Example: Incorrect Overriding in Java

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84103 Example: Incorrect Overriding
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class Point { int x = 0, y = 0, color;
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This example is an extended variation of that in the preceding section:
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int getX() { return x; } int getY() { return y; }
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the class RealPoint hides the declarations of the int instance variables x and y of class Point with its own float instance variables x and y, and overrides the method move of class Point with its own move method It also overloads the name move with another method with a different signature ( 842) In this example, the members of the class RealPoint include the instance variable color inherited from the class Point, the float instance variables x and y declared in RealPoint, and the two move methods declared in RealPoint Which of these overloaded move methods of class RealPoint will be chosen for any particular method invocation will be determined at compile time by the overloading resolution procedure described in 1512
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void move(int dx, int dy) { x += dx; y += dy; }
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void move(int dx, int dy) { x += dx; y += dy; }
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8410 Examples of Method Declarations
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class RealPoint extends Point { float x = 00f, y = 00f;
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CLASSES
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void move(int dx, int dy) { move((float)dx, (float)dy); } void move(float dx, float dy) { x += dx; y += dy; } float getX() { return x; } float getY() { return y; } }
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84104 Example: Overriding versus Hiding
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class Point {
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This example corrects the errors of the example in the preceding section:
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class RealPoint extends Point { float x = 00f, y = 00f;
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Here the overriding methods getX and getY in class RealPoint have the same return types as the methods of class Point that they override, so this code can be successfully compiled
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int x = 0, y = 0; int getX() { return x; } int getY() { return y; } int color;
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void move(int dx, int dy) { x += dx; y += dy; }
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void move(int dx, int dy) { move((float)dx, (float)dy); } void move(float dx, float dy) { x += dx; y += dy; } int getX() { return (int)Mathfloor(x); } int getY() { return (int)Mathfloor(y); }
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Here the class Point provides methods getX and getY that return the values of its elds x and y; the class RealPoint then overrides these methods by declaring methods with the same signature The result is two errors at compile time, one for each method, because the return types do not match; the methods in class Point return values of type int, but the wanna-be overriding methods in class RealPoint return values of type float
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CLASSES
Examples of Method Declarations
Consider, then, this test program:
class Test { public static void main(String[] args) { RealPoint rp = new RealPoint(); Point p = rp; rpmove(171828f, 414159f); pmove(1, -1); show(px, py); show(rpx, rpy); show(pgetX(), pgetY()); show(rpgetX(), rpgetY()); }
static void show(int x, int y) { Systemoutprintln("(" + x + ", " + y + ")"); } static void show(float x, float y) { Systemoutprintln("(" + x + ", " + y + ")"); } }
The output from this program is:
(0, 0) (27182798, 314159) (2, 3) (2, 3)
The rst line of output illustrates the fact that an instance of RealPoint actually contains the two integer elds declared in class Point; it is just that their names are hidden from code that occurs within the declaration of class RealPoint (and those of any subclasses it might have) When a reference to an instance of class RealPoint in a variable of type Point is used to access the eld x, the integer eld x declared in class Point is accessed The fact that its value is zero indicates that the method invocation pmove(1, -1) did not invoke the method move of class Point; instead, it invoked the overriding method move of class RealPoint The second line of output shows that the eld access rpx refers to the eld x declared in class RealPoint This eld is of type float, and this second line of output accordingly displays oating-point values Incidentally, this also illustrates the fact that the method name show is overloaded; the types of the arguments in the method invocation dictate which of the two de nitions will be invoked The last two lines of output show that the method invocations pgetX() and rpgetX() each invoke the getX method declared in class RealPoint Indeed, there is no way to invoke the getX method of class Point for an instance of class