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This is a follow-up question to some previous questions about String initialization in Java.
After some small tests in Java, I'm facing the following question:
Why can I execute this statement
String concatenated = str2 + " a_literal_string";
when str2 a String object initialized to null (String str2 = null;) but I cannot call the method toString() on str2? Then how does Java do the concatenation of a null String object and a string literal?
By the way, I tried also to concatenate an Integer initialized to null and the string literal "a_literal_string" and I've got the same thing that is "null a_literal_string" in the console. So whichever kind of null gives the same thing?
PS : System.out.println(concatenated); gives null a_literal_string as output in the console.
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To concatenate null to a string, use the + operator. Let's say the following is our string. String str = "Demo Text"; We will now see how to effortlessly concatenate null to string.
String literalsThe code concatenates the smaller strings to create the long string literal. The parts are concatenated into a single string at compile time.
When SET CONCAT_NULL_YIELDS_NULL is ON, concatenating a null value with a string yields a NULL result. For example, SELECT 'abc' + NULL yields NULL .
There are two ways to concatenate strings in Java: By + (String concatenation) operator. By concat() method.
This line:
String concatenated = str2 + " a_literal_string";
is compiled into something like
String concatenated = new StringBuilder().append(str2)
.append(" a_literal_string")
.toString();
This gives "null a_literal_string" (and not NullPointerException) because StringBuilder.append is implemented using String.valueOf, and String.valueOf(null) returns the string "null".
I tried also to concatenate an Integer initialized to null and the string literal "a_literal_string" and I've got the same thing
This is for the same reason as above. String.valueOf(anyObject) where anyObject is null will give back "null".
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