java - 如何为一个类定义多个 equals() 函数

Question

我想在名为 MyObject 的类中重写“public boolean equals(Object obj)”函数，用于名称和年龄

public class MyObject{
       private String name;
       private int age;
}

我怎么可以？

@balusC:

这个呢？

vo  = new MyObject() {
                    public boolean equals(Object obj) {
                        return ((MyObject)obj).name().equals(this.getName());

                    }


vo  = new MyObject() {
                    public boolean equals(Object obj) {
                        return ((MyObject)obj).age() == (this.getAge());

Answer 1

你的问题有点含糊，但如果唯一的目的是根据你想使用的属性使用不同的排序算法，那么最好使用 Comparator .

public class Person {
    private String name;
    private int age;

    public static Comparator COMPARE_BY_NAME = new Comparator<Person>() {
        public int compare(Person one, Person other) {
            return one.name.compareTo(other.name);
        }
    }

    public static Comparator COMPARE_BY_AGE = new Comparator<Person>() {
        public int compare(Person one, Person other) {
            return one.age > other.age ? 1
                 : one.age < other.age ? -1
                 : 0; // Maybe compare by name here? I.e. if same age, then order by name instead.
        }
    }

    // Add/generate getters/setters/equals()/hashCode()/toString()
}

您可以按如下方式使用:

List<Person> persons = createItSomehow();

Collections.sort(persons, Person.COMPARE_BY_NAME);
System.out.println(persons); // Ordered by name.

Collections.sort(persons, Person.COMPARE_BY_AGE);
System.out.println(persons); // Ordered by age.

至于实际的 equals() 实现，我宁愿让它在两个 Person 对象在技术上或自然上相同时返回 true。您可以为此使用数据库生成的 PK 来比较技术身份:

public class Person {
    private Long id;

    public boolean equals(Object object) {
        return (object instanceof Person) && (id != null) 
             ? id.equals(((Person) object).id) 
             : (object == this);
    }
}

或者只是比较每个属性以比较自然身份:

public class Person {
    private String name;
    private int age;

    public boolean equals(Object object) {
        // Basic checks.
        if (object == this) return true;
        if (object == null || getClass() != object.getClass()) return false;

        // Property checks.
        Person other = (Person) object;
        if (name == null ? other.name != null : !name.equals(other.name)) return false;
        if (age != other.age) return false;

        // All passed.
        return true;
    }
}

在覆盖 equals() 时，不要忘记覆盖 hashCode()。

另见:

• Object ordering
• Sorting an ArrayList of objects
• Overriding equals() and hashCode()