关于Java中HashCode方法的深入理解

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 )
}

//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);