这是对泛型的介绍性讨论的延续， 此处的先前部分可以在此处找到。
在上一篇文章中，我们讨论了关于类型参数的递归边界。 我们看到了递归绑定如何帮助我们重用了车辆比较逻辑。 在该文章的结尾，我建议当我们不够小心时，可能会发生类型混合。 今天我们将看到一个例子。
如果有人错误地通过以下方式创建了Vehicle的子类，则可能会发生混合：
/*** Definition of Vehicle*/
public abstract class Vehicle<E extends Vehicle<E>> implements Comparable<E> {// other methods and propertiespublic int compareTo(E vehicle) {// method implementation}
}/*** Definition of Bus*/
public class Bus extends Vehicle<Bus> {}/*** BiCycle, new subtype of Vehicle*/
public class BiCycle extends Vehicle<Bus> {}/*** Now this class’s compareTo method will take a Bus type* as its argument. As a result, you will not be able to compare* a BiCycle with another Bicycle, but with a Bus.*/
cycle.compareTo(anotherCycle); // This will generate a compile time error
cycle.compareTo(bus); // but you will be able to do this without any error枚举不会发生这种类型的混淆，因为JVM负责子类化和为枚举类型创建实例，但是如果我们在代码中使用此样式，则必须小心。
让我们谈谈递归边界的另一个有趣的应用。 考虑以下类别：
public class MyClass {private String attrib1;private String attrib2;private String attrib3;private String attrib4;private String attrib5;public MyClass() {}public String getAttrib1() {return attrib1;}public void setAttrib1(String attrib1) {this.attrib1 = attrib1;}public String getAttrib2() {return attrib2;}public void setAttrib2(String attrib2) {this.attrib2 = attrib2;}public String getAttrib3() {return attrib3;}public void setAttrib3(String attrib3) {this.attrib3 = attrib3;}public String getAttrib4() {return attrib4;}public void setAttrib4(String attrib4) {this.attrib4 = attrib4;}public String getAttrib5() {return attrib5;}public void setAttrib5(String attrib5) {this.attrib5 = attrib5;}
}如果我们要创建此类的实例，则可以执行以下操作：
MyClass mc = new MyClass();
mc.setAttrib1("Attribute 1");
mc.setAttrib2("Attribute 2");上面的代码创建该类的实例并初始化属性。 如果我们可以在此处使用方法链接 ，那么我们可以编写：
MyClass mc = new MyClass().setAttrib1("Attribute 1").setAttrib2("Attribute 2");显然比第一个版本好得多。 但是，要启用这种方法链接，我们需要通过以下方式修改MyClass ：
public class MyClass {private String attrib1;private String attrib2;private String attrib3;private String attrib4;private String attrib5;public MyClass() {}public String getAttrib1() {return attrib1;}public MyClass setAttrib1(String attrib1) {this.attrib1 = attrib1;return this;}public String getAttrib2() {return attrib2;}public MyClass setAttrib2(String attrib2) {this.attrib2 = attrib2;return this;}public String getAttrib3() {return attrib3;}public MyClass setAttrib3(String attrib3) {this.attrib3 = attrib3;return this;}public String getAttrib4() {return attrib4;}public MyClass setAttrib4(String attrib4) {this.attrib4 = attrib4;return this;}public String getAttrib5() {return attrib5;}public MyClass setAttrib5(String attrib5) {this.attrib5 = attrib5;return this;}
}然后我们将可以对此类的实例使用方法链接。 但是，如果我们想在涉及继承的地方使用方法链接，那么事情就会变得混乱：
public abstract class Parent {private String attrib1;private String attrib2;private String attrib3;private String attrib4;private String attrib5;public Parent() {}public String getAttrib1() {return attrib1;}public Parent setAttrib1(String attrib1) {this.attrib1 = attrib1;return this;}public String getAttrib2() {return attrib2;}public Parent setAttrib2(String attrib2) {this.attrib2 = attrib2;return this;}public String getAttrib3() {return attrib3;}public Parent setAttrib3(String attrib3) {this.attrib3 = attrib3;return this;}public String getAttrib4() {return attrib4;}public Parent setAttrib4(String attrib4) {this.attrib4 = attrib4;return this;}public String getAttrib5() {return attrib5;}public Parent setAttrib5(String attrib5) {this.attrib5 = attrib5;return this;}
}public class Child extends Parent {private String attrib6;private String attrib7;public Child() {}public String getAttrib6() {return attrib6;}public Child setAttrib6(String attrib6) {this.attrib6 = attrib6;return this;}public String getAttrib7() {return attrib7;}public Child setAttrib7(String attrib7) {this.attrib7 = attrib7;return this;}
}/*** Now try using method chaining for instances of Child* in the following way, you will get compile time errors.*/
Child c = new Child().setAttrib1("Attribute 1").setAttrib6("Attribute 6");这样做的原因是，即使Child从其父级继承了所有的setter，所有这些setter方法的返回类型也都是Parent类型，而不是Child类型。 因此，第一个设置器将返回类型为Parent的引用，调用setAttrib6会导致编译错误，因为它没有任何此类方法。
我们可以通过在Parent上引入通用类型参数并在其上定义递归绑定来解决此问题。 它的所有子项从其扩展时都将自己作为类型参数传递，从而确保setter方法将返回其类型的引用：
public abstract class Parent<T extends Parent<T>> {private String attrib1;private String attrib2;private String attrib3;private String attrib4;private String attrib5;public Parent() {}public String getAttrib1() {return attrib1;}@SuppressWarnings("unchecked")public T setAttrib1(String attrib1) {this.attrib1 = attrib1;return (T) this;}public String getAttrib2() {return attrib2;}@SuppressWarnings("unchecked")public T setAttrib2(String attrib2) {this.attrib2 = attrib2;return (T) this;}public String getAttrib3() {return attrib3;}@SuppressWarnings("unchecked")public T setAttrib3(String attrib3) {this.attrib3 = attrib3;return (T) this;}public String getAttrib4() {return attrib4;}@SuppressWarnings("unchecked")public T setAttrib4(String attrib4) {this.attrib4 = attrib4;return (T) this;}public String getAttrib5() {return attrib5;}@SuppressWarnings("unchecked")public T setAttrib5(String attrib5) {this.attrib5 = attrib5;return (T) this;}
}public class Child extends Parent<Child> {private String attrib6;private String attrib7;public String getAttrib6() {return attrib6;}public Child setAttrib6(String attrib6) {this.attrib6 = attrib6;return this;}public String getAttrib7() {return attrib7;}public Child setAttrib7(String attrib7) {this.attrib7 = attrib7;return this;}
}请注意，我们已经明确地施放此为T类型，因为编译器不知道这种转换是否是可能的，即使它是因为牛逼的定义是由父<T>界。 同样，由于我们将对象引用转换为T ，因此编译器将发出未经检查的警告。 为了抑制这种情况，我们在设置器上方使用了@SuppressWarnings（“ unchecked”） 。
经过上述修改，这样做是完全有效的：
Child c = new Child().setAttrib1("Attribute 1").setAttrib6("Attribute 6");当以这种方式编写方法设置器时，我们应注意不要将递归边界用于任何其他目的，例如从父级访问子级状态，因为这会使父级暴露其子类的内部细节，并最终破坏封装。
通过这篇文章，我完成了泛型的基本介绍。 我在本系列中没有讨论太多的事情，因为我认为它们已经超出了介绍的范围。
直到下一次。
翻译自: https://www.javacodegeeks.com/2014/07/an-introduction-to-generics-in-java-part-6.html
