Java DataStructures 问题装箱/拆箱

Question

I am studying for a data structures exam tommorrow and need to know what lines in the following code are correct and which aren't and why

Object  obj = new Integer(42);
Integer iObj = 43;
iObj = obj;

Answer 1

HelloWorld.java:19: error: incompatible types: Object cannot be converted to Integer
        iObj = obj;
               ^
1 error

The above fails because the compile time type of iObj and obj are not matching. This is a signature of strongly typed languages. Similar code in Javascript would work fine.

Answer 2

Answer to your question is here under:

 Object  obj = new Integer(42); //auto boxing  // true
 Integer iObj = 43; //direct intialization   //true
 iObj = obj  // false
 iObj = (Integer) obj; // manual boxing 

iObj = obj is false because obj is a reference to the Object and iObj is of Interger . Object is the parent of all and so Integer type iObj is child of obj and hence false.

 In short, child can be auto- boxed to parent but the vice-versa is not possible

Answer 3

所有这些行都是正确的首先你创建对象和 tgen 你创建整数并将值分配给该整数对象这行代码是完全正确的