java - 混淆自动装箱与泛型的使用

Question

据我所知，整数示例自动装箱用法是:

Integer iOb2 = 88; // auto-boxing
Integer iOb = new Integer(88) // is it auto-boxing ? I think no
                              // if it is auto-boxing what about above line?

上面的代码片段有效。不过，你能回答第二行是否自动装箱吗？使用泛型，我无法得到预期的结果。

// A very simple generic class. 
// Here, T is a type parameter that
// will be replaced by a real type
// when an object of type Gen is created.
class Gen<T> {
  T ob; // declare an object of type T

  // Pass the constructor a reference to 
  // an object of type T.
  Gen(T o) {
    ob = o;
  }

  // Return ob, which is of type T.
  T getob() {
    return ob;
  }
}

// Demonstrate the generic class.
class HelloWorld {
  public static void main(String args[]) {
    // Create a Gen reference for Integers. 
    Gen<Integer> iOb; 
    Integer iOb2;
    // Create a Gen<Integer> object and assign its
    // reference to iOb.  Notice the use of autoboxing 
    // to encapsulate the value 88 within an Integer object.
    //iOb = 88; //error
    iOb2 = 88;

    // Get the value in iOb. Notice that
    // no cast is needed.  The type is already known.
    //int v = iOb.getob();
    System.out.println("value: " + iOb2);

    System.out.println();

    // Create a Gen object for Strings.
    Gen<String> strOb = new Gen<String>("Generics Test");

    // Get the value of strOb. Again, notice
    // that no cast is needed.
    String str = strOb.getob();
    System.out.println("value: " + str);
  }
}

对于这个通用代码，为什么整数值不引用类型包装类型，即 Gen<Integer> ？既然如此，就应该如此。不应该吗？

Answer 1

Integer iOb2 = 88由编译器实现为 Integer iOb2 = Integer.valueOf(88) 。这就是自动装箱。

Integer iOb = new Integer(88)只是你在构建一个 Integer目的。不自动装箱。

自动装箱仅用于自动将基本类型转换为其等效类型 Object版本，例如int至Integer 。所有自动装箱操作均使用 valueOf() 完成方法，该方法是出于此特定目的而添加到 Java 5 中的(Boolean 除外，该方法已存在)。

因此，iOb = 88无效，因为 88是 int这与 Gen<Integer> 不兼容。 .

如果您写 iOb = new Gen<Integer>(88) ，那么您将在对象创建之前导致自动装箱，因为构造函数需要 Integer但您提供的是 int .

证据

证明自动装箱使用 valueOf() ，我创建了以下代码:

Boolean   a = true;
Character b = '1';
Byte      c = 1;
Short     d = 1;
Integer   e = 1;
Long      f = 1L;
Float     g = 1f;
Double    h = 1d;

使用 javap -c 进行拆卸生成的命令(为了清晰起见添加了空行):

 0: iconst_1
 1: invokestatic  #19                 // Method java/lang/Boolean.valueOf:(Z)Ljava/lang/Boolean;
 4: astore_1

 5: bipush        49
 7: invokestatic  #25                 // Method java/lang/Character.valueOf:(C)Ljava/lang/Character;
10: astore_2

11: iconst_1
12: invokestatic  #30                 // Method java/lang/Byte.valueOf:(B)Ljava/lang/Byte;
15: astore_3

16: iconst_1
17: invokestatic  #35                 // Method java/lang/Short.valueOf:(S)Ljava/lang/Short;
20: astore        4

22: iconst_1
23: invokestatic  #40                 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
26: astore        5

28: lconst_1
29: invokestatic  #45                 // Method java/lang/Long.valueOf:(J)Ljava/lang/Long;
32: astore        6

34: fconst_1
35: invokestatic  #50                 // Method java/lang/Float.valueOf:(F)Ljava/lang/Float;
38: astore        7

40: dconst_1
41: invokestatic  #55                 // Method java/lang/Double.valueOf:(D)Ljava/lang/Double;
44: astore        8