Removing from Java ArrayList without iteration

问题

I am working on an assignment, and I can't find a way to make this work properly. I have an ArrayList with information on 5 students. I need to construct a method that will remove one of the objects, but I do not need/want iteration, as it is a static removal. I have the following code:

import java.util.ArrayList;
import java.lang.String; 

public class Roster {
static ArrayList<Student> myRoster = new ArrayList<>();

public static void main(String[] args){

//Add Student Data
    Roster.add("1","John","Smith");
    add("2","Suzan","Erickson");
    add("3","Jack","Napoli");
    add("4","Erin","Black");
    add("5","Jason","Rapp");

remove("3");
remove("3");

}

public static void add(String studentID, String fName, String lName){
    Student newStudent = new Student(studentID, fName, lName);
myRoster.add(newStudent);
}

public static void remove(String studentID){
    if (myRoster.contains(studentID)){
        Roster.remove(studentID);
    }
    else {
        System.out.println("This student ID does not exist.");
    }
}

When I compile the code, none of the entries are removed, and the else statement fires off for both remove calls.

回答1:

myRoster is a List of Student objects. List contains and remove will work against only like objects, not String values - it has no idea how to match a String against a Student.

As an alternative, you could make use of List's Stream support and filter the List for elements which match the studentID and then remove all those elements, for example...

public static void remove(String studentID) {
    List<Student> matches = myRoster.stream().filter(student -> student.getStudentID().equals(studentID)).collect(Collectors.toList());
    myRoster.removeAll(matches);
}

but I do not need/want iteration

Just so you understand, there is still an iteration been performed, it's just that you're not doing it yourself, the API is

回答2:

It sounds as though perhaps you should be using a map rather than a list, e.g.

Map<Integer, Student> sMap = new HashMap<>();
sMap.put(1, new Student("1", "John","Smith"));
sMap.put(2, new Student("2", "Suzan","Erickson"));
sMap.put(3, new Student("3", "Jack","Napoli"));
sMap.put(4, new Student("4", "Erin","Black"));
sMap.put(5, new Student("5", "Jason","Rapp"));

Then, if you need to remove a student from the map, you can use the following one liner:

sMap.remove(3);

回答3:

ArrayList.contains(Object) and .remove(Object) uses Object.equals to iterate the whole list and find the object. So

If you insist using ArrayList, you should override the equals method of Student, and the hashCode method too, making them use studentId only. You should also modify the Roster.remove method, making it call myRoster.contains instead of calling itself again.
Below the surface it still is iterate.

回答4:

Going off of MadProgrammer's answer, there is no reason to open your own stream to collect students to remove when you can call Collection's removeIf

myRoster.removeIf(s -> studentID.equals(s.getstudentID()));

来源：https://stackoverflow.com/questions/49524813/removing-from-java-arraylist-without-iteration

标签

java

arraylist