openjdk源码:http://hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/5b86f66575b7
小例子
public class TestHashCode {
public static void main(String[] args) {
TestHashCode test1 = new TestHashCode();
TestHashCode test2 = new TestHashCode();
System.out.println("test1.hashcode:" + test1.hashCode());
System.out.println("test2.hashcode:" + test2.hashCode());
System.out.println("test1.identityHashCode:" + System.identityHashCode(test1));
System.out.println("test2.identityHashCode:" + System.identityHashCode(test2));
}
}
输出:
test1.hashcode:366712642
test2.hashcode:1829164700
test1.identityHashCode:366712642
test2.identityHashCode:1829164700
可以看到同一个对象调用
hashCode方法和identityHashCode的结果是一样的.
稍微改动一下
public class TestHashCode {
public int hashCode() {
return 3;
}
public static void main(String[] args) {
TestHashCode test1 = new TestHashCode();
TestHashCode test2 = new TestHashCode();
System.out.println("test1.hashcode:" + test1.hashCode());
System.out.println("test2.hashcode:" + test2.hashCode());
System.out.println("test1.identityHashCode:" + System.identityHashCode(test1));
System.out.println("test2.identityHashCode:" + System.identityHashCode(test2));
}
}
输出:
test1.hashcode:3
test2.hashcode:3
test1.identityHashCode:366712642
test2.identityHashCode:1829164700
问题:
可以看到如果子类重写了
hashCode方法之后,调用自身的hashCode的方法时运行时多态就会调用自身的已经重写的hashCode方法,因此对于hashcode方法时结果的改变在我们的预期之内,但是对于System.identityHashCode(Object obj)方法没有改变?如果他们之间存在什么关系,我们需要看一下?
identityHashCode 和 hashCode()
看看
identityHashCode的定义
需要注意两点:1.定义里面写着不论类是否重写了
hashCode()方法,它都会返回默认的哈希值(其实就是本地方法的值)
2.是属于System类里面的static方法
/**
* Returns the same hash code for the given object as
* would be returned by the default method hashCode(),
* whether or not the given object's class overrides
* hashCode().
* The hash code for the null reference is zero.
*
* @param x object for which the hashCode is to be calculated
* @return the hashCode
* @since JDK1.1
*/
public static native int identityHashCode(Object x);
对应的本地代码和代码链接
JNIEXPORT jint JNICALL
Java_java_lang_System_identityHashCode(JNIEnv *env, jobject this, jobject x)
{
return JVM_IHashCode(env, x);
}
看看
hashCode()方法
注意与identityHashCode的区别:1.
hashCode()是Object类的方法,注意一个类都会直接或者间接继承Object类
public native int hashCode();
对应的本地方法代码和代码链接:
static JNINativeMethod methods[] = {
{"hashCode", "()I", (void *)&JVM_IHashCode},
{"wait", "(J)V", (void *)&JVM_MonitorWait},
{"notify", "()V", (void *)&JVM_MonitorNotify},
{"notifyAll", "()V", (void *)&JVM_MonitorNotifyAll},
{"clone", "()Ljava/lang/Object;", (void *)&JVM_Clone},
};
所以从上面我们知道他们两个方法其实调用的是同一个方法
JVM_IHashCode即可,然后再回头看一下之前的代码,我相信你一定会明白了的.
参考:
1.https://www.cnblogs.com/godtrue/p/6395098.html
2.java1.8源码和openjdk源码
