今天，我將討論較少使用的空對象模式。在面向對象編程中，我們經常處理空對象。空對象是指沒有任何引用的對象，或者定義爲中性/空功能/行爲的對象。在訪問任何成員或調用任何方法時，需要檢查這些null對象，以確保它們不是null。這是因爲成員或方法通常不能在null對象上調用。
Null Object Pattern
- null對象設計模式描述了null對象的使用及其在系統中的行爲。
- 空對象模式處理空對象。
- 我們不檢查空對象，而是定義空行爲或調用不做行爲。
- 這些null對象還可以用於在數據不可用的情況下提供默認行爲。
- 這種方法相對於可工作的默認實現的優點是null對象是非常可預測的，並且沒有副作用——它什麼也不做。
- null對象模式還可以用作測試的存根，以防測試無法使用資源。
在使用Null對象模式之前，我們應該瞭解:
- 這種模式應謹慎使用。它可以使錯誤出現在正常的程序執行中。
- 我們不應該僅僅爲了避免null檢查和使代碼更易於閱讀而實現這種模式。實際上，如果代碼移動到另一個地方，比如null對象類，那麼讀取它就會比較困難。
- 我們必須執行額外的測試，以確保沒有任何地方需要指定null而不是null對象。
讓我們看一個例子來更好地理解這個模式。
Example of Null Objects
創建一個抽象類(或接口)來指定各種功能。在這個示例中，我使用了shape接口。請注意，我在接口中也創建了isNull()方法。有一個方法很好，我喜歡它，因爲我可以更好地識別和控制空定義的對象。該方法將爲所有的具體類返回false。而且，它只會爲null對象類返回true。
package design.nullobject;
public interface Shape {
double area();
double perimeter();
void draw();
// nice to have method to indicate null object
boolean isNull();
}您將需要創建一個具體的類來擴展這個類或實現接口。每個具體類將定義功能的特定版本。我定義了三種形狀:圓形、矩形和三角形。這些具體的類將定義不同類型的形狀。
下面是Circle類的代碼:
package design.nullobject;
public class Circle implements Shape {
// sides
private final double radius;
public Circle() {
this(1.0d);
}
public Circle(double radius) {
this.radius = radius;
}
@Override public double area() {
// Area = π r^2
return Math.PI * Math.pow(radius, 2);
}
@Override public double perimeter() {
// Perimeter = 2πr
return 2 * Math.PI * radius;
}
@Override public void draw() {
System.out.println("Drawing Circle with area: " + area() + " and perimeter: " + perimeter());
}
@Override
public boolean isNull() {
return false;
}
}下面的是 Rectangle 類的代碼：
package design.nullobject;
public class Rectangle implements Shape {
// sides
private final double width;
private final double length;
public Rectangle() {
this(1.0d ,1.0d);
}
public Rectangle(double width, double length) {
this.width = width;
this.length = length;
}
@Override
public double area() {
// A = w * l
return width * length;
}
@Override
public double perimeter() {
// P = 2(w + l)
return 2 * (width + length);
}
@Override
public void draw() {
System.out.println("Drawing Rectangle with area: " + area() + " and perimeter: " + perimeter());
}
@Override
public boolean isNull() {
return false;
}
}Triangle 類:
package design.nullobject;
public class Triangle implements Shape {
// sides
private final double a;
private final double b;
private final double c;
public Triangle() {
this(1.0d, 1.0d, 1.0d);
}
public Triangle(double a, double b, double c) {
this.a = a;
this.b = b;
this.c = c;
}
@Override
public double area() {
// Using Heron's formula:
// Area = SquareRoot(s * (s - a) * (s - b) * (s - c))
// where s = (a + b + c) / 2, or 1/2 of the perimeter of the triangle
double s = (a + b + c) / 2;
return Math.sqrt(s * (s - a) * (s - b) * (s - c));
}
@Override
public double perimeter() {
// P = a + b + c
return a + b + c;
}
@Override public void draw() {
System.out.println("Drawing Triangle with area: " + area() + " and perimeter: " + perimeter());
}
@Override
public boolean isNull() {
return false;
}
}現在，最重要的一步是創建一個空對象類，它擴展抽象類或接口並定義“不做任何事情”行爲。“不做”行爲類似於在數據不可用的情況下的默認行爲。
package design.nullobject;
public class NullShape implements Shape {
// no sides
@Override
public double area() {
return 0.0d;
}
@Override
public double perimeter() {
return 0.0d;
}
@Override
public void draw() {
System.out.println("Null object can't be draw");
}
@Override
public boolean isNull() {
return true;
}
}現在，我們定義Factory類來創建各種類型的形狀。
爲這個示例創建ShapeFactory類。
package design.nullobject;
public class ShapeFactory {
public static Shape createShape(String shapeType) {
Shape shape = null;
if ("Circle".equalsIgnoreCase(shapeType)) {
shape = new Circle();
} else if ("Rectangle".equalsIgnoreCase(shapeType)) {
shape = new Rectangle();
} else if ("Triangle".equalsIgnoreCase(shapeType)) {
shape = new Triangle();
} else {
shape = new NullShape();
}
return shape;
}
}爲了簡單起見，我沒有收到ShapeFactory方法中形狀邊的參數。因此，工廠正在創建具有固定邊值的不同Shape對象。
最後一步，創建一個主類來執行和測試代碼:
package design.nullobject;
import design.nullobject.ShapeFactory;
public class ShapeMain {
public static void main(String[] args) {
String[] shapeTypes = new String[] { "Circle", null, "Triangle", "Pentagon", "Rectangle", "Trapezoid"};
for (String shapeType : shapeTypes) {
Shape shape = ShapeFactory.createShape(shapeType);
// no null-check required since shape factory always creates shape objects
System.out.println("Shape area: " + shape.area());
System.out.println("Shape Perimeter: " + shape.perimeter());
shape.draw();
System.out.println();
}
}
}下面是代碼的輸出:
Shape area: 3.141592653589793
Shape Perimeter: 6.283185307179586
Drawing Circle with area: 3.141592653589793 and perimeter: 6.283185307179586
Shape area: 0.0
Shape Perimeter: 0.0
Null object can't be draw
Shape area: 0.4330127018922193
Shape Perimeter: 3.0
Drawing Triangle with area: 0.4330127018922193 and perimeter: 3.0
Shape area: 0.0
Shape Perimeter: 0.0
Null object can't be draw
Shape area: 1.0
Shape Perimeter: 4.0
Drawing Rectangle with area: 1.0 and perimeter: 4.0
Shape area: 0.0
Shape Perimeter: 0.0
Null object can't be draw在Java 8中，我們有java.util.Optional處理空引用的類。這個類最初來自於Guava API。
Optional
該類的目的是提供一個類型級別的解決方案，用於表示可選值，而不是使用空引用。
下面，我將演示java.util.Optional的一些有用api。有幾個靜態api來創建可選對象:
Optional.empty() : 要創建一個空的可選對象，使用空API:
@Test
public void optionalEmptyTest() {
Optional<Shape> empty = Optional.empty();
assertFalse(empty.isPresent());
} Optional.of(): 要創建一個非空的可選對象，如果我們確信我們有一個我們想讓它成爲可選的對象，使用API。如果您使用null調用這個API，它將拋出null指針異常 ( NullPointerException )。
@Test
public void optionalOfTest() {
Shape shape = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.of(shape);
assertTrue(nonEmpty.isPresent());
}
@Test(expected = NullPointerException.class)
public void optionalOfWithNullTest() {
Shape shape = null;
Optional.of(shape);
}Optional.ofNullable():在上面的代碼中，當我們處於空或非空狀態時，這兩個api都很有用。如果我們不確定對象，對象可能爲空，也可能不爲空。爲此，使用 ofNullable API。
@Test
public void optionalOfNullableTest() {
Shape shape1 = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.ofNullable(shape1);
assertTrue(nonEmpty.isPresent());
Shape shape2 = null;
Optional<Shape> empty = Optional.ofNullable(shape2);
assertFalse(empty.isPresent());
}Additional APIs
isPresent():此方法返回true，且僅當包裝在可選對象中的對象不是空的(非空)。
@Test
public void optionalIsPresentTest() {
Shape shape = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.of(shape);
assertTrue(nonEmpty.isPresent());
Optional<Shape> empty = Optional.empty();
assertFalse(empty.isPresent());
}ifPresent(): 這使我們能夠在包裝值上運行代碼，如果發現它是非空的。
@Test
public void optionalIfPresentTest() {
Shape shape = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.of(shape);
nonEmpty.ifPresent(circle -> circle.draw());
Optional<Shape> empty = Optional.empty();
empty.ifPresent(circle -> circle.draw());
}get():如果包裝對象不爲空，則返回一個值。否則，它將拋出一個沒有此類元素異常(NoSuchElementException)。
@Test
public void optionalGetTest() {
Shape shape = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.ofNullable(shape);
assertNotNull(nonEmpty.get());
}
@Test(expected = NoSuchElementException.class)
public void optionalGetWithNullTest() {
Shape shape = null;
Optional<Shape> empty = Optional.ofNullable(shape);
empty.get();
}orElse(): 這用於檢索包裝在可選實例中的對象。該方法有一個參數作爲默認對象。使用orElse，如果包裝對象存在，則返回包裝對象，如果包裝對象不存在，則返回給orElse的參數。
@Test
public void optionalOrElseTest() {
Shape shape = ShapeFactory.createShape("Rectangle");
Shape shape1 = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.of(shape1);
assertEquals(shape1, nonEmpty.orElse(shape));
Optional<Shape> empty = Optional.empty();
empty.ifPresent(circle -> circle.draw());
assertEquals(shape, empty.orElse(shape));
}orElseGet() : 這個方法與orElse類似。唯一的區別是，如果可選對象不存在，它不接受返回的對象，而是接受一個supplier functional接口，該接口被調用並返回調用的值。
@Test
public void optionalOrElseGetTest() {
Shape shape = ShapeFactory.createShape("Rectangle");
Shape shape1 = ShapeFactory.createShape("Circle");
Optional<Shape> nonEmpty = Optional.of(shape1);
assertEquals(shape1, nonEmpty.orElseGet(() -> ShapeFactory.createShape("Rectangle")));
Optional<Shape> empty = Optional.empty();
empty.ifPresent(circle -> circle.draw());
// comparing the area of the shape since orElseGet will create another instance of rectangle
assertEquals(shape.area(), empty.orElseGet(() -> ShapeFactory.createShape("Rectangle")).area(), 0.001d);
}orElseThrow(): 這與orElse類似。唯一的區別是，它添加了一種處理缺失值的新方法。當包裝對象不存在時，它不會返回默認值，而是拋出一個給定的(作爲參數)異常。
@Test(expected = IllegalArgumentException.class)
public void optionalOrElseThrowWithNullTest() {
Shape shape = null;
Optional<Shape> empty = Optional.ofNullable(shape);
empty.orElseThrow(IllegalArgumentException::new);
}現在，我們可以使用Optional來編寫ShapeFactory和Main，如下所示:
package design.nullobject;
public class ShapeFactoryJava8 {
public static Optional<Shape> createShape(String shapeType) {
Shape shape = null;
if ("Circle".equalsIgnoreCase(shapeType)) {
shape = new Circle();
} else if ("Rectangle".equalsIgnoreCase(shapeType)) {
shape = new Rectangle();
} else if ("Triangle".equalsIgnoreCase(shapeType)) {
shape = new Triangle();
} else {
// no need to have NullShape anymore
shape = null;
}
// using ofNullable because shape may be not null.
return Optional.ofNullable(shape);
}
}執行和測試代碼的主類:
package design.nullobject;
import java.util.Arrays;
import java.util.Optional;
public class ShapeMainJava8 {
public static void main(String[] args) {
String[] shapeTypes = new String[] { "Circle", null, "Triangle", "Pentagon", "Rectangle", "Trapezoid"};
Arrays.asList(shapeTypes).stream().forEach(shapeType -> {
Optional<Shape> optionalShape = ShapeFactoryJava8.createShape(shapeType);
optionalShape.ifPresent((shape) -> {
// null-check is done by ifPresent of Optional
System.out.println("Shape area: " + shape.area());
System.out.println("Shape Perimeter: " + shape.perimeter());
shape.draw();
System.out.println();
});
});
}
}下面是代碼的輸出:
Shape area: 3.141592653589793
Shape Perimeter: 6.283185307179586
Drawing Circle with area: 3.141592653589793 and perimeter: 6.283185307179586
Shape area: 0.4330127018922193
Shape Perimeter: 3.0
Drawing Triangle with area: 0.4330127018922193 and perimeter: 3.0
Shape area: 1.0
Shape Perimeter: 4.0
Drawing Rectangle with area: 1.0 and perimeter: 4.0