面试必问hashCode与equals
hashCode
和equals用来标识对象,两个方法协同工作可用来判断两个对象是否相等。这两方法来源于:java.lang.Object
//本地方法
public native int hashCode();
public boolean equals(Object obj) {
return (this == obj);
}
众所周知,根据生产的哈希将数据散列开来,可以是存取元素更快,对象通过调用Object.hashCode();
生成哈希值;由于不可避免地存在哈希值冲突的情况,因此,当hashCode
相同时,还需要进行equals方法比较,但是,若hashCode
不同,将直接判定两个对象不相等,跳过equals,这加快了冲突处理效率。
Object类定义中对hashCode
和equals要求如下:
如果两个对象的equals的结果相等,则两个对象的hashCode的返回结果必须是相同的。
如果两个兑现搞的hashCode相等,equals不一定相等,即就是两个对象不一定相等。
任何时候覆写equals,都必须同时覆写hashCode方法。
在Map和Set类集合中,用到这两个方法时,首先判断hashCode
的值,如果hash值相等,则在判断equals的结果,HashMap
中get方法判断如下:
if (e.hash == hash &&((k = e.key) == key
|| (key != null && key.equals(k))))
return e;
先进性hash值比较,然后进行equals比较。如果hash只不相等则equals方法都不用比较了。
这样一来就是要求hashCode
方法要求就高了很多,一个优秀的哈希算法应尽可能地让元素均匀分布,降低冲突概念,即就是尽量在equals不相等的情况下hash值也不相等,这样使用&&或者||短路操作一旦生效,会极大地提高程序的执行效率。如果自定义Map的键,那么必须重写equals和hashCode
方法。
此外,因为Set存储的是不重复的对象,依据hashCode
和equals进行判断,所以Set存储的自定义对象也必须重写这两个方法。此时,如果重写equals而不重写hashCode
,到底会有什么影响呢?
请看下面的代码:
public class Test {
public static void main(String[] args) {
Set<EqualsDemo> hashSet = new HashSet<>();
hashSet.add(new EqualsDemo(1, "Java后端技术栈"));
hashSet.add(new EqualsDemo(1, "Java后端技术栈"));
hashSet.add(new EqualsDemo(1, "Java后端技术栈"));
System.out.println(hashSet.size());
}
}
输出结果:3
看源码发现Object中的hashCode
方法是native方法:
public native int hashCode();
那就只能去看虚拟机中的源码了,源码下载地址:
openJDK 7
下载地址1:http://download.java.net/openjdk/jdk7
网上找找比什么盘,CSDN
上都可以找到,并且下载的快,自己看着办咯。
进入openjdk\jdk\src\share\classes\java\lang
目录下,可以看到 Object.java
源码,打开
/*
* Class: java_lang_Object
* Method: hashCode
* Signature: ()I
*/
JNIEXPORT jint JNICALL Java_java_lang_Object_hashCode
(JNIEnv *, jobject);
打开openjdk\jdk\src\share\native\java\lang\
目录,查看Object.c文件,可以看到hashCode()
的方法被注册成有JVM_IHashCode
方法指针来处理:
#include <stdio.h>
#include <signal.h>
#include <limits.h>
#include "jni.h"
#include "jni_util.h"
#include "jvm.h"
#include "java_lang_Object.h"
static JNINativeMethod methods[] = {
//hashcode的方法指针JVM_IHashCode
{"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_IHashCode
方法指针在 openjdk\hotspot\src\share\vm\prims\jvm.cpp
中定义,如下:
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
如上可以看出,JVM_IHashCode
方法中调用了ObjectSynchronizer::FastHashCode
方法
ObjectSynchronizer::fashHashCode()
方法在 openjdk\hotspot\src\share\vm\runtime\synchronizer.cpp
文件中实现。
// hashCode() generation :
//
// Possibilities:
// * MD5Digest of {obj,stwRandom}
// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
// * A DES- or AES-style SBox[] mechanism
// * One of the Phi-based schemes, such as:
// 2654435761 = 2^32 * Phi (golden ratio)
// HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
// * A variation of Marsaglia's shift-xor RNG scheme.
// * (obj ^ stwRandom) is appealing, but can result
// in undesirable regularity in the hashCode values of adjacent objects
// (objects allocated back-to-back, in particular). This could potentially
// result in hashtable collisions and reduced hashtable efficiency.
// There are simple ways to "diffuse" the middle address bits over the
// generated hashCode values:
//
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 = 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 = 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;
}
// ObjectSynchronizer::FastHashCode方法的实现,该方法最终会返回我们期望已久的hashcode
intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
if (UseBiasedLocking) {
// NOTE: many places throughout the JVM do not expect a safepoint
// to be taken here, in particular most operations on perm gen
// objects. However, we only ever bias Java instances and all of
// the call sites of identity_hash that might revoke biases have
// been checked to make sure they can handle a safepoint. The
// added check of the bias pattern is to avoid useless calls to
// thread-local storage.
if (obj->mark()->has_bias_pattern()) {
// Box and unbox the raw reference just in case we cause a STW safepoint.
Handle hobj (Self, obj) ;
// Relaxing assertion for bug 6320749.
assert (Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(),
"biases should not be seen by VM thread here");
BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
obj = hobj() ;
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
}
// hashCode() is a heap mutator ...
// Relaxing assertion for bug 6320749.
assert (Universe::verify_in_progress() ||
!SafepointSynchronize::is_at_safepoint(), "invariant") ;
assert (Universe::verify_in_progress() ||
Self->is_Java_thread() , "invariant") ;
assert (Universe::verify_in_progress() ||
((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
ObjectMonitor* monitor = NULL;
markOop temp, test;
intptr_t hash;
markOop mark = ReadStableMark (obj);
// object should remain ineligible for biased locking
assert (!mark->has_bias_pattern(), "invariant") ;
if (mark->is_neutral()) {
hash = mark->hash(); // this is a normal header
if (hash) { // if it has hash, just return it
return hash;
}
hash = get_next_hash(Self, obj); // allocate a new hash code
temp = mark->copy_set_hash(hash); // merge the hash code into header
// use (machine word version) atomic operation to install the hash
test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
if (test == mark) {
return hash;
}
// If atomic operation failed, we must inflate the header
// into heavy weight monitor. We could add more code here
// for fast path, but it does not worth the complexity.
} else if (mark->has_monitor()) {
monitor = mark->monitor();
temp = monitor->header();
assert (temp->is_neutral(), "invariant") ;
hash = temp->hash();
if (hash) {
return hash;
}
// Skip to the following code to reduce code size
} else if (Self->is_lock_owned((address)mark->locker())) {
temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
assert (temp->is_neutral(), "invariant") ;
hash = temp->hash(); // by current thread, check if the displaced
if (hash) { // header contains hash code
return hash;
}
// WARNING:
// The displaced header is strictly immutable.
// It can NOT be changed in ANY cases. So we have
// to inflate the header into heavyweight monitor
// even the current thread owns the lock. The reason
// is the BasicLock (stack slot) will be asynchronously
// read by other threads during the inflate() function.
// Any change to stack may not propagate to other threads
// correctly.
}
// 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;
}
以上这么多代码的核心部分:
mark=monitor->header;
asset(mark->is_neutral(),"invariant");
hash=mark->hash();
intptr_t hash() const{
return mask_bits(value()>>hash_shift,hash_mask);
}
这里也就印证了:hashCode
就是根据对象的地址进行相关计算得到int类型数值的。
上面EqualsDemo
没有写hashCode
,所以导致最后的结果是3,如果不想存储重复的元素,那么需要在EqualsDemo
类中重写hashCode
方法,代码如下:
@Override
public int hashCode() {
return id+ name.hashCode();
}
最后运行Test类,得出的结果:1
因为上面的name是String类型,并且String类重写了hashCode
方法了,所以这里就直接使用了。
equals的实现方式与类的具体业务逻辑有关,但又各不相同,因而应尽量分享源码来确定其判断结果,比如下面的代码:
public class MyListDemo {
public static void main(String[] args) {
LinkedList<Integer> linkedList = new LinkedList<>();
linkedList.add(1);
ArrayList<Integer> arrayList = new ArrayList<>();
arrayList.add(1);
if (linkedList.equals(arrayList)) {
System.out.println("equal");
} else {
System.out.println("not equal");
}
}
}
输出:equal
两个不同的集合,结果输出相等。让我们来看看这两个集合的equals方法是怎么实现的:
看其源码发现两个集合都是使用AbstractList
中的equals方法(JDK
版本是1.8),每个版本可能有差别。
面试题
两个对象的equals为true,则两个对象的hashCode
相等。
两个对象的hashCode
相等,两个对象的equals不一定为true。