[Java] When hash is used, HashCode and Equals are overwritten.

1. SourceSometimes, data objects are often stored in hashmap or hashset, and you want to represent data objects by ID. This overwrites the hashcode and equals methods. 2. Code

public class Goods {    public Goods(int id) {        this.id = id;    }    public int id;    public String name;    public String image;    public double price;    public static void main(String[] args) {        Set<Goods> set = new HashSet<Goods>();        Goods a = new Goods(0);        Goods b = new Goods(0);        set.add(a);        set.add(b);        HashMap<Goods, Integer> map = new HashMap<Goods, Integer>();        map.put(a, 0);        map.put(b, 0);        System.out.println("Goods");        System.out.println("set num = " + set.size());        System.out.println("map key set num = " + map.keySet().size());        System.out.println("a.hashCode() = " + a.hashCode());        System.out.println("b.hashCode() = " + b.hashCode());    }};

When goods set num = 2 Map key set num = 2 is not overloaded, it is treated as two objects even if the IDs are the same.

public class GoodsWithHashAndEquals {    public GoodsWithHashAndEquals(int id) {        this.id = id;    }    public int id;    public String name;    public String image;    public double price;    @Override    public int hashCode() {        return id;    }    @Override    public boolean equals(Object obj) {        return obj.hashCode() == hashCode();    }    public static void main(String[] args) {        Set<GoodsWithHashAndEquals> set = new HashSet<GoodsWithHashAndEquals>();        GoodsWithHashAndEquals a = new GoodsWithHashAndEquals(0);        GoodsWithHashAndEquals b = new GoodsWithHashAndEquals(0);        set.add(a);        set.add(b);        HashMap<GoodsWithHashAndEquals, Integer> map = new HashMap<GoodsWithHashAndEquals, Integer>();        map.put(a, 0);        map.put(b, 0);        System.out.println("GoodsWithHashAndEquals");        System.out.println("set num = " + set.size());        System.out.println("map key set num = " + map.keySet().size());        System.out.println("a.hashCode() = " + a.hashCode());        System.out.println("b.hashCode() = " + b.hashCode());    }}

After goodswithhashandequals set num = 1 Map key set num = 1 is overloaded, the IDs are the same and are treated as an object. 3. ReferenceMajor bugs caused by cached key values in map hashcode: equals () and hashcode () methods in Java 4. Link to this articleHttp://blog.csdn.net/xiaodongrush/article/details/9419831