Java自定义实现equals()方法过程解析

public class MyDate implements Comparable<MyDate> {
  private final int year;
  private final int month;
  private final int day;
  public MyDate(int year, int month, int day) {
    this.year = year;
    this.month = month;
    this.day = day;
  }
  @Override
  public int compareTo(MyDate o) {
    throw new NotImplementedException();
  }

  public boolean equals(Date that) {
    if (this.day != that.day) {
      return false;
    }
    if (this.month != that.month) {
      return false;
    }
    if (this.year != that.year) {
      return false;
    }
    return true;
  }
}

//定义为final类型：允许子类直接使用父类equals()方法是不安全的
public final class MyDate implements Comparable<MyDate> {
  private final int year;
  private final int month;
  private final int day;
  public MyDate(int year, int month, int day) {
    this.year = year;
    this.month = month;
    this.day = day;
  }
  @Override
  public int compareTo(MyDate o) {
    throw new NotImplementedException();
  }

  @Override
  //规定参数必须是Object类型
  public boolean equals(Object obj) {
    //检查是否相同引用
    if (obj == this) {
      return true;
    }
    //检查null
    if (obj == null) {
      return false;
    }
    //getClass()判断的是准确的运行时类型，instanceof的类型可以是父类或接口
    if (obj.getClass() != this.getClass()) {
      return false;
    }
    //这里类型转换一定是安全的
    MyDate that = (MyDate) obj;
    //确认关键字段都相等
    if (this.day != that.day) {
      return false;
    }
    if (this.month != that.month) {
      return false;
    }
    if (this.year != that.year) {
      return false;
    }
    return true;
  }
}