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| CHAPTER 5: Conversions and Promotions |
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Casting conversion is applied to the operand of a cast operator (S15.15): the type of the operand expression must be converted to the type explicitly named by the cast operator. Casting contexts allow the use of an identity conversion (S5.1.1), a widening primitive conversion (S5.1.2), a narrowing primitive conversion (S5.1.3), a widening reference conversion (S5.1.4), or a narrowing reference conversion (S5.1.5). Thus casting conversions are more inclusive than assignment or method invocation conversions: a cast can do any permitted conversion other than a string conversion.
Some casts can be proven incorrect at compile time; such casts result in a compile-time error.
A value of a primitive type can be cast to another primitive type by identity conversion, if the types are the same, or by a widening primitive conversion or a narrowing primitive conversion.
A value of a primitive type cannot be cast to a reference type by casting conversion, nor can a value of a reference type be cast to a primitive type.
The remaining cases involve conversion between reference types. The detailed rules for compile-time correctness checking of a casting conversion of a value of compile-time reference type S (source) to a compile-time reference type T (target) are as follows:
See Chapter 8 for the detailed specifications of classes, Chapter 9 for interfaces, and Chapter 10 for arrays.
If a cast to a reference type is not a compile-time error, there are two cases:
If a run-time exception is thrown, it is a ClassCastException (S11.5.1.1, S20.22).
Here are some examples of casting conversions of reference types, similar to the example in S5.2:
public class Point { int x, y; }
public interface Colorable { void setColor(int color); }
public class ColoredPoint extends Point implements Colorable
{
int color;
public void setColor(int color) { this.color = color; }
}
final class EndPoint extends Point { }
class Test {
public static void main(String[] args) {
Point p = new Point();
ColoredPoint cp = new ColoredPoint();
Colorable c;
// The following may cause errors at run time because
// we cannot be sure they will succeed; this possibility
// is suggested by the casts:
cp = (ColoredPoint)p; // p might not reference an
// object which is a ColoredPoint
// or a subclass of ColoredPoint
c = (Colorable)p; // p might not be Colorable
// The following are incorrect at compile time because
// they can never succeed as explained in the text:
Long l = (Long)p; // compile-time error #1
EndPoint e = new EndPoint();
c = (Colorable)e; // compile-time error #2
}
}
Here the first compile-time error occurs because the class types Long and Point are unrelated (that is, they are not the same, and neither is a subclass of the other), so a cast between them will always fail.
The second compile-time error occurs because a variable of type EndPoint can never reference a value that implements the interface Colorable . This is because EndPoint is a final type, and a variable of a final type always holds a value of the same run-time type as its compile-time type. Therefore, the run-time type of variable e must be exactly the type EndPoint , and type EndPoint does not implement Colorable .
Here is an example involving arrays (Chapter 10):
class Point {
int x, y;
Point(int x, int y) { this.x = x; this.y = y; }
public String toString() { return "("+x+","+y+")"; }
}
public interface Colorable { void setColor(int color); }
public class ColoredPoint extends Point implements Colorable
{
int color;
ColoredPoint(int x, int y, int color) {
super(x, y); setColor(color);
}
public void setColor(int color) { this.color = color; }
public String toString() {
return super.toString() + "@" + color;
}
}
class Test {
public static void main(String[] args) {
Point[] pa = new ColoredPoint[4];
pa[0] = new ColoredPoint(2, 2, 12);
pa[1] = new ColoredPoint(4, 5, 24);
ColoredPoint[] cpa = (ColoredPoint[])pa;
System.out.print("cpa: {");
for (int i = 0; i < cpa.length; i++)
System.out.print((i == 0 ? " " : ", ") + cpa[i]);
System.out.println(" }");
}
}
This example compiles without errors and produces the output:
cpa: { (2,2)@12, (4,5)@24, null, null }
The following example uses casts to compile, but it throws exceptions at run time, because the types are incompatible:
public class Point { int x, y; }
public interface Colorable { void setColor(int color); }
public class ColoredPoint extends Point implements Colorable
{
int color;
public void setColor(int color) { this.color = color; }
}
class Test {
public static void main(String[] args) {
Point[] pa = new Point[100];
// The following line will throw a ClassCastException:
ColoredPoint[] cpa = (ColoredPoint[])pa;
System.out.println(cpa[0]);
int[] shortvec = new int[2];
Object o = shortvec;
// The following line will throw a ClassCastException:
Colorable c = (Colorable)o;
c.setColor(0);
}
}
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