我有一个HashMap，其中键是一个类，值是一个整数。我需要检查该类的对象是否已存在于地图中。我使用containsKey()，但由于某些原因，当我在sideDish和equals()中包含属性hashCode()时，它不起作用。这是我的类代码：
OrderItem类：
@ToString
@Entity
@Table(name="OrderItem")
public class OrderItem implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Getter @Setter
private Long id;
@ManyToOne
@Getter @Setter
private Food food;
@ManyToMany
@Getter @Setter
private List<SideDish> sideDishes;
public OrderItem() {}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((food == null) ? 0 : food.hashCode());
result = prime * result + ((sideDishes == null) ? 0 : sideDishes.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
OrderItem other = (OrderItem) obj;
if (food == null) {
if (other.food != null)
return false;
} else if (!food.equals(other.food))
return false;
if (sideDishes == null) {
if (other.sideDishes != null)
return false;
} else if (!sideDishes.equals(other.sideDishes))
return false;
return true;
}
}
食品类：
@ToString
@Entity
@Table(name="Food")
public class Food implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Getter @Setter
private Long id;
@Column(nullable = false, unique = true)
@NotNull(message = "Name cannot be null.")
@Getter @Setter
private String name;
@ManyToMany
@Getter @Setter
private List<SideDish> sidedishes;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((foodtype == null) ? 0 : foodtype.hashCode());
result = prime * result + ((id == null) ? 0 : id.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Food other = (Food) obj;
if (foodtype == null) {
if (other.foodtype != null)
return false;
} else if (!foodtype.equals(other.foodtype))
return false;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
SideDish课程：
@Entity
@ToString(exclude= {"id","dishtype"})
@Table(name="SideDish")
public class SideDish implements Serializable, Comparable<SideDish>{
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Getter @Setter
private Long id;
@Getter @Setter
@Column(nullable = false, unique = true)
private String name;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((id == null) ? 0 : id.hashCode());
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
SideDish other = (SideDish) obj;
if (id == null) {
if (other.id != null)
return false;
} else if (!id.equals(other.id))
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}
}
出于某种原因，如果我从sideDish课程中的equals()和hashCode()删除OrderItem属性，则效果非常好。
但我还需要sideDish作为对象标识的一部分进行检查。
以下是我如何使用它：
HashMap<OrderItem, Integer> orderItemsToSend = new HashMap<OrderItem, Integer>();
for (Order order : orders) {
for (OrderItem orderItem : order.getOrderItems()) {
int numSimilarOrders = getNumOfSimilarOrders(orderItem, orders);
if(!orderItemsToSend.containsKey(orderItem)) {
orderItemsToSend.put(orderItem, numSimilarOrders);
}else {
System.out.println("Vec je dodat item koji isti kao: " + orderItem.getFood().getName());
}
}
}
答案 0 :(得分：0)
在OrderItem课程中，hashCode()和equals()都取决于属性List<SideDish> sideDishes。
因此，如果sideDishes发生变化，hashCode()也会发生变化（等等也是如此）。
HashMap同时使用hashCode()和equals()来存储和查找键对象。它使用了一个名为“hash buckets”的概念。如果您将密钥放入HashMap，然后hashCode()更改，则该对象将位于错误的哈希存储区中，您将无法找到它试。
键是用于查找目的的东西 - 这就是“密钥”一词的含义。无论是在数据库中还是在散列映射中，密钥的重要质量都是不可变的。所以在Java中，这意味着更改其hashCode()的对象会导致错误的密钥。
有点像文件系统通过文件名的散列进行查找，但后来你改变了文件名，但它没有更新散列。您只能通过使用旧名称进行查找来找到该文件。
这个简单的测试程序将说明这一点。
我们在HashMap中存储了2个对象，然后更改了hashCode()。地图仍然包含两个对象，但现在无法找到其中一个或用于查找。
解决方案是使用一些简单的不可变对象作为键，例如其数据库ID的Long。
示例输出低于代码。
public class HashTest {
static class Hashable {
String name;
@Override
public int hashCode() {
return ((name == null) ? 0 : name.hashCode());
}
@Override
public boolean equals(Object object) {
return (object instanceof Hashable) && equals((Hashable) object);
}
private boolean equals(Hashable that) {
return Objects.equals(this.name, that.name);
}
@Override
public String toString() {
// Use identityHashCode() so we can really see which object is which
return "[" + name + ":" + System.identityHashCode(this) + "]";
}
}
public static void main(String[] args) {
Hashable one = new Hashable();
one.name = "one";
Hashable two = new Hashable();
two.name = "one";
print(one, two);
two.name = "two";
print(one, two);
HashMap<Hashable, Integer> map = new HashMap<>();
map.put(one, 1);
map.put(two, 2);
find(map, one, two);
one.name = "two"; // Let's confuse things
print(one, two);
find(map, one, two);
}
private static void print(Hashable one, Hashable two) {
System.out.print("Names:" + one.name + ":" + two.name);
System.out.print("\tHashcodes:" + one.hashCode() + ":" + two.hashCode());
System.out.println("\tEquals:" + one.equals(two));
}
private static void find(HashMap<Hashable, Integer> map, Hashable one, Hashable two) {
System.out.print(map);
System.out.print("\tFound: " + map.get(one));
System.out.println("\tFound: " + map.get(two));
}
}
示例输出：
Names:one:one Hashcodes:110182:110182 Equals:true
Names:one:two Hashcodes:110182:115276 Equals:false
{[one:366712642]=1, [two:1829164700]=2} Found: 1 Found: 2
Names:two:two Hashcodes:115276:115276 Equals:true
{[two:366712642]=1, [two:1829164700]=2} Found: 2 Found: 2