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小編給大家分享一下java中為什么重寫(xiě)equals時(shí)必須重寫(xiě)hashCode方法，相信大部分人都還不怎么了解，因此分享這篇文章給大家參考一下，希望大家閱讀完這篇文章后大有收獲，下面讓我們一起去了解一下吧！
創(chuàng)新互聯(lián)公司一直在為企業(yè)提供服務(wù)，多年的磨煉，使我們?cè)趧?chuàng)意設(shè)計(jì)，成都營(yíng)銷(xiāo)網(wǎng)站建設(shè)到技術(shù)研發(fā)擁有了開(kāi)發(fā)經(jīng)驗(yàn)。我們擅長(zhǎng)傾聽(tīng)企業(yè)需求，挖掘用戶(hù)對(duì)產(chǎn)品需求服務(wù)價(jià)值，為企業(yè)制作有用的創(chuàng)意設(shè)計(jì)體驗(yàn)。核心團(tuán)隊(duì)擁有超過(guò)十年以上行業(yè)經(jīng)驗(yàn)，涵蓋創(chuàng)意，策化，開(kāi)發(fā)等專(zhuān)業(yè)領(lǐng)域，公司涉及領(lǐng)域有基礎(chǔ)互聯(lián)網(wǎng)服務(wù)達(dá)州托管服務(wù)器、app軟件開(kāi)發(fā)公司、手機(jī)移動(dòng)建站、網(wǎng)頁(yè)設(shè)計(jì)、網(wǎng)絡(luò)整合營(yíng)銷(xiāo)。
先讓我們來(lái)看看Object類(lèi)源碼
/**
* 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}.
*
* The general contract of {@code hashCode} is:
*
-
*
- Whenever it is invoked on the same object more than once during * an execution of a Java application, the {@code hashCode} method * must consistently return the same integer, provided no information * used in {@code equals} comparisons on the object is modified. * This integer need not remain consistent from one execution of an * application to another execution of the same application. *
- If two objects are equal according to the {@code equals(Object)} * method, then calling the {@code hashCode} method on each of * the two objects must produce the same integer result. *
- It is not required that if two objects are unequal * according to the {@link java.lang.Object#equals(java.lang.Object)} * method, then calling the {@code hashCode} method on each of the * two objects must produce distinct integer results. However, the * programmer should be aware that producing distinct integer results * for unequal objects may improve the performance of hash tables. *
* 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();
/** * Indicates whether some other object is "equal to" this one. ** The {@code equals} method implements an equivalence relation * on non-null object references: *
-
*
- It is reflexive: for any non-null reference value * {@code x}, {@code x.equals(x)} should return * {@code true}. *
- It is symmetric: for any non-null reference values * {@code x} and {@code y}, {@code x.equals(y)} * should return {@code true} if and only if * {@code y.equals(x)} returns {@code true}. *
- It is transitive: for any non-null reference values * {@code x}, {@code y}, and {@code z}, if * {@code x.equals(y)} returns {@code true} and * {@code y.equals(z)} returns {@code true}, then * {@code x.equals(z)} should return {@code true}. *
- It is consistent: for any non-null reference values * {@code x} and {@code y}, multiple invocations of * {@code x.equals(y)} consistently return {@code true} * or consistently return {@code false}, provided no * information used in {@code equals} comparisons on the * objects is modified. *
- For any non-null reference value {@code x}, * {@code x.equals(null)} should return {@code false}. *
* The {@code equals} method for class {@code Object} implements * the most discriminating possible equivalence relation on objects; * that is, for any non-null reference values {@code x} and * {@code y}, this method returns {@code true} if and only * if {@code x} and {@code y} refer to the same object * ({@code x == y} has the value {@code true}). *
* Note that it is generally necessary to override the {@code hashCode} * method whenever this method is overridden, so as to maintain the * general contract for the {@code hashCode} method, which states * that equal objects must have equal hash codes. * * @param obj the reference object with which to compare. * @return {@code true} if this object is the same as the obj * argument; {@code false} otherwise. * @see #hashCode() * @see java.util.HashMap */ public boolean equals(Object obj) { return (this == obj); }
hashCode：是一個(gè)native方法，返回的是對(duì)象的內(nèi)存地址，
equals：對(duì)于基本數(shù)據(jù)類(lèi)型，==比較的是兩個(gè)變量的值。對(duì)于引用對(duì)象，==比較的是兩個(gè)對(duì)象的地址。
接下來(lái)我們看下hashCode的注釋
1.在 Java 應(yīng)用程序執(zhí)行期間，在對(duì)同一對(duì)象多次調(diào)用 hashCode 方法時(shí)，必須一致地返回相同的整數(shù)，前提是將對(duì)象進(jìn)行 equals 比較時(shí)所用的信息沒(méi)有被修改。
從某一應(yīng)用程序的一次執(zhí)行到同一應(yīng)用程序的另一次執(zhí)行，該整數(shù)無(wú)需保持一致。
2.如果根據(jù) equals(Object) 方法，兩個(gè)對(duì)象是相等的，那么對(duì)這兩個(gè)對(duì)象中的每個(gè)對(duì)象調(diào)用 hashCode 方法都必須生成相同的整數(shù)結(jié)果。
3.如果根據(jù) equals(java.lang.Object) 方法，兩個(gè)對(duì)象不相等，那么兩個(gè)對(duì)象不一定必須產(chǎn)生不同的整數(shù)結(jié)果。
但是，程序員應(yīng)該意識(shí)到，為不相等的對(duì)象生成不同整數(shù)結(jié)果可以提高哈希表的性能。
從hashCode的注釋中我們看到，hashCode方法在定義時(shí)做出了一些常規(guī)協(xié)定，即
1，當(dāng)obj1.equals(obj2) 為 true 時(shí)，obj1.hashCode() == obj2.hashCode()
2，當(dāng)obj1.equals(obj2) 為 false 時(shí)，obj1.hashCode() != obj2.hashCode()
hashcode是用于散列數(shù)據(jù)的快速存取，如利用HashSet/HashMap/Hashtable類(lèi)來(lái)存儲(chǔ)數(shù)據(jù)時(shí)，都是根據(jù)存儲(chǔ)對(duì)象的hashcode值來(lái)進(jìn)行判斷是否相同的。如果我們將對(duì)象的equals方法重寫(xiě)而不重寫(xiě)hashcode，當(dāng)我們?cè)俅蝞ew一個(gè)新的對(duì)象的時(shí)候，equals方法返回的是true，但是hashCode方法返回的就不一樣了，如果需要將這些對(duì)象存儲(chǔ)到結(jié)合中（比如：Set，Map ...）的時(shí)候就違背了原有集合的原則，下面讓我們通過(guò)一段代碼看下。
/**
* @see Person
* @param args
*/
public static void main(String[] args)
{
HashMap map = new HashMap();
Person p = new Person("jack",22,"男");
Person p1 = new Person("jack",22,"男");
System.out.println("p的hashCode:"+p.hashCode());
System.out.println("p1的hashCode:"+p1.hashCode());
System.out.println(p.equals(p1));
System.out.println(p == p1);
map.put(p,888);
map.put(p1,888);
map.forEach((key,val)->{
System.out.println(key);
System.out.println(val);
});
} equals和hashCode方法的都不重寫(xiě)
public class Person
{
private String name;
private int age;
private String sex;
Person(String name,int age,String sex){
this.name = name;
this.age = age;
this.sex = sex;
}
}p的hashCode:356573597 p1的hashCode:1735600054 false false com.blueskyli.練習(xí).Person@677327b6 com.blueskyli.練習(xí).Person@1540e19d
只重寫(xiě)equals方法
public class Person
{
private String name;
private int age;
private String sex;
Person(String name,int age,String sex){
this.name = name;
this.age = age;
this.sex = sex;
}
@Override public boolean equals(Object obj)
{
if(obj instanceof Person){
Person person = (Person)obj;
return name.equals(person.name);
}
return super.equals(obj);
}
}p的hashCode:356573597 p1的hashCode:1735600054 true false com.blueskyli.練習(xí).Person@677327b6 com.blueskyli.練習(xí).Person@1540e19d
equals和hashCode方法都重寫(xiě)
public class Person
{
private String name;
private int age;
private String sex;
Person(String name,int age,String sex){
this.name = name;
this.age = age;
this.sex = sex;
}
@Override public boolean equals(Object obj)
{
if(obj instanceof Person){
Person person = (Person)obj;
return name.equals(person.name);
}
return super.equals(obj);
}
@Override public int hashCode()
{
return name.hashCode();
}
}p的hashCode:3254239 p1的hashCode:3254239 true false com.blueskyli.練習(xí).Person@31a7df
我們知道m(xù)ap是不允許存在相同的key的，由上面的代碼可以知道，如果不重寫(xiě)equals和hashCode方法的話(huà)會(huì)使得你在使用map的時(shí)候出現(xiàn)與預(yù)期不一樣的結(jié)果，具體equals和hashCode如何重寫(xiě)，里面的邏輯如何實(shí)現(xiàn)需要根據(jù)現(xiàn)實(shí)當(dāng)中的業(yè)務(wù)來(lái)規(guī)定。
總結(jié)：
1，兩個(gè)對(duì)象，用==比較比較的是地址，需采用equals方法（可根據(jù)需求重寫(xiě)）比較。
2，重寫(xiě)equals()方法就重寫(xiě)hashCode()方法。
3，一般相等的對(duì)象都規(guī)定有相同的hashCode。
4，String類(lèi)重寫(xiě)了equals和hashCode方法，比較的是值。
5，重寫(xiě)hashcode方法為了將數(shù)據(jù)存入HashSet/HashMap/Hashtable（可以參考源碼有助于理解）類(lèi)時(shí)進(jìn)行比較
以上是“java中為什么重寫(xiě)equals時(shí)必須重寫(xiě)hashCode方法”這篇文章的所有內(nèi)容，感謝各位的閱讀！相信大家都有了一定的了解，希望分享的內(nèi)容對(duì)大家有所幫助，如果還想學(xué)習(xí)更多知識(shí)，歡迎關(guān)注創(chuàng)新互聯(lián)行業(yè)資訊頻道！
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文章起源：http://fisionsoft.com.cn/article/ispocs.html
