如何重寫hashcode和equals方法

public class Object { 
public native int hashCode(); 
    /**
     * Indicates whether some other object is "equal to" this one.
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.
     * <li>It is <i>symmetric</i>: for any non-null reference values
     *     {@code x} and {@code y}, {@code x.equals(y)}
     *     should return {@code true} if and only if
     *     {@code y.equals(x)} returns {@code true}.
     * <li>It is <i>transitive</i>: for any non-null reference values
     *     {@code x}, {@code y}, and {@code z}, if
     *     {@code x.equals(y)} returns {@code true} and
     *     {@code y.equals(z)} returns {@code true}, then
     *     {@code x.equals(z)} should return {@code true}.
     * <li>It is <i>consistent</i>: for any non-null reference values
     *     {@code x} and {@code y}, multiple invocations of
     *     {@code x.equals(y)} consistently return {@code true}
     *     or consistently return {@code false}, provided no
     *     information used in {@code equals} comparisons on the
     *     objects is modified.
     * <li>For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.
     * </ul>
     * <p>
     * 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}).
     * <p>
     * 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.
     *
     * @param   obj   the reference object with which to compare.
     * @return  {@code true} if this object is the same as the obj
     *          argument; {@code false} otherwise.
     * @see     #hashCode()
     * @see     java.util.HashMap
     */
    public boolean equals(Object obj) {
        return (this == obj);
    }
}

package jianlejun.study; 
public class Student {
	private String name;// 姓名
	private String sex;// 性別
	private String age;// 年齡
	private float weight;// 體重
	private String addr;// 地址
	
	// 重寫hashcode方法
	@Override
	public int hashCode() {
		int result = name.hashCode();
		result = 17 * result + sex.hashCode();
		result = 17 * result + age.hashCode();
		return result;
	}
 
	// 重寫equals方法
	@Override
	public boolean equals(Object obj) {
		if(!(obj instanceof Student)) {
       // instanceof 已經處理了obj = null的情況
			return false;
		}
		Student stuObj = (Student) obj;
		// 地址相等
		if (this == stuObj) {
			return true;
		}
		// 如果兩個物件姓名、年齡、性別相等，我們認為兩個物件相等
		if (stuObj.name.equals(this.name) && stuObj.sex.equals(this.sex) && stuObj.age.equals(this.age)) {
			return true;
		} else {
			return false;
		}
	}
 
	public String getName() {
		return name;
	}
 
	public void setName(String name) {
		this.name = name;
	}
 
	public String getSex() {
		return sex;
	}
 
	public void setSex(String sex) {
		this.sex = sex;
	}
 
	public String getAge() {
		return age;
	}
 
	public void setAge(String age) {
		this.age = age;
	}
 
	public float getWeight() {
		return weight;
	}
 
	public void setWeight(float weight) {
		this.weight = weight;
	}
 
	public String getAddr() {
		return addr;
	}
 
	public void setAddr(String addr) {
		this.addr = addr;
	}
 
}

Set set = new HashSet();
	set.add(s1);
	set.add(s2);
	System.out.println(set);

@Override
	public int hashCode() {
		int result = name.hashCode();
		result = 17 * result + sex.hashCode();
		result = 17 * result + age.hashCode();
		return result;
	}

Map m = new HashMap();
	m.put(s1, s1);
	m.put(s2, s2);
	System.out.println(m);
	System.out.println(((Student)m.get(s1)).getAddr());
 
輸出結果：
{jianlejun.study.Student@43c2ce69=jianlejun.study.Student@43c2ce69}
222

		Map<String,Value> map1 = new HashMap<String,Value>();
        String s1 = new String("key");
        String s2 = new String("key");
        Value value = new Value(2);
        map1.put(s1, value);
        System.out.println("s1.equals(s2):"+s1.equals(s2));
        System.out.println("map1.get(s1):"+map1.get(s1));
        System.out.println("map1.get(s2):"+map1.get(s2));

        Map<Key,Value> map2 = new HashMap<Key,Value>();
        Key k1 = new Key("A");
        Key k2 = new Key("A");
        map2.put(k1, value);
        System.out.println("k1.equals(k2):"+k1.equals(k2));
        System.out.println("map2.get(k1):"+map2.get(k1));
        System.out.println("map2.get(k2):"+map2.get(k2));

static class Key{
        private String k;
        public Key(String key){
            this.k=key;
        }
        //如果不重寫hashCode，只重寫了equals，會造成相同值被放入不同的桶中
//        @Override
//        public int hashCode() {
//            return k.hashCode();
//        }

        @Override
        public boolean equals(Object obj) {
            if(obj instanceof Key){
                Key key=(Key)obj;
                return k.equals(key.k);
            }
            return false;
        }
    }
    
 static class Value{
        private int v;

        public Value(int v){
            this.v=v;
        }

        @Override
        public String toString() {
            return "類Value的值－－>"+v;
        }
    }

		Map<Integer, Integer> map3 =new HashMap();
        while (true){
            boolean flag = false;
            for (int i = 0; i < 1000; i++) {
                if(!map3.containsKey(i)){
                    map3.put(i, i);
                    flag = true;
                }
            }
            if (flag == false) {
                break;
            }
            System.out.println("map3的容量" + map3.size());
        }

        Map<Key2, Integer> map4 =new HashMap();
        while (true){
            boolean flag = false;
            for (int i = 0; i < 1000; i++) {
                if(!map4.containsKey(new Key2(i))){
                    map4.put(new Key2(i), i);
                    flag = true;
                }
            }
            if (flag == false) {
                break;
            }
            System.out.println("map4的容量" + map4.size());
        }

    static class Key2{
        Integer id;

        Key2(Integer id) {
            this.id = id;
        }

        @Override
        public int hashCode() {
            return id.hashCode();
        }
        //不重寫equals就會導致一直認為沒有相同的值，就會一直插入。
//        @Override
//        public boolean equals(Object obj) {
//            if(obj instanceof Key2){
//                Key2 key2 =(Key2)obj;
//                return id.equals(key2.id);
//            }
//            return false;
//        }
    }