类型擦除

学过C++模板的，在使用Java泛型的时候，会感觉到有点不疑问，例如：（1）无法定义一个泛型数组、无法调用泛型参数对象中对应的方法（当然，通过extends关键字是可以做到，只是比较麻烦）；（2）ArrayList<Integer>和ArrayList<String>在运行时的类型是相同的。Java中的泛型有这些问题，是它的实现机制决定的，即“类型擦除”。

类型擦除的定义：编译通过后，准备进入JVM运行时，就不再有类型参数的概念，换句话说：每定义一个泛型类型，JVM会自动提供一个对应的原生类；

public class Holder4<T> {

    private T a;
    private T b;
    private T c;

    public Holder4(T a, T b, T c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public T getA() {
        return a;
    }

    public T getB() {
        return b;
    }

    public T getC() {
        return c;
    }

    public void setA(T a) {
        this.a = a;
    }

    public void setB(T b) {
        this.b = b;
    }

    public void setC(T c) {
        this.c = c;
    }


 public static void main(String[] args) {
     Holder4<Automobile> holder4 = new Holder4<>(new Automobile(),new Automobile(), new Automobile());

     Automobile a = holder4.getA(); //编译器帮忙转型，不需要显式转型
     Automobile b = holder4.getB();
     Automobile c = holder4.getC();
 }
}

在Java中，每定义一个泛型类型，就会自动提供一个对应的原始类型，例如：

public class Holder4Raw {

    private Object a;
    private Object b;
    private Object c;

    public Holder4Raw(Object a, Object b, Object c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public Object getA() {
        return a;
    }

    public Object getB() {
        return b;
    }

    public Object getC() {
        return c;
    }

    public void setA(Object a) {
        this.a = a;
    }

    public void setB(Object b) {
        this.b = b;
    }

    public void setC(Object c) {
        this.c = c;
    }

    public static void main(String[] args) {
        Holder4Raw holder4Raw = new Holder4Raw(new Automobile(),new Automobile(), new Automobile());

        Automobile a = (Automobile) holder4Raw.getA();  //显示的转型
        Automobile b = (Automobile) holder4Raw.getB();
        Automobile c = (Automobile) holder4Raw.getC();
    }
}

为什么选择这种实现机制？
- 在Java诞生10年后，才想实现类似于C++模板的概念，即泛型；
- Java的类库是Java生态中非常宝贵的财富，必须保证向后兼容（即现有的代码和类文件依旧合法）和迁移兼容（泛化的代码和非泛化的代码可互相调用）基于上面这两个背景和考虑，Java设计者采取了“类型擦除”这种折中的实现方式。

Java泛型依赖编译器实现，只存在于编译期，JVM中没有泛型的概念；那么，编译器做了什么工作呢？（1）set方法是编译期检查；（2）get方法的返回值进行转型，编译器插入了一个checkcast语句。

我们通过字节码进行观察，可以看出：（1）Holder4和Holder4Raw两个类的字节码完全相同；（2）在main函数的33、41和49行就是编译器插入的checkcast语句；

public class org.java.learn.generics.Holder4<T> {
  public org.java.learn.generics.Holder4(T, T, T);
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: aload_0
       5: aload_1
       6: putfield      #2                  // Field a:Ljava/lang/Object;
       9: aload_0
      10: aload_2
      11: putfield      #3                  // Field b:Ljava/lang/Object;
      14: aload_0
      15: aload_3
      16: putfield      #4                  // Field c:Ljava/lang/Object;
      19: return

  public T getA();
    Code:
       0: aload_0
       1: getfield      #2                  // Field a:Ljava/lang/Object;
       4: areturn

  public T getB();
    Code:
       0: aload_0
       1: getfield      #3                  // Field b:Ljava/lang/Object;
       4: areturn

  public T getC();
    Code:
       0: aload_0
       1: getfield      #4                  // Field c:Ljava/lang/Object;
       4: areturn

  public void setA(T);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #2                  // Field a:Ljava/lang/Object;
       5: return

  public void setB(T);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #3                  // Field b:Ljava/lang/Object;
       5: return

  public void setC(T);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #4                  // Field c:Ljava/lang/Object;
       5: return

  public static void main(java.lang.String[]);
    Code:
       0: new           #5                  // class org/java/learn/generics/Holder4
       3: dup
       4: new           #6                  // class org/java/learn/generics/Automobile
       7: dup
       8: invokespecial #7                  // Method org/java/learn/generics/Automobile."<init>":()V
      11: new           #6                  // class org/java/learn/generics/Automobile
      14: dup
      15: invokespecial #7                  // Method org/java/learn/generics/Automobile."<init>":()V
      18: new           #6                  // class org/java/learn/generics/Automobile
      21: dup
      22: invokespecial #7                  // Method org/java/learn/generics/Automobile."<init>":()V
      25: invokespecial #8                  // Method "<init>":(Ljava/lang/Object;Ljava/lang/Object;Ljava/lang/Object;)V
      28: astore_1
      29: aload_1
      30: invokevirtual #9                  // Method getA:()Ljava/lang/Object;
      33: checkcast     #6                  // class org/java/learn/generics/Automobile，get方法的转型
      36: astore_2
      37: aload_1
      38: invokevirtual #10                 // Method getB:()Ljava/lang/Object;
      41: checkcast     #6                  // class org/java/learn/generics/Automobile，get方法的转型
      44: astore_3
      45: aload_1
      46: invokevirtual #11                 // Method getC:()Ljava/lang/Object;
      49: checkcast     #6                  // class org/java/learn/generics/Automobile，get方法的转型
      52: astore        4
      54: return
}