JAVA | equals 的正确姿势

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职员（Employee）及其子类（Manager），当判断对象是否等同取决于所有属性时，建议使用

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if (getClass() != otherObject.getClass()) return false;

而不是

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if (!(otherObject instanceof Employee)) return false;

如果当父类能够明确判断对象是否相同的条件时，可以使用instanceof。比如，实际中职员都有一个唯一ID，只要id相同即是同一个职员。那么将父类Employee的equals方法设计为判断ID值是否相同即可，当然hashCode方法也要由id来决定。

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@Override
public int hashCode() {
    return Objects.hash(this.id);
}
@Override
public boolean equals(Object otherObject) {
    if (this == otherObject) return true;
    if (otherObject == null) return false;
    if (!(otherObject instanceof Employee)) return false;
	
    Employee other = (Employee) otherObject;
    return this.id == other.id;
}

全部代码：

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class Employee {
    private String name;
    private double salary;
    private LocalDate hireDay;
	
    public Employee(String name, double salary, int year, int month, int day) {
        this.name = name;
        this.salary = salary;
        this.hireDay = LocalDate.of(year, month, day);
    }
    public String getName() {
        return this.name;
    }
    public double getSalary() {
        return this.salary;
    }
    public LocalDate getHireDay() {
        return this.hireDay;
    }
	
    public void raiseSalary(double byPercent) {
        double raise = this.salary * byPercent / 100;
        this.salary += raise;
    }
	
    @Override
    public int hashCode() {
        return Objects.hash(this.name, this.salary, this.hireDay);
    }
	
	@Override
    public boolean equals(Object otherObject) {
        if (this == otherObject) return true;
        if (otherObject == null) return false;
        if (getClass() != otherObject.getClass()) return false;
		
        Employee other = (Employee) otherObject;
        return Objects.equals(this.name, other.name) && this.salary == other.salary 
            && Objects.equals(this.hireDay, other.hireDay);
    }
    @Override
    public String toString() {
        return getClass().getName() + " [name=" + name + ", salary=" + salary 
            + ", hireDay=" + hireDay + "]";
    }
}
class Manager extends Employee {
    private double bonus;
	
    public Manager(String name, double salary, int year, int month, int day) {
        super(name, salary, year, month, day);
        bonus = 0;
    }
	
    public void setBonus(double bonus) {
        this.bonus = bonus;
    }
    public double getSalary() {
        double baseSalary = super.getSalary();
        return baseSalary + bonus;
    }
	
    @Override
    public int hashCode() {
        return super.hashCode() + 17 * Double.hashCode(bonus);
    }
	
    @Override
    public boolean equals(Object otherObject) {
        if (!super.equals(otherObject)) return false;
		
        Manager other = (Manager) otherObject;
        return bonus == other.bonus;
    }
}