java的equals重写

java的equals方法一般情况下需要重写，以保证能够比较两个实例对象是否一致，注意重写equals方法时还要重写hashCode方法。

equals方法的注意事项：1）此方法应为继承自Object的方法（可用@Override标示）。2）如果子类要重写此方法，在最后判断时同样要调用父类方法判断父类中数据是否相等。3）equals中比较的内容应有序、完整。4）应重写hashCode方法。

下面以Employee类为例介绍（Cloneable接口在此可忽略）：

package ch05;

import java.util.Date;
import java.util.GregorianCalendar;
import java.util.Objects;

/**
 * @author qiaowei
 *
 */
public class Employee implements Cloneable {
	public Employee(String name, double salary) {
		GregorianCalendar calendar = new GregorianCalendar();
		
		mName = name;
		mSalary = salary;
		mHireDay = calendar.getTime();
	}
	
	public Employee(String n, double s, int year, int month, int day) {
		mName = n;
		mSalary = s;
		GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
		mHireDay = calendar.getTime();
	}

	@Override
	public boolean equals(Object o) {
        /** 
         * equals must five steps:
         *   1:two variable are point to the same object.
         *   2:obj is null.
         *   3:variables is the same Class.
         *   3.5: use instanceof test.
         *   4:cast obj.
         *   5:equals each field's value(== for primitive; equals for object).
         * */
		if (this == o) {
			return true;
		}

		if ((null == o) ||
				(this.getClass() != o.getClass())) {
			return false;
		}

		if ( !(o instanceof Employee)) {
			return false;
		} else {
			Employee e = (Employee)o;
			/*return ((mName.equals(e.getName()) &&
					mSalary == e.getSalary()) &&
					mHireDay.equals(e.getDate()));*/
			
                        //类类型用Objects.equals方法比较，基础类型用==比较
			return (super.equals(e)) && 
                                        (Objects.equals(mName, e.getName()) &&
					mSalary == e.getSalary() &&
					Objects.equals(mHireDay, e.getDate()));
		}
	}
	
	@Override
	public int hashCode() {
                //以下两种方法都可正确
		/*return (7 * Objects.hashCode(name) +
				11 * new Double(salary).hashCode() +
				13 * Objects.hashCode(hireDay));*/
		
		return Objects.hash(mName,
				mSalary,
				mHireDay);
	}

	//实现Cloneable接口
	@Override
	public Employee clone() throws CloneNotSupportedException {
		Employee cloned = (Employee)super.clone();
		cloned.mName = mName;
		cloned.mSalary = mSalary;
		cloned.mHireDay = (Date)mHireDay.clone();

		return cloned;
	}
	
	public final String getName() {
		return mName;
	}
	
	public double getSalary() {
		return mSalary;
	}
	
	public Date getDate() {
		return (Date)mHireDay.clone();
	}
	
	public void raiseSalary(double byPercent) {
		double raise = mSalary * byPercent / 100;
		mSalary += raise;
	}

	private String mName;
	private double mSalary;
	private Date mHireDay;
}

Manager类（Employee的子类）重写equals和hashCode方法：

package ch05;

import java.util.Objects;

/**
 * @author qiaowei
 * 重写类的equals方法，在重写同时需重写hashCode方法
 */
public class Manager extends Employee{
	public Manager(String n, double s, int year, int month, int day) {
		super(n, s, year, month, day);
		
		mBouns = 0;
	}
	
	@Override
	public boolean equals(Object otherObj) {
		if (this == otherObj) {
			return true;
		}
		
		if (null == otherObj) {
			return false;
		}
		
		if (getClass() != otherObj.getClass()) {
			return false;
		}
		
		if ( !(otherObj instanceof Manager)) {
			return false;
		} else {
			Manager manager = (Manager)otherObj;
			
			/*return (manager.getName() == getName() &&
					manager.getSalary() == getSalary() &&
					manager.getDate() == getDate() &&
					manager.bonus == bonus);*/
			//注意父类数据同样要判断是否相等
			return (super.equals(otherObj) &&
					mBouns == manager.mBouns);
		}
	}
	
	@Override
	public int hashCode() {
		/*return (super.hashCode() +
				17 * new Double(mBouns).hashCode());*/
		//父类数据同样要用hash
		return Objects.hash(super.hashCode(), mBouns);
	}
	
	public static void main(String[] args) {
		Manager m01 = new Manager("jowei000", 1000, 1981, 2, 5);
		Manager m02 = new Manager("jowei000", 1000, 1981, 2, 15);
		
		//比较实例是否相等
		//System.out.println(m01.equals(m02));
		System.out.println(m01.hashCode());
		System.out.println(m02.hashCode());
	}
	
	private double mBouns;
}