blocks|key|1174091|text|在您的情况下，判断对象的一个特定实例是否包含在HashSet中的唯一方法是迭代HashSet的内容，并比较对象标识(使用==操作符而不是equals()方法)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1174092|类似于：|1174093|boolean+isObjectInSet(Object+object,+Set<?+extends+Object>+set)+{
+++boolean+result+=+false;

+++for(Object+o+:+set)+{
+++++if(o+==+object)+{
+++++++result+=+true;
+++++++break;
+++++}
+++}

+++return+result;
}|code-block|syntax|javascript|1174094|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

In your situation, the only way to tell if a particular instance of an object is contained in the HashSet, is to iterate the contents of the HashSet, and compare the object identities ( using the == operator instead of the equals() method).

Something like: 

<pre><code>boolean isObjectInSet(Object object, Set&lt;? extends Object&gt; set) {
 boolean result = false;

 for(Object o : set) {
 if(o == object) {
 result = true;
 break;
 }
 }

 return result;
}
</code></pre>

blocks|key|1439747|text|检查对象是否是同一个对象的方法是将它们与==进行比较，以查看对象引用是否相等。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1439748|亲切的问候，弗兰克|1439749|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

The way to check if objects are the same object is by comparing them with == to see that the object references are equal.

Kind Greetings,
Frank

blocks|key|1174049|text|您还必须重写hashCode方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1174050|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

You will have to override the hashCode method also.

blocks|key|2676792|text|HashSet+contains使用equals方法来确定是否包含对象，并且在HashSet中不保留重复项。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2676793|假设您的equals和hashcode只使用name字段..。|2676794|HashSet<MyObject>+objectSet+=+new+HashSet<MyObject>();
MyObject+name1Object+=+new+MyObject("name1");

objectSet.add(new+MyObject("name1"));
objectSet.add(name1Object);
objectSet.add(new+MyObject("name2"));
//HashSet+now+contains+2+objects,+name1Object+and+the+new+name2+object
//HashSets+do+not+hold+duplicate+objects+(name1Object+and+the+new+object+with+name1+would+be+considered+duplicates)

objectSet.contains(new+MyObject("name1"))+//+returns+true
objectSet.contains(name1Object)+++++++++++//+returns+true
objectSet.contains(new+MyObject("name2"))+//+returns+true
objectSet.contains(new+MyObject("name3"))+//+returns+false|code-block|syntax|javascript|2676795|如果您想要检查HashSet中的对象是否是您要比较的确切对象，则必须将其提取出来并使用==直接进行比较。|2676796|for+(MyObject+o+:+objectSet)
{
++++if+(o+==+name1Object)
++++{
++++++++return+true;
++++}
}|2676797|如果您对特定对象做了很多这样的操作，那么使用HashMap可能会更容易，因此您不必在列表中迭代以获取特定的命名对象。也许值得为你研究一下，因为这样你就可以做这样的事情：|2676798|(objectMap.get("name")+==+myNameObject)+//+with+a+HashMap<String,+MyNameObject>+where+"name"+is+the+key+string.|2676799|entityMap^0|8|8|0|4|6|B|8|M|4|0|0|17|2|0|0|M|7|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]|$9|14|A|15|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|16|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|17|8|@$9|18|A|19|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|1A|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|S|3|T|5|J|7|1E|8|@]|D|@]|E|$K|L]]|$1|U|3|-4|5|6|7|1F|8|@]|D|@]|E|$]]]|V|$]]

HashSet <code>contains</code> uses the equals method to determine if the object is contained - and duplicates are not kept within the HashSet.

Assuming your <code>equals</code> and <code>hashcode</code> are only using a <code>name</code> field...

<pre><code>HashSet&lt;MyObject&gt; objectSet = new HashSet&lt;MyObject&gt;();
MyObject name1Object = new MyObject("name1");

objectSet.add(new MyObject("name1"));
objectSet.add(name1Object);
objectSet.add(new MyObject("name2"));
//HashSet now contains 2 objects, name1Object and the new name2 object
//HashSets do not hold duplicate objects (name1Object and the new object with name1 would be considered duplicates)

objectSet.contains(new MyObject("name1")) // returns true
objectSet.contains(name1Object) // returns true
objectSet.contains(new MyObject("name2")) // returns true
objectSet.contains(new MyObject("name3")) // returns false
</code></pre>

If you wanted to check if the object in the HashSet is the exact object you are comparing you would have to pull it out and compare it directly using <code>==</code>

<pre><code>for (MyObject o : objectSet)
{
 if (o == name1Object)
 {
 return true;
 }
}
</code></pre>

If you do this alot for specific objects it might be easier to use a <code>HashMap</code> so you don't have to iterate through the list to grab a specific named Object. May be worth looking into for you because then you could do something like this:

<pre><code>(objectMap.get("name") == myNameObject) // with a HashMap&lt;String, MyNameObject&gt; where "name" is the key string.
</code></pre>

blocks|key|1439754|text|试试这个..。只考虑对象的一个属性“名称”来保持唯一性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1439755|@Override
public+int+hashCode()+{
++++final+int+prime+=+31;
++++int+result+=+1;
++++result+=+prime+*+result+%2B+(name+==+null+?+0+:+name.hashCode());
++++return+result;
}

@Override
public+boolean+equals(Object+obj)+{
++++if+(this+==+obj)+{
++++++++return+true;
++++}
++++if+(obj+==+null)+{
++++++++return+false;
++++}
++++if+(getClass()+!=+obj.getClass())+{
++++++++return+false;
++++}
++++User+other+=+(User)+obj;
++++if+(name+==+null)+{
++++++++if+(other.name+!=+null)+{
++++++++++++return+false;
++++++++}
++++}+else+if+(!name.equals(other.name))+{
++++++++return+false;
++++}
++++return+true;
}|code-block|syntax|javascript|1439756|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

try this.. 
Considering only one property 'name' of your Objects to maintain uniqueness.

<pre><code>@Override
public int hashCode() {
 final int prime = 31;
 int result = 1;
 result = prime * result + (name == null ? 0 : name.hashCode());
 return result;
}

@Override
public boolean equals(Object obj) {
 if (this == obj) {
 return true;
 }
 if (obj == null) {
 return false;
 }
 if (getClass() != obj.getClass()) {
 return false;
 }
 User other = (User) obj;
 if (name == null) {
 if (other.name != null) {
 return false;
 }
 } else if (!name.equals(other.name)) {
 return false;
 }
 return true;
}
</code></pre>

blocks|key|1179156|text|我使用重写的equal方法创建了自己的类，该方法只检查名称(类中的属性)是否相等。|type|blockquote|depth|inlineStyleRanges|entityRanges|data|1179157|这违反了.equals的约定，无论它看起来多么方便，您都绝不能这样做。|unstyled|offset|length|1179158|相反，如果您希望通过某个属性(如名称)索引和查找元素，请使用HashMap<Name,+YourType>查找它们。或者，使用TreeSet并传递给它一个只比较名称的比较器。然后可以删除不正确的equals方法。|style|CODE|1179159|如果您想要通过引用相等找到对象，那么有三种方法：|1179160|你的目标没有内在的或有用的平等概念。
不要实现equals。让它保持默认状态。然后，您可以使用HashSet查找引用相等性，并使用HashMap或TreeSet对它们进行任何特定属性的索引。|ordered-list-item|1179161|您的对象确实有一个有用的、通用的平等概念，但无论如何您都希望有效地找到等效实例。
几乎从来都不是这样的。但是，您可以使用例如Apache+IdentityMap。|1179162|你不关心效率。
使用for循环和==每一个元素。|1179163|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#equals(java.lang.Object))|1|https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html|2|https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html|3|http://commons.apache.org/proper/commons-collections/javadocs/api-3.2.1/org/apache/commons/collections/map/IdentityMap.html^0|0|4|7|0|0|U|N|2P|6|1R|7|1|2B|3|2|0|0|N|6|1B|7|0|1X|B|3|0|G|2|0^^$0|@$1|2|3|4|5|6|7|17|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|18|8|@]|9|@$E|19|F|1A|1|1B]]|A|$]]|$1|G|3|H|5|D|7|1C|8|@$E|1D|F|1E|I|J]|$E|1F|F|1G|I|J]]|9|@$E|1H|F|1I|1|1J]|$E|1K|F|1L|1|1M]]|A|$]]|$1|K|3|L|5|D|7|1N|8|@]|9|@]|A|$]]|$1|M|3|N|5|O|7|1O|8|@$E|1P|F|1Q|I|J]|$E|1R|F|1S|I|J]]|9|@]|A|$]]|$1|P|3|Q|5|O|7|1T|8|@]|9|@$E|1U|F|1V|1|1W]]|A|$]]|$1|R|3|S|5|O|7|1X|8|@$E|1Y|F|1Z|I|J]]|9|@]|A|$]]|$1|T|3|-4|5|D|7|20|8|@]|9|@]|A|$]]]|U|$V|$5|W|X|Y|A|$Z|10]]|11|$5|W|X|Y|A|$Z|12]]|13|$5|W|X|Y|A|$Z|14]]|15|$5|W|X|Y|A|$Z|16]]]]

<blockquote>
 I made my own class with an overridden equals method which just checks, if the names (attributes in the class) are equal.
</blockquote>

This breaks the contract of <a href="https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#equals(java.lang.Object)" rel="nofollow">.equals</a>, and you must never do it no matter how convenient it seems. 

Instead, if you want to index and look up elements by a certain attribute such as the name, use a <code>HashMap&lt;Name, YourType&gt;</code> to find them. Alternatively, use a <a href="https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html" rel="nofollow">TreeSet</a> and pass it a <a href="https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html" rel="nofollow">Comparator</a> that compares the name only. You can then remove the incorrect <code>equals</code> method.

There are then three ways if you want to find objects by reference equality:

<ol>
<li>Your objects have no inherent or useful notion of equality.

Don't implement <code>equals</code>. Leave it to its default. You can then use a <code>HashSet</code> to look for reference equality, and a HashMap or TreeSet to index them by any specific attributes.</li>
<li>Your objects do have a useful, universal notion of equality, but you want to find equivalent instances efficiently anyways.

This is almost never the case. However, you can use e.g. an Apache <a href="http://commons.apache.org/proper/commons-collections/javadocs/api-3.2.1/org/apache/commons/collections/map/IdentityMap.html" rel="nofollow">IdentityMap</a>. </li>
<li>You don't care about efficiency. 

Use a for loop and <code>==</code> every element.</li>
</ol>

I made my own class with an overridden equals method which just checks, if the names (attributes in the class) are equal. Now I store some instances of that class in a HashSet so that there are no instances with the same names in the HashSet.

My Question: How is it possible to check if the HashSet contains such an object. .contains() wont work in that case, because it works with the .equals() method. I want to check if it is really the same object.

edit:

<pre><code>package testprogram;

import java.util.HashSet;
import java.util.Set;

public class Example {
 private static final Set&lt;Example&gt; set = new HashSet&lt;Example&gt;();
 private final String name;
 private int example;

 public Example(String name, int example) {
 this.name = name;
 this.example = example;
 set.add(this);
 }

 public boolean isThisInList() {
 return set.contains(this);
 //will return true if this is just equal to any instance in the list
 //but it should not
 //it should return true if the object is really in the list
 }

 public boolean remove() {
 return set.remove(this);
 }

 //Override equals and hashCode
}
</code></pre>

Sorry, my english skills are not very well. Please feel free to ask again if you don't understand what I mean.

Java HashSet contains Object

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我使用重写的equal方法创建了自己的类，该方法只检查名称(类中的属性)是否相等。现在，我将该类的一些实例存储在HashSet中，以便在HashSet中不存在具有相同名称的实例。我的问题是:如何能够检查HashSet是否包含这样的对象。.contains()在这种情况下不能工作，因为它与.equals()方法一起工作。我想检查它是否真的是同一个对象。编辑：package testprogram;i

问HashSet包含对象
EN

回答 6

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问HashSet包含对象
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