Re: [問題] pass-by-value的問題

[csihcs (非天夜翔)]

※ 引述《newjoy (職業格鬥家)》之銘言：
: public class test {
:         private int index = 0;
:         public test()
:         {
:                 this.index = 0;
:         }
:         public void setIndex(int index)
:         {
:                 this.index = index;
:         }
:         public int getIndex()
:         {
:                 return this.index;
:         }
:         public static void assignT(test t)
:         {
:                 t = new test();
:         }
:          public static void main(String[] args){
:                 test t1 = new test();
:                 t1.setIndex(9);
:                 assignT(t1);
:                 System.out.println("value : "+t1.getIndex());
:      }
: }
: 如果傳入function 裡面的是 object的 reference,
: 那為什麼執行assignT之後
: 印出來的值一樣是9而不是0??
: 假設 t1 的位址是 0xAA
: 那傳進 assignT 裡的 t 會是copy的一份 0xAA的 object 位址
: 然後 t = new test(); 會把0xAA的內容指向一個新的 test實體,
: 這樣理解有錯嗎?
------------------------------------------------------------------------------
after  "assignT(t1);"
before "t = new Test();"

┌─────┐0x00f00000 in stack memory
│0x10f10000│t1 @ main
└┼────┘
  └─────┐┌───────────┐
              ↓↓                      │
┌─────┐0x10f10000 in heap memory │
│..........│an instance of class test │
└─────┘which is created @ main   │
  ┌──────────────────┘
┌┼────┐0x00f20000 in stack memory
│0x10f10000│t @ assignT [when main call assignT]
└─────┘the value is passed by main
------------------------------------------------------------------------------
after "t = new Test();"

┌─────┐0x00f00000 in stack memory
│0x10f10000│t1 @ main
└┼────┘
  └─────┐
              ↓
┌─────┐0x10f10000 in heap memory
│..........│an instance of class test
└─────┘which is created @ main

┌─────┐0x00f20000 in stack memory
│0x10f30000│t @ assignT [after "t = new Test();"]
└┼────┘the value is assigned
  └─────┐
              ↓
┌─────┐0x10f30000 in heap memory
│..........│an instance of class test
└─────┘which is created @ assignT
------------------------------------------------------------------------------
back to main

┌─────┐0x00f00000 in stack memory
│0x10f10000│t1 @ main
└┼────┘
  └─────┐
              ↓
┌─────┐0x10f10000 in heap memory
│..........│an instance of class test
└─────┘which is created @ main

┌─────┐0x10f30000 in heap memory
│..........│an instance of class test
└─────┘which is created @ assignT
              It becomes a garbage.
              The reason is nobody can access this instance.
------------------------------------------------------------------------------

如果我沒有理解錯的話，
除了， main 自己以外，
沒有其它的 method 可以改變 t1 @ main 的值。

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推 Apohades:這圖案好精美
推 wendly777:簡單的說就是要把reference與instance搞清楚
→ wendly777:即使改成reference，不懂指標的人還是不懂reference