While creating an array, we can pass short, char, byte, int. So, [why] is int[] a = new int['a'] valid? It doesn't throw a compile time error.
What does an array declaration in this form mean?
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user2864740
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kwishna
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`'a'` is a `char` literal. In Java, `char` is a numeric type, so expressions of that type can be used where a number is required, including to specify the size of an array. – John Bollinger Sep 14 '18 at 20:34
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1@Oleksandr I think this question is different enough from that. – Sweeper Sep 14 '18 at 20:36
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@Sweeper And it's still effectively the same answer: https://stackoverflow.com/a/8549716/2864740 D: (Although the answers given here are more precise and better formatted.) – user2864740 Sep 14 '18 at 20:42
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When I Tried To Find The Length Of This Array. I Got 97. I Means int[] a = new int['a']; Is Actually Creating An Array With Length 97. 97 Is ASCII Value Of 'a'. – kwishna Sep 14 '18 at 20:43
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1@ABD Please consider accepting the answer that has been the most helpful to you by clicking on that checkmark. – Sweeper Sep 14 '18 at 21:50
5 Answers
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From JLS Sec 15.10.1:
The type of each dimension expression within a DimExpr must be a type that is convertible (§5.1.8) to an integral type, or a compile-time error occurs.
Each dimension expression undergoes unary numeric promotion (§5.6.1). The promoted type must be int, or a compile-time error occurs.
And from JLS Sec 5.6.1:
If the operand is of compile-time type Byte, Short, Character, or Integer, it is subjected to unboxing conversion (§5.1.8). The result is then promoted to a value of type int by a widening primitive conversion (§5.1.2) or an identity conversion (§5.1.1).
and
Otherwise, if the operand is of compile-time type byte, short, or char, it is promoted to a value of type int by a widening primitive conversion (§5.1.2).
So, any of Byte, Short, Character, Integer, byte, short, char or int are acceptable.
Hence 'a', a char literal, is allowed, and is promoted to the int value 97.
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Andy Turner
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Like most times with this kind of questions, the answer lies in the Java Language Specification:
§ 5.1.2. Widening Primitive Conversion
19 specific conversions on primitive types are called the widening primitive conversions:
bytetoshort,int,long,float, ordoubleshorttoint,long,float, ordoublechartoint,long,float, ordoubleinttolong,float, ordoublelongtofloatordoublefloattodouble
(Emphasis mine)
It may or may not be intuitive, but a char is actually an integral type, and the Java rules specify that primitive integral types may be converted to an integral type with a lower or higher-capacity. In this case, it's called a widening primitive conversion.
Andy's answer provides more references to the Java Language Specification, and explicitly states that an expression as in your question is valid.
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MC Emperor
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According to the JLS §15.10.1 Array Creation Expressions,
The type of each dimension expression within a DimExpr must be a type that is convertible (§5.1.8) to an integral type, or a compile-time error occurs.
Each dimension expression undergoes unary numeric promotion (§5.6.1). The promoted type must be int, or a compile-time error occurs.
Here, unary numeric promotion occurs. Here is an excerpt from §5.6.1:
...if the operand is of compile-time type byte, short, or char, it is promoted to a value of type int by a widening primitive conversion (§5.1.2).
Therefore, new int['a'] is valid.
If you are wondering how exactly is a char converted to an int, here is the relevant bits from §5.1.2:
A widening conversion of a char to an integral type T zero-extends the representation of the char value to fill the wider format.
Also note that characters are all encoded as integers.
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In java char is integer type (hello C++). This code do same int[] a = new int[97];
Valerij Zakharenko
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