問題描述

在閱讀HashMap的源代碼時，我在public V put(K key, V value)中看到了這個片段:

While reading the source code for HashMap, I came across this snippet in public V put(K key, V value):

for (Entry<K,V> e = table[i]; e != null; e = e.next) {
    Object k;
    if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
        V oldValue = e.value;
        e.value = value;
        e.recordAccess(this);
        return oldValue;
    }
}

為什么要將 e.key 分配給 k 進行比較?為什么不直接比較，比如:

Why assign e.key to k for comparing? Why not compare directly, like:

if (e.hash == hash && (e.key == key || key.equals(e.key))

-------- 更新 ------------

------------------- UPDATE ------------------------

根據@seand的回答，我做了更詳細的調查:

According to the answer from @seand, I do more detail investigation:

import com.test.Test;

    public class Main {
        public static void main(String[] args) {
            Test t = new Test();
            int a = t.a;
            int b = a;
        }
    }

類 Test 有一個 int 字段；

class Test has a int filed a;

使用 javap -c Main 獲取類文件內容:

Using javap -c Main to get the class file content:

  public static void main(java.lang.String[]);
Code:
   0: new           #2                  // class test/Test
   3: dup           
   4: invokespecial #3                  // Method test/Test."<init>":()V
   7: astore_1      
   8: aload_1       
   9: getfield      #4                  // Field test/Test.a:I
  12: istore_2      
  13: iload_2       
  14: istore_3      
  15: return    

int a = t.a 表示

8:[load the t object]
9:[access the field a]
12:[store the value to a]

參考jvm規范獲取[getfield的信息]

Refer to jvm specification get information of [getfield]

int b = a 表示:

13:[load the local variable]
14:[store the value to b];

訪問局部變量似乎比訪問類字段更合理.

It seems reasonable to access the local variable than the class field.

推薦答案

我猜這是一種優化，可以節省對 e.key 的額外查找.(雖然它實際上不是使用invokevirtual 的方法調用，但它可以節省一定程度的間接性).由于這是一個使用非常頻繁的庫函數，作者可能會使用他們能想到的所有技巧來獲得最大性能.您還可以看到它如何在 k = e.key 中檢查對象身份，這可以避免成本稍高的 equals() 調用.

My guess is it's an optimization which saves an extra lookup to e.key. (Though it's not actually a method call that's using invokevirtual, it may save a level of indirection). Since this is a very heavily used library function the authors likely used every trick they could think of for maximum performance. You can also see how it checks for object identity in k = e.key which may avoid a slightly more costly equals() call.

這篇關于為什么不直接比較 e.key 而不是將其分配給變量?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！