173 lines
7.2 KiB
Plaintext
173 lines
7.2 KiB
Plaintext
[[painless-casting]]
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=== Casting
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Casting is the conversion of one type to another. Implicit casts are casts that
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occur automatically, such as during an assignment operation. Explicit casts are
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casts where you use the casting operator to explicitly convert one type to
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another. This is necessary during operations where the cast cannot be inferred.
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To cast to a new type, precede the expression by the new type enclosed in
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parentheses, for example
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`(int)x`.
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The following sections specify the implicit casts that can be performed and the
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explicit casts that are allowed. The only other permitted cast is casting
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a single character `String` to a `char`.
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*Grammar:*
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[source,ANTLR4]
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----
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cast: '(' TYPE ')' expression
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----
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[[numeric-casting]]
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==== Numeric Casting
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The following table shows the allowed implicit and explicit casts between
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numeric types. Read the table by row. To find out if you need to explicitly
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cast from type A to type B, find the row for type A and scan across to the
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column for type B.
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IMPORTANT: Explicit casts between numeric types can result in some data loss. A
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smaller numeric type cannot necessarily accommodate the value from a larger
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numeric type. You might also lose precision when casting from integer types
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to floating point types.
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|====
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| | byte | short | char | int | long | float | double
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| byte | | implicit | implicit | implicit | implicit | implicit | implicit
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| short | explicit | | explicit | implicit | implicit | implicit | implicit
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| char | explicit | explicit | | implicit | implicit | implicit | implicit
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| int | explicit | explicit | explicit | | implicit | implicit | implicit
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| long | explicit | explicit | explicit | explicit | | implicit | implicit
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| float | explicit | explicit | explicit | explicit | explicit | | implicit
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| double | explicit | explicit | explicit | explicit | explicit | explicit |
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|====
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Example(s)
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[source,Java]
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----
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int a = 1; // Declare int variable a and set it to the literal
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// value 1
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long b = a; // Declare long variable b and set it to int variable
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// a with an implicit cast to convert from int to long
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short c = (short)b; // Declare short variable c, explicitly cast b to a
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// short, and assign b to c
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byte d = a; // ERROR: Casting an int to a byte requires an explicit
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// cast
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double e = (double)a; // Explicitly cast int variable a to a double and assign
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// it to the double variable e. The explicit cast is
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// allowed, but it is not necessary.
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----
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[[reference-casting]]
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==== Reference Casting
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A reference type can be implicitly cast to another reference type as long as
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the type being cast _from_ is a descendant of the type being cast _to_. A
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reference type can be explicitly cast _to_ if the type being cast to is a
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descendant of the type being cast _from_.
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*Examples:*
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[source,Java]
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----
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List x; // Declare List variable x
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ArrayList y = new ArrayList(); // Declare ArrayList variable y and assign it a
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// newly allocated ArrayList [1]
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x = y; // Assign Arraylist y to List x using an
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// implicit cast
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y = (ArrayList)x; // Explicitly cast List x to an ArrayList and
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// assign it to ArrayList y
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x = (List)y; // Set List x to ArrayList y using an explicit
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// cast (the explicit cast is not necessary)
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y = x; // ERROR: List x cannot be implicitly cast to
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// an ArrayList, an explicit cast is required
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Map m = y; // ERROR: Cannot implicitly or explicitly cast [2]
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// an ArrayList to a Map, no relationship
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// exists between the two types.
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----
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[1] `ArrayList` is a descendant of the `List` type.
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[2] `Map` is unrelated to the `List` and `ArrayList` types.
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[[def-type-casting]]
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==== def Type Casting
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All primitive and reference types can always be implicitly cast to
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`def`. While it is possible to explicitly cast to `def`, it is not necessary.
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However, it is not always possible to implicitly cast a `def` to other
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primitive and reference types. An explicit cast is required if an explicit
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cast would normally be required between the non-def types.
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*Examples:*
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[source,Java]
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----
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def x; // Declare def variable x and set it to null
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x = 3; // Set the def variable x to the literal 3 with an implicit
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// cast from int to def
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double a = x; // Declare double variable a and set it to def variable x,
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// which contains a double
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int b = x; // ERROR: Results in a run-time error because an explicit cast is
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// required to cast from a double to an int
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int c = (int)x; // Declare int variable c, explicitly cast def variable x to an
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// int, and assign x to c
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----
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[[boxing-unboxing]]
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==== Boxing and Unboxing
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Boxing is where a cast is used to convert a primitive type to its corresponding
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reference type. Unboxing is the reverse, converting a reference type to the
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corresponding primitive type.
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There are two places Painless performs implicit boxing and unboxing:
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* When you call methods, Painless automatically boxes and unboxes arguments
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so you can specify either primitive types or their corresponding reference
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types.
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* When you use the `def` type, Painless automatically boxes and unboxes as
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needed when converting to and from `def`.
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The casting operator does not support any way to explicitly box a primitive
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type or unbox a reference type.
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If a primitive type needs to be converted to a reference type, the Painless
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reference type API supports methods that can do that. However, under normal
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circumstances this should not be necessary.
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*Examples:*
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[source,Java]
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----
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Integer x = 1; // ERROR: not a legal implicit cast
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Integer y = (Integer)1; // ERROR: not a legal explicit cast
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int a = new Integer(1); // ERROR: not a legal implicit cast
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int b = (int)new Integer(1); // ERROR: not a legal explicit cast
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----
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[[promotion]]
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==== Promotion
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Promotion is where certain operations require types to be either a minimum
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numerical type or for two (or more) types to be equivalent.
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The documentation for each operation that has these requirements
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includes promotion tables that describe how this is handled.
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When an operation promotes a type or types, the resultant type
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of the operation is the promoted type. Types can be promoted to def
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at compile-time; however, at run-time, the resultant type will be the
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promotion of the types the `def` is representing.
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*Examples:*
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[source,Java]
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----
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2 + 2.0 // Add the literal int 2 and the literal double 2.0. The literal
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// 2 is promoted to a double and the resulting value is a double.
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def x = 1; // Declare def variable x and set it to the literal int 1 through
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// an implicit cast
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x + 2.0F // Add def variable x and the literal float 2.0.
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// At compile-time the types are promoted to def.
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// At run-time the types are promoted to float.
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----
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