问题描述
在阅读HashMap的源代码时，我在public V put(K key, V value)中看到了这个片段:
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;
}
}
为什么要将 e.key 分配给 k 进行比较?为什么不直接比较，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 ------------------------
根据@seand的回答，我做了更详细的调查:
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;
}
}
类 Test 有一个 int 字段；
class Test has a int filed a;
使用 javap -c Main 获取类文件内容:
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]
参考jvm规范获取[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];
访问局部变量似乎比访问类字段更合理.
It seems reasonable to access the local variable than the class field.
推荐答案
我猜这是一种优化，可以节省对 e.key 的额外查找.(虽然它实际上不是使用invokevirtual 的方法调用，但它可以节省一定程度的间接性).由于这是一个使用非常频繁的库函数，作者可能会使用他们能想到的所有技巧来获得最大性能.您还可以看到它如何在 k = e.key 中检查对象身份，这可以避免成本稍高的 equals() 调用.
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.
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