为什么包装类对象的标识符不能用作引用变量

Question

My question involves wrapper classes. I know that when we store a primitive type literal by using wrapper classes, we are storing it as a object of that wrapper class so object's identifier will be a reference variable (somehow like a pointer in c++). For instance, in Integer wi = new Integer("56") , wi is a reference variable. But if that is true:

1. Why can I do wi++ or wi +=2 ? Why does compiler deal with those reference variables like normal primitive variables? Doesn't a reference variable store reference of a object?

2. Given Integer wi = new Integer("56") and int pi = 56 , why does (wi == pi) returns true. Isn't wi supposed to store a reference (address)?

And another question: When a reference variable is passed to a method as parameter it counts as passing by reference so the modifiction that happens to that reference variable should affect it's value but it doesn't:

public class Main {
  void show(Integer x){
    x *=100 ;
  }

  void goo(int x){
    x *=100 ;
  }

  public static void main(String[] args) {
    Main mn = new Main() ;
    Integer wi = new Integer("86");
    int pi = 86 ;

    mn.goo(pi);
    System.out.println(pi); //output = 86

    mn.show(wi);
    System.out.println(wi); //output = 86, shouldn't it be 8600?
  }
}

Answer 1

the statement mn.goo(pi) passes the copy of value 86 while mn.show(wi) passes the copy of reference variable which holds the same object.

1. why can i do this? wi++ or wi +=2 .i mean why does compiler deal with those reference vriables like normal primitve variables?(doesn't a reference variable store reference of a object?)

Because of the concept of autoboxing and auto-unboxing , wi is converted to primitive , incremented, then then converted back to Wrapper

2.or if we have==>" Integer wi = new Integer("56") " and "int pi = 56" . why does (wi == pi) returns true. isn't wi supposed to store refernce (address)

This is because for Integer wrapper classes, the == will return true for the value till 128 . This is by design

For your doubts regarding passign primitives and object references, Please study these programs

class PassPrimitiveToMethod
{
    public static void main(String [] args)
    {
        int a = 5;
        System.out.println("Before Passing value to modify() a = " + a);
        PassPrimitiveToMethod p = new PassPrimitiveToMethod();
        p.modify(a);
        System.out.println("After passing value to modify() a = " + a);
        // the output is still the same because the copy of the value is passed to the method and not the copy of the bits like in refrence variables
        // hence unlike the reference variables the value remains unchanged after coming back to the main method

    }   


    void modify(int b)
    {
        b = b + 1;
        System.out.println("Modified number  b = " + b);
        // here the value passed is the copy of variable a
        // and only the copy is modified here not the variable 
    }       

}

The output is

Before Passing value to modify() a = 5
Modified number  b = 6
After passing value to modify() a = 5

Passing object reference to method

class PassReferenceToMethod
{
    public static void main(String [] args)
    {
        Dimension d = new Dimension(5,10);
        PassReferenceToMethod p = new PassReferenceToMethod();
        System.out.println("Before passing the reference d.height = " + d.height);
        p.modify(d);            // pass the d reference variable
        System.out.println("After passing the reference d.height = " + d.height);
        // the value changes because we are passing the refrence only which points to the single and same object
        // hence the values of the object are modified 
    } 

    void modify(Dimension dim)
    {
        dim.height = dim.height + 1;
    }   


}

The output is

class PassReferenceToMethod
{
    public static void main(String [] args)
    {
        Dimension d = new Dimension(5,10);
        PassReferenceToMethod p = new PassReferenceToMethod();
        System.out.println("Before passing the reference d.height = " + d.height);
        p.modify(d);            // pass the d reference variable
        System.out.println("After passing the reference d.height = " + d.height);
        // the value changes because we are passing the refrence only which points to the single and same object
        // hence the values of the object are modified 
    } 

    void modify(Dimension dim)
    {
        dim.height = dim.height + 1;
    }   


}

The output is

Before passing the reference d.height = 10
After passing the reference d.height = 11

Answer 2

The java compiler automatically inserts intValue and Integer.valueOf calls to convert between int and Integer . For example, here's a code snippet from the question:

void show(Integer x){
  x *=100 ;
}

And here is what really happens:

void show(Integer x) {
  int unboxed = x.intValue();
  unboxed *= 100;
}

As you can see, the line x *= 100 does not really change the Integer object you pass in, it only changes the int value extracted from that Integer object.

In a similar way, the code wi == pi from the question actually means wi.intValue() == pi , which explains your observation.

Answer 3

Java uses the "call by value" concept as described in detail here So in your case x *=100 ; in method show only updates the local variable

Answer 4

编译器unboxes wi到原始数据type.So现在，这是一个基本数据类型，因为一切都在Java是按值传递，在形式参数的改变不会影响实际arguements。

mn.show(wi);