Java TreeSet:remove和contains()不起作用

Question

我已经向TreeSet添加了一些简单的对象,但是当我调用TreeSet的remove()和contains()方法时,它们不起作用.但是,当我遍历集合时,对象被打印出来.当对象唯一性基于对象名称属性时,应将Employee对象添加到集合中.Id属性是应该排序的值,但不是唯一的.

public class Employee {
    private String name;
    private int id;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
    return name;
    }

    public void setName(String name) {
    this.name = name;
    }

 // Two objects are considered equal if their names are equal
    @Override
    public boolean equals(Object o) {
    if (o == null)
        return false;
    if (this == o)
        return true; 
    if (o.getClass() == this.getClass()) {
        Employee p = ( Employee) o;
        if (p.getName() != null && this.getName() != null)
        return this.getName().equals(p.getName());
        else
        return false;
    } else {
        return false;
    }
    }
} 

//*******************************************************

public class EmployeeComp implements Comparator<Employee> {

    // Sort Ids, but allow duplicates, hence this function is never returning 0
    @Override
    public int compare(Employee p1, Employee p2) {
    int re = 0;

    boolean scoreLt = (p1.getId() > p2.getId());
    boolean scoreGt = (p1.getId() < p2.getId());

    if(scoreLt)
        re = -1;
    if(scoreGt)
        re = 1;
    else 
        re = -1;                       
         return re;                 
    }    
}
//*******************************************************
// Collection shall store unique names with ordered values like:
// Alex, 923
// Toni, 728
// Eddi, 232
// Peter, 232
// Eddi, 156  *** not allowed
import java.util.TreeSet;


public class Main {
    private static EmployeeComp comp = new EmployeeComp(); 
    private static TreeSet<Employee> employees = new TreeSet<Employee>(comp); 

    public static void main(String[] args) {

    Employee p1 = new Employee();
    p1.setName("Eddi");
    p1.setId(232);

    Employee p2 = new Employee();
    p2.setName("Toni");
    p2.setId(728);

    Employee p3 = new Employee();
    p3.setName("Peter");
    p3.setId(232);

    Employee p4 = new Employee();
    p4.setName("Alex");
    p4.setId(923);

    employees.add(p1);
    employees.add(p2);
    employees.add(p3);
    employees.add(p4);

    // Here, contains() and remove() should check the object address
    // and not perform their actions based on compareTo

       } 
}

Answer 1

一TreeSet,插入/移除根据的结果Comparable,而不是.equals()/ .hashCode()!

这意味着,顺便说一句,你的对象Set确实实现了Comparable(如果他们没有,每次你尝试并插入一个成员时,你都会受到a ClassCastException)的欢迎.

更准确,TreeSet是一个实现SortedSet.

如果你想要一个.equals()/ .hashCode()-compatible集,请使用例如a HashSet.

为了说明,这里发生了什么BigDecimal(几个小时前在这里发布):

final BigDecimal one = new BigDecimal("1");
final BigDecimal oneDotZero = new BigDecimal("1.0");

final Set<BigDecimal> hashSet = new HashSet<>();
// BigDecimal implements Comparable of itself, so we can use that
final Set<BigDecimal> treeSet = new TreeSet<>();

hashSet.add(one);
hashSet.add(oneDotZero);
// hashSet's size is 2: one.equals(oneDotZero) == false

treeSet.add(one);
treeSet.add(oneDotZero);
// treeSet's size is... 1! one.compareTo(oneDotZero) == 0

引述的Javadoc Comparable,这意味着BigDecimal的.compareTo()是'不相一致.equals()’.

**编辑**关于OP想要什么:

• a Collection不接受重复名称;
• 排序视图,Collection将根据用户的ID进行排序.

如上所述,你不能拥有一个同时兼顾两者的集合.解决方案:

• 对于第一个,a HashSet;
• 对于第二个,将该副本设置为a ArrayList,然后使用Collections.sort().

这意味着.equals()并且.hashCode()必须仅对名称起作用,而自定义Comparator将作用于id.除了Comparator自定义之外别无选择,因为它是一个.equals()在任何情况下都不一致的比较器.

至于提议的代码,存在问题.

第一:Employee覆盖.equals()但不是.hashCode().因此,Employee打破.equals()合同(其中一部分是如果两个对象相等,则它们必须具有相同的哈希码).更.hashCode()重要的是,HashSet对于工作至关重要.固定:

@Override
public int hashCode()
{
    return name == null ? 0 : name.hashCode();
}

@Override
public boolean equals(final Object obj)
{
    if (obj == null)
        return false;
    if (this == obj)
        return false;
    if (!(obj instanceof Employee))
        return false;
    final Employee other = (Employee) obj;
    return name == null ? other.name == null
        : name.equals(other.name);
}

第二:比较器同样破坏,Employee因为它打破了Comparator合同(对于任何o1和o2,o1.compareTo(o2) == - o2.compareTo(o1)).固定:

public final class EmployeeComp
    implements Comparator<Employee>
{
    @Override
    public int compare(final Employee o1, final Employee o2)
    {
        final int id1 = o1.getId(), id2 = o2.getId();
        if (id1 == id2)
            return 0;
        return id1 > id2 ? 1 : -1;
    }
}

然后,如何获取该集的排序副本:

// "set" contains the unique employees
final List<Employee> sorted = new ArrayList<Employee>(set);
Collections.sort(list, new EmployeeComp());

DONE.