hashmap

问题 Consider the following: HashMap<String, Object> hm = new HashMap<>(); final String prefix = "My objects "; int counter = 0; void put(Object value) { hm.put(prefix+(counter++), value); } Given that the key of each entry starts with the same string and only differs by a number appended to it, is this likely to lead to more collisions? I'm trying to decide whether this way of creating unique keys is a good idea from a performance perspective. 回答1: No it will not. And that is not necessarily

问题 i'm trying to retrieve data from a hashmap with multiple values for 1 key and set it to a listview,but instead of setting the values into the listview and displaying the listview,all that is displayed is the array(without the key). The code is as follows: ListView lv = (ListView)findViewById(R.id.list); //hashmap of type `HashMap<String, List<String>>` HashMap<String, List<String>> hm = new HashMap<String, List<String>>(); List<String> values = new ArrayList<String>(); for (int i = 0; i < j;

问题 I have a question, if we need to pass custom object as a key, we just need to override equals and hashcode methods or even the class should be immutable? because string is immutable so we prefer it as a key in hashmap, so with that logic i have raised this question ? 回答1: It depends on your hashing function. All data that is being hashed should be immutable. Otherwise you will lose access to your value once you alter one of those fields, until you pass another object which hashes the same way

问题 I have some data at a certain reference in Firebase and I'm retrieving it using addChildEventListener() and putting it in a HashMap<String, Object> like this: final HashMap<String, Object> map1 = new HashMap<>(); ref.child(requestID).child(key).addChildEventListener(new ChildEventListener() { @Override public void onChildAdded(DataSnapshot dataSnapshot, String s) { if (dataSnapshot.getValue() != null) { map1.put(dataSnapshot.getKey(), dataSnapshot.getValue()); Log.d("map1", map1.toString());

问题 In the Java implementation, I found transient Entry[] table; which is initiated in constructor as table = new Entry[capacity]; I know and understand that creating generic array is not allowed but then what I fail to understand is that how the whole thing works. I mean when we do something like HashMap<Integer, String> hMap = new HashMap<Integer, String>(); How does above codes leads to creating an Entry array of type <Integer, String> Well, few people are not able to understand what I am

问题 Here's the code that I have: HashMap<String, String> inst1 = new HashMap(instructorIO.getInstructors()); instListModel = new DefaultListModel<String>(inst1.values()); I get errors: DepartmentProject.java:69: error: constructor DefaultListModel in class DefaultListModel<E> cannot be applied to given types; required: no arguments found: Collection<String> reason: actual and formal argument lists differ in length where E is a type-variable: E extends Object declared in class DefaultListModel Can

问题 (i'm new in Java) ... I want to store some class fields values in a HashMap then write it to a file (the path is passed as an argument) and then restore HashMap and fetch needed info. In my constructor called Carte, i get an exception that file is not found , anyway it's in the right place and saved data is in my xml file. Any ideas on this point An exception has occured : java.io.FileNotFoundException: users/stefan/desktop/lol.xml (No such file or directory) // Salveaza toate obiectele

问题 I am trying to benchmark getting keys from a Rust hash map. I have the following benchmark: #[bench] fn rust_get(b: &mut Bencher) { let (hash, keys) = get_random_hash::<HashMap<String, usize>>(&HashMap::with_capacity, &rust_insert_fn); let mut keys = test::black_box(keys); b.iter(|| { for k in keys.drain(..) { hash.get(&k); } }); } where get_random_hash is defined as: fn get_random_hash<T>( new: &Fn(usize) -> T, insert: &Fn(&mut T, String, usize) -> (), ) -> (T, Vec<String>) { let mut keys =

问题 Here is some sample code...I always seem to get a ClassCastException...anybody point out what I am doing wrong? package com.query; import java.util.ArrayList; import java.util.Collection; import java.util.HashMap; import java.util.Map; import java.util.Set; import java.util.TreeSet; import org.junit.Test; import com.google.common.collect.Sets; public class ClassCastExceptionTest { @Test public void test() { A<SomeType> a1 = new A<SomeType>(1); A<SomeType> a2 = new A<SomeType>(2); A<SomeType>

问题 I have written the below code to find out the key(String) that has the maximum value(Integer) using TreeMap in JAVA. public static void maxprofitItem(int[] costs, int[] prices, int[] sales,String[] items) { TreeMap<String,Integer>map=new TreeMap<String,Integer>(); int[] profits=new int[items.length]; int maxvalue; for(int i=0;i<items.length;i++){ profits[i]=sales[i]*prices[i]-costs[i]*sales[i]; if(profits[i]>0){ map.put(items[i],profits[i]); } } Set setOfKeys = map.keySet(); Iterator iterator