing.

How do I check if two strings are equal in Java when either one of the strings can be a null?

You do not need to create a new method for that. You can use java.util.Objects#equals(Object a, Object b) from the Java standard library. Below is the definition as on JDK.

public static boolean equals(Object a, Object b) {
    return (a == b) || (a != null && a.equals(b));
}

Java string null check by != null or !str.equals(null)?

1st one is better (and the only option), because 2nd one will throw NPE, when your value is actually null. As simple as that.

Try this out:

String str = null;
str.equals(null);  // will throw `NPE`.

So basically, the test which you wanted to perform itself triggers a NullPointerException in the 2nd case. So, it is no choice.

Do I need to check null and while comparing String in Java?

There is no way that "Hello".equals(str)) would ever fail, a string literal simply can never be null. Same thing with "".equals(str).

The method Objects.equals(Object, Object) automatically checks nulls first.

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To answer your question, using if ("Hello".equals(str)) is just fine, but if you ever want to replace the string literal "Hello" by a variable, you should use Objects.equals(greeting, str).

Checking for an empty string is absolutely unnecessary.

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