Overriding toString()

Figure 479. Printing a Shape's info

Code	Output
final Shape shape = new Shape(2.0, 3.0); // Center coordinates IO.println(shape);	inherit.Shape@37d31475 Desired: (2.0\|3.0)

Code

Output

final Shape shape =
     new Shape(2.0, 3.0); // Center coordinates
IO.println(shape);

inherit.Shape@37d31475

Desired:

(2.0|3.0)

Figure 480. Overwriting toString()

‹›

Figure 481. Shape extending Object

‹›

Figure 482. Overriding toString() in class Shape.

Code	public class Shape { private int x, y; ... @Override public String toString() { return "(" + x + "\|" + y + ")"; } } IO.println(new Shape(2.0, 3.0));
Output	(2.0\|3.0)

Code

public class Shape {
  private int x, y;
    ...
  @Override public String toString() {
        return "(" + x + "|" + y + ")";
    }
}

IO.println(new Shape(2.0, 3.0));

Output

(2.0|3.0)

Figure 483. Logging Rectangle instances

Code	// Center coordinates (2\|3), width==5 and height==7 final Rectangle r = new Rectangle(2.0, 3.0, 5.0, 7.0); IO.println(r);
Output	Desired: (2.0\|3.0), width=5.0, height=7.0

Code

// Center coordinates (2|3), width==5 and height==7
final Rectangle r = new Rectangle(2.0, 3.0, 5.0, 7.0);
IO.println(r);

Output

Desired:

(2.0|3.0), width=5.0, height=7.0

Figure 484. Overriding toString() in Rectangle.

public class Rectangle extends Shape {
  private double width, height;
    ...
  @Override public String toString() {
    return super.toString() ❶ +
      ", width=" + width + ", height=" + height; ❷
  } ...

❶	The `super` keyword allows for calling the superclass `Shape`'s `toString()` method inserting the (2.0\|3.0) center coordinate.
❷	Append class `Rectangle`'s “own” width and height attribute values.

Figure 485. Rectangle extending Shape

‹›

Figure 486. Implementing Circle.toString()

Code	public class Circle extends Shape { private double radius; ... @Override public String toString() { return super.toString() + ", radius=" + radius; } } IO.println(new Circle(2.0, 3.0, 7.0));
Output	(2.0\|3.0), radius=7.0

Code

public class Circle extends Shape {
  private double radius;
    ...
  @Override public String toString() {
    return super.toString() + ", radius=" + radius;
  }
}

IO.println(new Circle(2.0, 3.0, 7.0));

Output

(2.0|3.0), radius=7.0

Figure 487. Shape inheritance hierarchy and toString()

exercise No. 179

`String` vs. `StringBuffer`

Consider two StringBuffer instances:

Code	Execution result
final StringBuffer a = new StringBuffer("test"), b = new StringBuffer("test"); IO.println(a.equals(b));	false

Strangely String instances behave differently:

Code	Execution result
final `String` a = new String("test"), b = new String("test"); IO.println(a.equals(b));	true

Explain this different behaviour.

Tip

Take a closer look at the String and StringBuffer API regarding the toString() method.

The String API reveals:

public boolean equals (Object anObject)
Compares this string to the specified object. The result is true if and only if the argument is
not null and is a String object that represents the same sequence of characters as this object.

Overrides: equals in class Object

Parameters:
anObject - The object to compare this String against

Returns:
true if the given object represents a String equivalent to this string, false otherwise

In a nutshell: The superclass Object.equals() method is being overridden: Two instances of String will be compared for equality character by character, i.e. by content as expected.

The StringBuffer does not override its parent classes Object.equals() method. Therefore comparing two of its instances results in:

public boolean equals (Object obj)
Indicates whether some other object is "equal to" this one.
...
Parameters:
obj - the reference object with which to compare.
Returns:
true if this object is the same as the obj argument; false otherwise.

The comparison is thus by object identity rather than by content.

exercise No. 180

Alternate implementation of opposite directions

Provide a different implementation of your opposite() from Compass direction neighbours based on values() and ordinal().

We observe the following ordinal values:

public enum Direction {

  N(   0, "north"),         // 0
  NE( 45, "north by east"), // 1
  E(  90, "east"),          // 2
  SE(135, "south by east"), // 3
  S( 180, "south"),         // 4
  SW(225, "south by west"), // 5
  W( 270, "west"),          // 6
  NW(315, "north by west"); // 7
...

public Direction opposite() {
    switch (this) {
      case N: return S;   // 0 → 4
      case NE: return SW; // 1 → 5
      case E: return W;   // 2 → 6
      case SE: return NW; // 3 → 7
      case S: return N;   // 4 → 0
      case SW: return NE; // 5 → 1
      case W: return E;   // 6 → 2
      case NW: return SE; // 7 → 3
      ...

We are thus require implementing an integer shift of (0, 1, 2, 3, 4, 5, 6, 7) → (4, 5, 6, 7, 0, 1, 2, 3) by four places:

public Direction opposite() {
   return values()[(ordinal() + 4) % 8];
}

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