Java HashMap containsKey returns false for existing object

Question

I have a HashMap for storing objects:

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

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(); 

I have:

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

but

fields.containsKey(bean) = false 

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; }   } 

Answer 1

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

Edit : I found the extract of javadoc in Map :

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.

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;   } } 

and the test :

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)); } 

Output :

true false 

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