hashmap | 易学教程

Java HashMap get works but containsKey does not

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问题 I am trying to locate a key in a HashMap. I can print the selected key by using 'get' but when I use 'containsKey' in an if statement, it is not found. I KNOW the key is present in the Map but it keeps returning false. Any ideas people? My code: public static boolean checkLowerStructuralSupport(Location location) { boolean hasSupport = false; Location supportingLocation = new Location(location.getX(), location.getY(), location.getZ() - 1); System.out.println(_levels.get(supportingLocation

Comparing two hashmaps for equal values and same key sets?

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问题 How can I best compare two HashMap s, if I want to find out if none of them contains different keys than the other, and if the values of that keys match each other. Map<objA, objB> mapA = new HashMap<objA, objB>(); mapA.put("A", "1"); mapA.put("B", "2"); Map<objA, objB> mapB = new HashMap<objA, objB>(); mapB.put("D", "4"); mapB.put("A", "1"); When comparing A with B, it should fail due to different keys B and D. How could I best compare non-sorted hashmaps? 回答1: Make an equals check on the

How HashTable and HashMap key-value are stored in the memory?

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问题 I understand there is a hashing technique is applied to a key to store its value in the memory address. But I don't understand how the collision is happening here? Which hashing algorithm does Java use to create a memory space ? Is it MD5? 回答1: The basic idea of HashMap is this: A HashMap is really an array of special objects that hold both Key and Value. The array has some amount of buckets (slots), say 16. The hashing algorithm is provided by the hashCode() method that every object has.

map vs. hash_map in C++

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问题 I have a question with hash_map and map in C++. I understand that map is in STL, but hash_map is not a standard. What's the difference between the two? 回答1: They are implemented in very different ways. hash_map ( unordered_map in TR1 and Boost; use those instead) use a hash table where the key is hashed to a slot in the table and the value is stored in a list tied to that key. map is implemented as a balanced binary search tree (usually a red/black tree). An unordered_map should give slightly

HashSet原理与 linkedHashSet

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http://blog.csdn.net/guoweimelon/article/details/50804799 HashSet是Java Map类型的集合类中最常使用的，本文基于Java1.8，对于HashSet的实现原理做一下详细讲解。（Java1.8源码： http://docs.oracle.com/javase/8/docs/api/ ）一、HashSet实现原理总结 HashSet的实现原理总结如下： ①是基于HashMap实现的，默认构造函数是构建一个初始容量为16，负载因子为0.75 的HashMap。封装了一个 HashMap 对象来存储所有的集合元素，所有放入 HashSet 中的集合元素实际上由 HashMap 的 key 来保存，而 HashMap 的 value 则存储了一个 PRESENT，它是一个静态的 Object 对象。 ②当我们试图把某个类的对象当成 HashMap的 key，或试图将这个类的对象放入 HashSet 中保存时，重写该类的 equals(Object obj) 方法和 hashCode() 方法很重要，而且这两个方法的返回值必须保持一致：当该类的两个的 hashCode() 返回值相同时，它们通过 equals() 方法比较也应该返回 true。通常来说，所有参与计算 hashCode() 返回值的关键属性，都应该用于作为

How to persist a HashMap with hibernate

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问题 Hello I am very new to the hibernate world and seem to have hit a roadblock. The object I need to store has a hashmap in it. private Map<String, SentimentFrequencyCounts> modelData = null; The thing is I will never need to search, sort or do any thing with this map I just need to save it with the object and load it when the object is loaded, so I was hoping there was some way that hibernate could just serializes it and then store it in a CLOB or BLOB field but I can not seem to find any way

How can I use a HashMap with f64 as key in Rust?

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问题 I want to use a HashMap<f64, f64> , for saving the distances of a point with known x and key y to another point. f64 as value shouldn't matter here, the focus should be on key. let mut map = HashMap<f64, f64>::new(); map.insert(0.4, f64::hypot(4.2, 50.0)); map.insert(1.8, f64::hypot(2.6, 50.0)); ... let a = map.get(&0.4).unwrap(); As f64 is neither Eq nor Hash , but only PartialEq , f64 is not sufficient as a key. I need to save the distances first, but also access the distances later by y.

Explain the timing causing HashMap.put() to execute an infinite loop

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问题 As a number of people have noted and encountered HashMap.put can go into an infinite execution loop when used concurrently (see GRIZZLY-1207, JGRP-525, possibly HHH-6414, and this SO answer). HashMap is clearly documented as not thread safe. Obviously, the correct fix is to use a thread-safe implementation of Map , ConncurrentHashMap in particular. I'm more curious about the concurrent timing that causes the infinite loop. I encountered this loop recently with a Java 7 JRE and would like to

HashMap to ListView

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问题 I have HashMap, how can I to put it in ListView? Which adapter need to use? public void showCinemas(HashMap<String, String> cinemas) { ...//What? list.setAdapter(adapter); } 回答1: Make simple adapter class: MyAdapter.java import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.BaseAdapter; import android.widget.TextView; import java.util.ArrayList; import java.util.Map; public class MyAdapter extends BaseAdapter { private final

How safe are Golang maps for concurrent Read/Write operations?

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问题 According to the Go blog, Maps are not safe for concurrent use: it's not defined what happens when you read and write to them simultaneously. If you need to read from and write to a map from concurrently executing goroutines, the accesses must be mediated by some kind of synchronization mechanism. (source: https://blog.golang.org/go-maps-in-action) Can anyone elaborate on this? Concurrent read operations seem permissible across routines, but concurrent read/write operations may generate a