collections

问题 I have one million rows of data in .txt format. the format is very simple. For each row: user1,value1 user2,value2 user3,value3 user1,value4 ... You know what I mean. For each user, it could appear many times, or appear only once (you never know). I need to find out all the values for each user. Because user may appear randomly, I used Hashmap to do it. That is: HashMap(key: String, value: ArrayList). But to add data to the arrayList, I have to constantly use HashMap get(key) to get the

问题 Java 9 comes with convenience factory methods for creating immutable lists. Finally a list creation is as simple as: List<String> list = List.of("foo", "bar"); But there are 12 overloaded versions of this method, 11 with 0 to 10 elements, and one with var args. static <E> List<E> of(E... elements) Same is the case with Set and Map . Since there is a var args method, what is the point of having extra 11 methods? What I think is that var-args create an array, so the other 11 methods can skip

问题 I'm new to Mac/iPhone programming and Objective-C. In C# and Java we have "generics", collection classes whose members can only be of the type declared. For example, in C# Dictionary<int, MyCustomObject> can only contain keys that are integers and values that are of type MyCustomObject. Does a similar mechanism exist in Objective-C? 回答1: In Xcode 7, Apple has introduced 'Lightweight Generics' to Objective-C. In Objective-C, they will generate compiler warnings if there is a type mismatch.

问题 I have an array list that contains Quote objects. I want to be able to sort alphabetically by name, by change, and by percent change. How can I sort my arraylist? package org.stocktwits.model; import java.io.Serializable; import java.text.DecimalFormat; public class Quote implements Serializable { private static final long serialVersionUID = 1L; public String symbol; public String name; public String change; public String percentChange; public String open; public String daysHigh; public

问题 I have an array list that contains Quote objects. I want to be able to sort alphabetically by name, by change, and by percent change. How can I sort my arraylist? package org.stocktwits.model; import java.io.Serializable; import java.text.DecimalFormat; public class Quote implements Serializable { private static final long serialVersionUID = 1L; public String symbol; public String name; public String change; public String percentChange; public String open; public String daysHigh; public

问题 How can I find an object, Carnet , in a ArrayList<Carnet> knowing its property codeIsin . List<Carnet> listCarnet = carnetEJB.findAll(); public class Carnet { private String codeTitre; private String nomTitre; private String codeIsin; // Setters and getters } 回答1: You can't without an iteration. Option 1 Carnet findCarnet(String codeIsIn) { for(Carnet carnet : listCarnet) { if(carnet.getCodeIsIn().equals(codeIsIn)) { return carnet; } } return null; } Option 2 Override the equals() method of

问题 Here's a nice pitfall I just encountered. Consider a list of integers: List<Integer> list = new ArrayList<Integer>(); list.add(5); list.add(6); list.add(7); list.add(1); Any educated guess on what happens when you execute list.remove(1) ? What about list.remove(new Integer(1)) ? This can cause some nasty bugs. What is the proper way to differentiate between remove(int index), which removes an element from given index and remove(Object o), which removes an element by reference, when dealing

问题 Here's a nice pitfall I just encountered. Consider a list of integers: List<Integer> list = new ArrayList<Integer>(); list.add(5); list.add(6); list.add(7); list.add(1); Any educated guess on what happens when you execute list.remove(1) ? What about list.remove(new Integer(1)) ? This can cause some nasty bugs. What is the proper way to differentiate between remove(int index), which removes an element from given index and remove(Object o), which removes an element by reference, when dealing

问题 This question already has answers here : How can I get the size of an array, a Collection, or a String in Java? (2 answers) Closed 5 years ago . import java.util.*; class Ball{ public static void main(String args[]) { ArrayList al = new ArrayList(); al.add(new Integer(1)); al.add(new Integer(2)); al.add(new Integer(3)); Object a[]= al.toArray(); for (int i=0; i<a.length; i++) { System.out.println("Contents are :"+ a[i]); } }} So I created an ArrayList and added a couple of elements to it.

问题 Closed. This question is off-topic. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 6 years ago . How can we implement a compare method that compares Employee s by their employee Id? public int compare(Employee emp1, Employee emp2) { throw new UnsupportedOperationException("Not supported yet."); if(emp1.getEmpid()<emp2.getEmpid()) { return -1; } else if(emp1.getEmpid()>emp2.getEmpid()) return 1; else return