CONVERSIONS AND PROMOTIONS

Assignment Conversion

5.2

//

The following cause compile time errors because

//

we cannot be sure they will succeed, depending on

//

the run time type of

p

; a run time check will be

//

necessary for the needed narrowing conversion and

//

must be indicated by including a cast:

cp = p;

// p

 might be neither a

ColoredPoint

//

nor a subclass of

ColoredPoint

c = p;

// p

 might not implement

Colorable

}

}

Here is another example involving assignment of array objects:

class Point { int x, y; }

class ColoredPoint extends Point { int color; }

class Test {

public static void main(String[] args) {

long[] veclong = new long[100];

Object o = veclong;

//

okay

Long l = veclong;

//

compile time error

short[] vecshort = veclong;

//

compile time error

Point[] pvec = new Point[100];

ColoredPoint[] cpvec = new ColoredPoint[100];

pvec = cpvec;

//

okay

pvec[0] = new Point();

//

okay at compile time,

//

but would throw an

//

exception at run time

cpvec = pvec;

//

compile time error

}

}

In this example:

The value of

veclong

 cannot be assigned to a

Long

 variable, because

Long

is a class type ( 20.8) other than

Object

. An array can be assigned only to a

variable of a compatible array type, or to a variable of type

Object

.

The value of

veclong

 cannot be assigned to

vecshort

, because they are

arrays of primitive type, and

short

 and

long

 are not the same primitive type.

The value of

cpvec

 can be assigned to

pvec

, because any reference that

could be the value of an expression of type

ColoredPoint

 can be the value of

a variable of type

Point

. The subsequent assignment of the new

Point

 to a

component of

pvec

 then would throw an

ArrayStoreException

 (if the pro 

gram were otherwise corrected so that it could be compiled), because a

ColoredPoint

array can't have an instance of

Point

 as the value of a com 

ponent.

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