hashmap | 易学教程

Continuously add data to Map

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问题 I need to add data to a Map or HashMap before a for...loop, add data to the Map during the for...loop and then create the document with all of the data after the loop. In Java for Android I used: Map<String, Object> createDoc = new HashMap<>(); createDoc.put("type", type); createDoc.put("title", title); for (int x = 0; x < sArray.size(); x++) { createDoc.put("data " + x,sArray.get(x)); } firebaseFirestoreDb.collection("WPS").add(createDoc); My question is, how would I create the document and

How to find highest key value from hashmap

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问题 iterate with max key value so that it will replace max string value. first My code is HashMap<String, String> mapp=new HashMap<String, String>(); mapp.put("ab","blue"); mapp.put("abc","black"); mapp.put("abcd","pink"); for (Iterator it = alltyp.iterator(); it.hasNext();) { String finalstring = (String) it.next(); Iterator it1=mapp.entrySet().iterator(); while(it1.hasNext()) { Map.Entry pairs = (Map.Entry) it1.next(); String key_ = (String) pairs.getKey(); String value_ = (String) pairs

Why it is necessary to override hashcode and equals method of key for Hashmap? [duplicate]

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问题 This question already has answers here : What issues should be considered when overriding equals and hashCode in Java? (11 answers) Closed 6 years ago . I created a HashMap having Student as key and String as value. Now everywhere i have read It is necessary to override equals and hashcode method if using as a key for hashmap. But I did not override it. And insert multiple key value pairs in hashmap. I am also able to fetch it back. So why it is necessary? 回答1: They are required when you want

重写HashMap的equals和hashCode方法

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首先定义HashMap的key，这个类中重写equals和hashcode方法： public class Key { String key; public String getKey() { return key; } public void setKey(String key) { this.key = key; } public Key(String key) { this.key = key; } @Override public boolean equals(Object o) { if (this == o) return true; if (o == null || getClass() != o.getClass()) return false; Key key1 = (Key) o; return Objects.equals(key, key1.key); } @Override public int hashCode() { return key.hashCode(); } } 然后创建使用： HashMap<Key,String> map=new HashMap(); Key key=new Key("hyb"); map.put(key,key.getKey()); Key key2=new Key("hyb"); System.out.println

Get a HashSet out of the keys of a HashMap?

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问题 I have a pretty big (100'000s of entries) HashMap . Now, I need a HashSet containing all the keys from this HashMap . Unfortunately, HashMap only has a keySet() method which returns a Set but not a HashSet . What would be an efficient way to generate such a HashSet using Java? 回答1: Why do you specifically need a HashSet? Any Set have the same interface, so typically can be used interchangeably, as good-practices requires that you use the Set interface for all of them. If you really need so,

Understanding HashMap<K,V>

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问题 Ok, here is the bit I do not understand. If you attempt to retrieve an object using the get() method and null is returned, it is still possible that null may be stored as the object associated with the key you supplied to the get() method. You can determine if this is the case by passing your key of the object to containsKey() method for map. This returns true if key is stored in the map So, how is containsKey() supposed to tell me if the value associated with the key supplied is null ? This

How to annotate Map<Entity,INTEGER> with JPA?

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问题 I have this map: Map<Owner, Integer> ownerSharesMap = new HashMap<>(); I know how to map a HashMap with @OneToMany relation that has a primitive type as key, but I googled a lot and didn't find a way to map the above HashMap with JPA. I see two options though: to change my HashMap to this: (in which the uuid is the key column of Owner ) Map<UUID, Integer> ownerIdSharesMap = new HashMap<>(); or create a class like this : public class Share{ int count; Owner owner } then persist this Set: Set

Can i have HashSets as the keys in a HashMap? Suggest an alternative if not

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问题 Edit: explained the problem properly now. I have a hashmap where i want to store sets of words seen together (key) and the lines in which they were seen together(value). This is the structure i came up with: HashMap<HashSet<String>, HashSet<Integer>> hm= ... for inputs: mango, banana, apple apple, banana peach, walrus walrus, peach As I read this, line by line, I make new temporary keys (hashsets not yet inserted into hashmap) from the combination of words in the line. Each temporary key is a

How can I create a JList that contains entries of a Hashtable of String and Object?

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问题 I want to create a JList that contains entries of a Hashtable of String and object : Hashtable<String,Object> the JList element should contain the hashtable entry and display the value of the entry key that is a string ... Is it possible ? How can it be done ? 回答1: Implement the ListModel interface by extending AbstractListModel . Use the derived model to create your JList . See also How to Use Lists. 回答2: Use the keyset of your Hashtable for the data in your JList : Hashtable<String, Object>

整理所学之HashMap | 第一篇

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本文参考： https://blog.csdn.net/yyyljw/article/details/80903391 https://www.jianshu.com/p/a89e9487a06c https://blog.csdn.net/woshimaxiao1/article/details/83661464 https://blog.csdn.net/eaphyy/article/details/84386313 所学浅薄，抛砖引玉。这里会涉及： 1. 哈希表 2. HashMap 的hash算法 static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); } 3. HashMap 查找数组下标为什么是 (n - 1) & hash 哈希表散列表（也叫哈希表），是根据关键码值(Key value)而直接进行访问的数据结构。也就是说，它通过把关键码值映射到表中一个位置来访问记录，以加快查找的速度。这个映射函数叫做散列函数，存放记录的数组叫做散列表。　　来让我们在研究哈希表之前，先看下其他数据结构　　数组：是在内存中存储相同数据类型的物理上连续的空间。特点：查找快，插入、删除等更新操作慢。　　线性链表：