java - HashMap...这里获取 null，我必须重载 hashcode() 以便我各自的对象

Question

    class Student2{
        int age;
        String name;

        Student2(int age,String name)
        {
            this.age=age;
            this.name=name;
        }

        /*
public int hashCode(){
            return age;     
        }
        */

        public boolean equals(Object o){
            Student2 s2=(Student2)o;
            return (this.age==s2.age && this.name==s2.name);
        }


    }

    public class HashMapDemo{




        public static void main(String a[])
        {   Map m=new HashMap();
        m.put(new Student2(10,"sameer"), 1);
        m.put(new Student2(11,"pagal"), 2);
        m.put(new Student2(12,"ullu"), 3);
        m.put(new Student2(13,"lullu"), 5);

        System.out.println(m.get(new Student2(11,"pagal")));


        }


    }

HashMap...这里获取 null，我必须重载 hashcode() 以便我各自的对象。我要在 hashCode 中添加什么。我的疑问是在 hashCode 方法中是否必须重写 hashCode 方法。

Answer 1

如果你想使用你的类作为键，你需要 hashCode 和 equals 方法。让您的 IDE 生成或使用:

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + age;
    result = prime * result + ( ( name == null ) ? 0 : name.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;
    Student2 other = (Student2) obj;
    if( age != other.age )
        return false;
    if( name == null ) {
        if( other.name != null )
            return false;
    } else if( !name.equals( other.name ) )
        return false;
    return true;
}