Java HashMap containsKey 为现有对象返回 false

Java HashMap containsKey returns false for existing object(Java HashMap containsKey 为现有对象返回 false)

问题描述

我有一个用于存储对象的 HashMap:

I have a HashMap for storing objects:

    private Map<T, U> fields = Collections.synchronizedMap(new HashMap<T, U>());

但是，当尝试检查密钥是否存在时，containsKey 方法返回 false.
实现了equals和hashCode方法，但是没有找到key.
调试一段代码时:

but, when trying to check existence of a key, containsKey method returns false.
equals and hashCode methods are implemented, but the key is not found.
When debugging a piece of code:

    return fields.containsKey(bean) && fields.get(bean).isChecked();

我有:

   bean.hashCode() = 1979946475 
   fields.keySet().iterator().next().hashCode() = 1979946475    
   bean.equals(fields.keySet().iterator().next())= true 
   fields.keySet().iterator().next().equals(bean) = true

但是

fields.containsKey(bean) = false

什么会导致这种奇怪的行为?

What could cause such strange behavioure?

public class Address extends DtoImpl<Long, Long> implements Serializable{

   <fields>
   <getters and setters>

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + StringUtils.trimToEmpty(street).hashCode();
    result = prime * result + StringUtils.trimToEmpty(town).hashCode();
    result = prime * result + StringUtils.trimToEmpty(code).hashCode();
    result = prime * result + ((country == null) ? 0 : country.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;
    Address other = (Address) obj;
    if (!StringUtils.trimToEmpty(street).equals(StringUtils.trimToEmpty(other.getStreet())))
        return false;
    if (!StringUtils.trimToEmpty(town).equals(StringUtils.trimToEmpty(other.getTown())))
        return false;
    if (!StringUtils.trimToEmpty(code).equals(StringUtils.trimToEmpty(other.getCode())))
        return false;
    if (country == null) {
        if (other.country != null)
            return false;
    } else if (!country.equals(other.country))
        return false;
    return true;
}


}

推荐答案

key插入map后，不能修改.

You shall not modify the key after having inserted it in the map.

我在 Map 中找到了 javadoc 的摘录 :

注意:如果将可变对象用作映射键，则必须非常小心.如果对象的值以影响等于比较的方式更改，而对象是映射中的键，则不会指定映射的行为.

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map.

一个简单的包装类的例子:

Example with a simple wrapper class:

public static class MyWrapper {

  private int i;

  public MyWrapper(int i) {
    this.i = i;
  }

  public void setI(int i) {
    this.i = i;
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    return i == ((MyWrapper) o).i;
  }

  @Override
  public int hashCode() {
    return i;
  }
}

和测试:

public static void main(String[] args) throws Exception {
  Map<MyWrapper, String> map = new HashMap<MyWrapper, String>();
  MyWrapper wrapper = new MyWrapper(1);
  map.put(wrapper, "hello");
  System.out.println(map.containsKey(wrapper));
  wrapper.setI(2);
  System.out.println(map.containsKey(wrapper));
}

输出:

true
false

注意:如果你不覆盖 hashcode() 那么你只会得到 true

Note : If you dont override hashcode() then you will get true only

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