package com.set;

public class Cat {
    private String name;
    private int month;
    private String species;
    //構造方法
    public Cat(String name, int month, String species) {
        this.name = name;
        this.month = month;
        this.species = species;
    }
    //get和set方法
    public String getName() {
        
 
return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getMonth() {
        return month;
    }
    public void setMonth(int month) {
        this.month = month;
    }
    public String getSpecies() {
        return species;
    }
    public void setSpecies(String species) {
        
 
this.species = species;
    }
    @Override
    public String toString() {
        return "Cat{" +
                "name='" + name + '\'' +
                ", month=" + month +
                ", species='" + species + '\'' +
                '}';
    }
}

package com.set;

import java.util.HashSet;
 
import java.util.Iterator;
import java.util.Set;

public class CatTest {
    public static void main(String[] args) {
        Cat huahua=new Cat("花花",12,"英國短毛貓");
        Cat fanfan=new Cat("凡凡",3,"中華田園貓");
        Set set=new HashSet();
        set.add(huahua);
        set.add(fanfan);
        Iterator it= set.iterator();
        while (it.hasNext()){
            System.out.println(it.next());
        }
        System.out.println("------------------------------");
        Cat huahua01=new Cat("花花",12,"英國短毛貓");
        set.add(huahua01);
        it=set.iterator();
        while(it.hasNext()){
            System.out.println(it.next());
        }
    }
}

　　add方法會呼叫hashCode、equals方法進行判斷，此時未重寫hashCode和equals方法，由於預設每個物件的hashCode值都不一樣，重複的物件huahua01將會新增到集合中：

    @Override
    public int hashCode() {
        final  int prime=31;
        int result=1;
        result=prime*result+month;
        result=prime*result+((name==null)?0:name.hashCode());
        result=prime*result+((species==null)?0:species.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if(this==obj)
            return true;
        if(obj.getClass()==Cat.class){
            Cat cat=(Cat) obj;
            return cat.getName().equals(name)
                    && (cat.getMonth()==month)
                    && (cat.getSpecies().equals(species));
        }
        return false;
    }

　　在重寫hashCode和equals方法後，重複的物件不會被新增，執行結果為：