public class Address {private String province;private String city;public String getProvince() {return province;}public void setProvince(String province) {this.province = province;}public String getCity() {return city;}public void setCity(String city) {this.city = city;}public Address() {}public Address(String province, String city) {this.province = province; this.city = city;}}

 1 public class TestAddress {
 2
 3     public static void main(String[] args) {
 4         Address address1 = new Address("广东","广州");
 5         Address address2 = new Address("广东", "广州");
 6
 7         System.out.println(address1 == address2);//false
 8         System.out.println(address1.equals(address2));//false
 9         System.out.println(address1.hashCode() == address2.hashCode());//false
10     }
11 }

    public boolean equals(Object obj) {return (this == obj);}

     * The {@code equals} method for class {@code Object} implements* the most discriminating possible equivalence relation on objects;* that is, for any non-null reference values {@code x} and* {@code y}, this method returns {@code true} if and only* if {@code x} and {@code y} refer to the same object* ({@code x == y} has the value {@code true}).

   / * Note that it is generally necessary to override the {@code hashCode}* method whenever this method is overridden, so as to maintain the* general contract for the {@code hashCode} method, which states* that equal objects must have equal hash codes.*/public boolean equals(Object obj) {return (this == obj);}

1     @Override
2     public boolean equals(Object obj) {
3         if(obj == this)
4             return true;
5         if(!(obj instanceof Address))
6             return false;
7         Address address = (Address)obj;
8         return address.getProvince().equals(province) && address.getCity().equals(city);
9     }

public class Address {private String province;private String city;public String getProvince() {return province;}public void setProvince(String province) {this.province = province;}public String getCity() {return city;}public void setCity(String city) {this.city = city;}public Address() {}public Address(String province, String city) {this.province = province; this.city = city;}@Overridepublic boolean equals(Object obj) {if(obj == this)return true;if(!(obj instanceof Address))return false;Address address = (Address)obj;return address.getProvince().equals(province) && address.getCity().equals(city);}@Overridepublic int hashCode() {int result = 17;result += 31 * province.hashCode();result += 31 * city.hashCode();return result;}
}

import java.util.HashMap;
import java.util.Map;public class TestAddress {public static void main(String[] args) {Address address1 = new Address("广东","广州");Address address2 = new Address("广东", "广州");System.out.println(address1 == address2);//falseSystem.out.println(address1.equals(address2));//trueSystem.out.println(address1.hashCode() == address2.hashCode());//true
        Address diff1 = new Address("四川","成都");Address diff2 = new Address("四川","绵阳");System.out.println(diff1 == diff2);//falseSystem.out.println(diff1.equals(diff2));//falseSystem.out.println(diff1.hashCode() == diff2.hashCode());//false
        Map<Address, Integer> hashMap = new HashMap<Address, Integer>();hashMap.put(address1, 1);hashMap.put(address2, 2);//address2的hashCode 和 address1 相同,因此 put 方法会覆盖 address1 对应的 Value值1System.out.println(hashMap.get(address1));//2System.out.println(hashMap.get(address2));//2
        hashMap.put(diff1, 1);hashMap.put(diff2, 2);System.out.println(hashMap.get(diff1));//1System.out.println(hashMap.get(diff2));//2
    }
}

    @Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((city == null) ? 0 : city.hashCode());result = prime * result + ((province == null) ? 0 : province.hashCode());return result;}@Overridepublic 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 (city == null) {if (other.city != null)return false;} else if (!city.equals(other.city))return false;if (province == null) {if (other.province != null)return false;} else if (!province.equals(other.province))return false;return true;}