Why am I able to concatenate a String to a null String but not a Character to a null Character?

Question

I am having a confusion here. I have String which I have assigned null to it. So when I concat a String anotherString="abc", it prints "nullabc". But if I try to do the same with a Character, as shown in code, I get a null pointer. I want to understand as to why its happening. I am new to Java and want to understand the nuances of the language.Any help appreciated. Thanks!

String nullString=null;
Character nullCharacter=null;
Character character=new Character('k');
Integer nullInteger=null;

System.out.println(nullString+"abcdefgh");//prints-"nullabcdefgh"
System.out.println(nullCharacter+character);// throws null pointer exception

GeertPt · Accepted Answer · 2021-11-24 12:24:40Z

2

The Character class is a wrapper class for the char primitive type, which is a 16-bit number.

If you add two primitive char variables together, it justs adds the numeric values.

    char a = 'a';
    char b = 'b';
    char c = (char)(a+b); // c == 'Ã'

The nullCharacter+character syntax using the Character wrapper object is only possible thanks to auto-unboxing (introduced in Java 1.5). It will implicitly convert it to a numeric addition:

    nullCharacter.charValue() + character.charValue() // throws NPE

On the other hand, adding two strings together nullString+"abcdefgh" is the String concatenation operation which was introduced at the very start of Java, and that operation handles null values differently. In theory it's compiler specific, but most compilers implement it as this:

new StringBuilder()
    .append(nullString)  // implicitly converts null to "null"
    .append("abcdefgh")
    .toString()

edited Nov 24, 2021 at 12:24

answered Nov 24, 2021 at 9:57

GeertPt

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2 Comments

Andy Turner Over a year ago

"char c = a+b;" you need a cast in there. Also, it's charValue(), not toCharValue().

abc123 Over a year ago

understood , didnt know there was auto-unboxing going on in there!

Abra · Accepted Answer · 2021-11-24 10:00:00Z

1

Java uses autoboxing (actually, in this case, unboxing) to convert the Character object to a char primitive and it fails since nullCharacter is null.

Autoboxing only affects classes that wrap primitives and so it does not apply to String.

If you convert the not-null Character (named character in your code) to a String, you will not get NullPointerException.

System.out.println(character.toString() + nullCharacter);

edited Nov 24, 2021 at 10:00

answered Nov 24, 2021 at 9:54

Abra

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