2019-08-12 13:55:44 4607浏览
今天千锋扣丁学堂Java培训老师给大家分享一篇关于Java中HashCode方法的相关资料，文中通过示例代码介绍的非常详细，下面我们一起来看一下吧。
package main
import "fmt"
func main() {
var a int= 20 /* 声明实际变量 */
var ip *int /* 声明指针变量 */
ip = &a /* 指针变量的存储地址 */
fmt.Printf("a 变量的地址是: %x\n", &a )
/* 指针变量的存储地址 */
fmt.Printf("ip 变量储存的指针地址: %x\n", ip )
/* 使用指针访问值 */
fmt.Printf("*ip 变量的值: %d\n", *ip )
}
很多人说对象的HashCode方法返回的就是对象的内存地址，包括我在《Java核心编程·卷I》的第5章内容中也发现说是HashCode其值就是对象的内存地址。
//Indicates whether some other object is "equal to" this one.
public boolean equals(Object obj) {
return (this == obj);
}
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
/**
* Returns a hash code value for the object. This method is
* supported for the benefit of hash tables such as those provided by
* {@link java.util.HashMap}.
* ...
* As much as is reasonably practical, the hashCode method defined by
* class {@code Object} does return distinct integers for distinct
* objects. (This is typically implemented by converting the internal
* address of the object into an integer, but this implementation
* technique is not required by the
* Java™ programming language.)
*
* @return a hash code value for this object.
* @see java.lang.Object#equals(java.lang.Object)
* @see java.lang.System#identityHashCode
*/
public native int hashCode();
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},
};
JVM_ENTRY(jint, JVM_IHashCode(JNIEnv* env, jobject handle))
JVMWrapper("JVM_IHashCode");
// as implemented in the classic virtual machine; return 0 if object is NULL
return handle == NULL ? 0 : ObjectSynchronizer::FastHashCode (THREAD, JNIHandles::resolve_non_null(handle)) ;
JVM_END
intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
if (UseBiasedLocking) {
......
// Inflate the monitor to set hash code
monitor = ObjectSynchronizer::inflate(Self, obj);
// Load displaced header and check it has hash code
mark = monitor->header();
assert (mark->is_neutral(), "invariant") ;
hash = mark->hash();
if (hash == 0) {
hash = get_next_hash(Self, obj);
temp = mark->copy_set_hash(hash); // merge hash code into header
assert (temp->is_neutral(), "invariant") ;
test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
if (test != mark) {
// The only update to the header in the monitor (outside GC)
// is install the hash code. If someone add new usage of
// displaced header, please update this code
hash = test->hash();
assert (test->is_neutral(), "invariant") ;
assert (hash != 0, "Trivial unexpected object/monitor header usage.");
}
}
// We finally get the hash
return hash;
}
static inline intptr_t get_next_hash(Thread * Self, oop obj) {
intptr_t value = 0 ;
if (hashCode == 0) {
// This form uses an unguarded global Park-Miller RNG,
// so it's possible for two threads to race and generate the same RNG.
// On MP system we'll have lots of RW access to a global, so the
// mechanism induces lots of coherency traffic.
value = os::random() ;
} else
if (hashCode == 1) {
// This variation has the property of being stable (idempotent)
// between STW operations. This can be useful in some of the 1-0
// synchronization schemes.
intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3 ;
value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
} else
if (hashCode == 2) {
value = 1 ; // for sensitivity testing
} else
if (hashCode == 3) {
value = ++GVars.hcSequence ;
} else
if (hashCode == 4) {
value = cast_from_oop<intptr_t>(obj) ;
} else {
// Marsaglia's xor-shift scheme with thread-specific state
// This is probably the best overall implementation -- we'll
// likely make this the default in future releases.
unsigned t = Self->_hashStateX ;
t ^= (t << 11) ;
Self->_hashStateX = Self->_hashStateY ;
Self->_hashStateY = Self->_hashStateZ ;
Self->_hashStateZ = Self->_hashStateW ;
unsigned v = Self->_hashStateW ;
v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
Self->_hashStateW = v ;
value = v ;
}
value &= markOopDesc::hash_mask;
if (value == 0) value = 0xBAD ;
assert (value != markOopDesc::no_hash, "invariant") ;
TEVENT (hashCode: GENERATE) ;
return value;
}
public String toString() {
return getClass().getName() + "@" + Integer.toHexString(hashCode());
}
public class Student {
private String name;
private String gender;
public Student(String name, String gender) {
this.name = name;
this.gender = gender;
}
//省略 Setter，Gettter
@Override
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof Student) {
Student anotherStudent = (Student) anObject;
if (this.getName() == anotherStudent.getName()
|| this.getGender() == anotherStudent.getGender())
return true;
}
return false;
}
}
public static void main(String[] args) {
Student student1 = new Student("小明", "male");
Student student2 = new Student("小明", "male");
System.out.println("equals结果：" + student1.equals(student2));
System.out.println("对象1的散列值：" + student1.hashCode() + "，对象2的散列值：" + student2.hashCode());
}
public static void main(String[] args) {
Student student1 = new Student("小明", "male");
Student student2 = new Student("小明", "male");
HashMap<Student, String> hashMap = new HashMap<>();
hashMap.put(student1, "小明");
String value = hashMap.get(student2);
System.out.println(value);
}
public static native int identityHashCode(Object x);
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