如何使用Java ArrayList删除对象的一个实例

Question

如果我有两个ArrayListlist和toremove，并且我想删除list上与toremove上的对象相等的所有对象。但只有一次(对象的一个实例(。我如何在Java中实现这一点？我是否必须手动循环浏览所有toremove对象，并在list中删除它？

我尝试过.removeAll(toremove);，但它删除了if对象的每个实例。

例如，如果我有book1 book1 book2 book3，而我的toRemove列表是book1 book2。输出将是CCD_ 11。

private List<Book>  removeList(List<Book> initial,List<Book> toremove) {
List<Book> list=initial;
for (Book book : List) {
}
return List;
}

Answer 1

只需使用此：

for (Book b : toremove) {
int index = list.indexOf(b);
if (index != -1) list.remove(index);
}

这将查找对象的索引，然后使用该索引将其删除，只要list中存在该书即可。

当然，正如sc0der所说，您必须重写Book类的equals方法，但我怀疑您已经这样做了，因为removeAll确实可以删除所有对象。

示例输入：

list = new ArrayList<>();
list.add(book1);
list.add(book1);
list.add(book2);
list.add(book1);
list.add(book3);
toremove = Arrays.asList(new Book[]{book1, book2});

示例输出：

list = [book1, book1, book3]

Answer 2

覆盖书本类中的equals()

static class Book {
int id;
String title;
public Book(int id, String title) {
this.id = id;
this.title = title;
}
@Override
public boolean equals(Object obj) {
Book other = (Book) obj;
return this.title.equals(other.title);
}
@Override
public String toString() {
return id + ": " + title;
}
}

并使用remove()删除列表中第一个出现的对象

public static void main(String[] a) {
Book b1 = new Book(1, "book1");
Book b2 = new Book(1, "book1");
Book b3 = new Book(3, "book2");
Book b4 = new Book(4, "book3");
List<Book> list = new ArrayList<>();
list.add(b1);
list.add(b2);
list.add(b3);
list.add(b4);
removeFirstOccurance(list, Arrays.asList(b1, b3));
System.out.println(list);
}
private static void removeFirstOccurance(List<Book> list, List<Book> toRemove) {
for (Book b : toRemove) {
list.remove(b);
}
}

，输出

[1: book1, 4: book3]

Answer 3

您可以按如下方式执行：

private static List<Book> removeList(List<Book> initial, List<Book> toRemove) {
List<Book> toBeReturned = new ArrayList<Book>(initial);
List<Book> deleted = new ArrayList<Book>();
Iterator<Book> itr = toBeReturned.iterator();
while (itr.hasNext()) {
Book a = itr.next();
for (Book b : toRemove) {
if (b.equals(a) && !deleted.contains(b)) {
deleted.add(b);
itr.remove();
}
}
}
return toBeReturned;
}

演示：

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;
class Book {
int id;
public Book(int id) {
this.id = id;
}
@Override
public int hashCode() {
return Objects.hash(id);
}
@Override
public boolean equals(Object obj) {
Book other = (Book) obj;
return this.id == other.id;
}
@Override
public String toString() {
return "Book [id=" + id + "]";
}
}
public class Main {
public static void main(String[] args) {
List<Book> initial = new ArrayList<Book>();
initial.add(new Book(1));
initial.add(new Book(1));
initial.add(new Book(1));
initial.add(new Book(2));
initial.add(new Book(2));
initial.add(new Book(3));
List<Book> toRemove = new ArrayList<Book>();
toRemove.add(new Book(1));
toRemove.add(new Book(2));
List<Book> list = removeList(initial, toRemove);
// Display list
list.stream().forEach(System.out::println);
}
private static List<Book> removeList(List<Book> initial, List<Book> toRemove) {
List<Book> toBeReturned = new ArrayList<Book>(initial);
List<Book> deleted = new ArrayList<Book>();
Iterator<Book> itr = toBeReturned.iterator();
while (itr.hasNext()) {
Book a = itr.next();
for (Book b : toRemove) {
if (b.equals(a) && !deleted.contains(b)) {
deleted.add(b);
itr.remove();
}
}
}
return toBeReturned;
}
}

输出：

Book [id=1]
Book [id=1]
Book [id=2]
Book [id=3]