【Video & Communication Platform】
http://study.163.com/course/introduction.htm?courseId=1004329008&utm_campaign=commission&utm_source=400000000155061&utm_medium=share
http://study.163.com/course/introduction.htm?courseId=1004638001&utm_campaign=commission&utm_source=400000000155061&utm_medium=share
https://gitee.com/happyangellxq520/spring-boot
à Spring Boot communication platform
http://412887952-qq-com.iteye.com/blog/2321532
Origin of need:
After publishing lombok -related articles, some netizens left a message, it is best to talk about @EqualsAndHashCode .
==============
There is an old saying : Whoever wins the hearts and minds of the people wins the world,
Today there is a cloud : those who have fans will win the world.
==============
The requirements of fans are still well met, and @EqualsAndHashCode is more complicated.
一、@EqualsAndHashCode
There are a few things to know about @EqualsAndHashCode :
( 1 ) This annotation will generate equals (Object other) and hashCode () methods.
( 2 ) It uses non-static, non-transient properties by default.
( 3 ) Some attributes can be excluded through the parameter exclude
( 4 ) Only which properties to use can be specified through the parameter of
( 5 ) By default, it only uses the properties defined in the class and does not call the methods of the parent class
( 6 ) The problem in ( 5 ) can be solved by callSuper=true , so that the method of the parent class is called in the method it generates.
Second, practice is more fluent
Well, there is no problem with the theoretical things, let's take a practical look.
2.1 This annotation will generate equals(Object other) and hashCode() methods
@EqualsAndHashCode()
public class Girl2 {
private int id;//主键.
private String name;//姓名.
private int age; //年龄.
private int weight;//体重.
private int height;//身高.
}
这里使用了@EqualsAndHashCode注解，那么一旦使用此注解之后会生成hashCode()和equals(Object other)方法，具体生成的代码如下：
public int hashCode() {
int PRIME = 59;
int result = 1;
result = result * 59 + this.id;
Object $name = this.name;
result = result * 59 + ($name == null ? 43 : $name.hashCode());
result = result * 59 + this.age;
result = result * 59 + this.weight;
result = result * 59 + this.height;
return result;
}
protected boolean canEqual(Object other) {
return other instanceof Girl2;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof Girl2)) {
return false;
}
Girl2 other = (Girl2) o;
if (!other.canEqual(this)) {
return false;
}
if (this.id != other.id) {
return false;
}
Object this$name = this.name;
Object other$name = other.name;
if (this$name == null ? other$name != null : !this$name.equals(other$name)) {
return false;
}
if (this.age != other.age) {
return false;
}
if (this.weight != other.weight) {
return false;
}
return this.height == other.height;
}
对于hashCode（）方法生成的时候，跟所有字段的hashCode有关系。
对于equals（）方法就是判断所有的字段是否相等了，如果所有的字段都相等的话，那么就返回true，如果有一个字段不相等的话，就返回false。
2.2 它默认使用非静态，非瞬态的属性。
这个是怎么理解呢？也就是说变量使用了静态了，或者是瞬态了，那么在生成equals和hashcode方法的时候，静态和瞬态的属性是不会参与的。如下的字段定义：
private static int myCode = 1; private transient int myCode2 = 2;
mycode和myCode2不会参与hashCode的生成，不会参与equasl方法的判断，所以生成的hashCode和equals方法和上面的是一样的。
2.3 可通过参数exclude排除一些属性
这个很好理解，就是有些字段不用参与比较，那么就可以排除掉，比如：我们可以判处name、age、weight、height属性，通过判断id就能判断两个女孩是否同一个女孩，如下的代码：
@EqualsAndHashCode(exclude={"name","age","weight","height"})
public class Girl2{
private static int myCode = 1;
private transient int myCode2 = 2;
private int id;
private String name;
private int age;
private int weight;
private int height;
}
那么对应生成的hashCode和equals方法如下：
public int hashCode() {
int PRIME = 59;
int result = 1;
result = result * 59 + this.id;
return result;
}
protected boolean canEqual(Object other) {
return other instanceof Girl2;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof Girl2)) {
return false;
}
Girl2 other = (Girl2) o;
if (!other.canEqual(this)) {
return false;
}
return this.id == other.id;
}
看到了，方法中只有和id有关系。看到这里有人会有疑问：这要是有n多个字段，但是equals只跟id有关系，那么排除很多的字段，感觉还是很麻烦的，其实有更简单的方式，接着往下看吧。（如果排除的字段比较少，可以使用这种方式）
2.4 可通过参数of指定仅使用哪些属性
这里的意思，可以通过of指定我们要的属性，比如：只和id有关系，那么就可以写成如下的代码：
@EqualsAndHashCode(of={"id"})
public class Girl2{
private static int myCode = 1;
private transient int myCode2 = 2;
private int id;
private String name;
private int age;
private int weight;
private int height;
}
生成的代码的如下：
public int hashCode() {
int PRIME = 59;
int result = 1;
result = result * 59 + this.id;
return result;
}
protected boolean canEqual(Object other) {
return other instanceof Girl2;
}
public boolean equals(Object o) {
if (o == this) {
return true;
}
if (!(o instanceof Girl2)) {
return false;
}
Girl2 other = (Girl2) o;
if (!other.canEqual(this)) {
return false;
}
return this.id == other.id;
}
细心的小伙伴会发现，这里生成的代码和在2.3是一样的。对于of和exclude只要还是看属性的多少，如果of配置的属性过多，那么就考虑使用exclude；如果exclude排除的属性过多，那么就应该使用of 。
2.5 使用callSuper=true解决继承问题
（1）默认仅使用该类中定义的属性且不调用父类的方法
这个的意思，就是默认情况下，子类的equals和hashCode方法，不会调用父类的super.hashCode方法。如下代码：
父类代码：
@EqualsAndHashCode
public class Person {
private int id;
}
子类代码：
@EqualsAndHashCode
public class Girl3 extends Person {
private String name;
private int age;
private int weight;
private int height;
}
那么生成的代码为：
public int hashCode() {
int PRIME = 59;
int result = 1;
Object $name = this.name;
result = result * 59 + ($name == null ? 43 : $name.hashCode());
result = result * 59 + this.age;
result = result * 59 + this.weight;
result = result * 59 + this.height;
return result;
}
protected boolean canEqual(Object other) {
return other instanceof Girl3;
}
public boolean equals(Object o) {
if (o == this) {
return true;
} else if (!(o instanceof Girl3)) {
return false;
} else {
Girl3 other = (Girl3)o;
if (!other.canEqual(this)) {
return false;
} else {
Object this$name = this.name;
Object other$name = other.name;
if (this$name == null) {
if (other$name != null) {
return false;
}
} else if (!this$name.equals(other$name)) {
return false;
}
if (this.age != other.age) {
return false;
} else if (this.weight != other.weight) {
return false;
} else if (this.height != other.height) {
return false;
} else {
return true;
}
}
}
我们会发现这里的的hashCode和equals方法只使用到了子类Girl3的属性，并没有和父类的相关。那么这样就会存在一个问题，id不一样，但是其它属性都一样的情况下，那么就会判断是同一个对象，然后实际上一旦id不一样了，是两个对象，这时候就需要使用到了callSuper=true的属性了，很简单，只需要在Girl3修改如下的代码：
@EqualsAndHashCode(callSuper=true)
public class Girl3 extends Person {
//省略其它代码;
}
那么生成的代码如下：
protected boolean canEqual(Object other) {
return other instanceof Girl3;
}
public int hashCode() {
int PRIME = 59;
int result = super.hashCode();
Object $name = this.name;
result = result * 59 + ($name == null ? 43 : $name.hashCode());
result = result * 59 + this.age;
result = result * 59 + this.weight;
result = result * 59 + this.height;
return result;
}
public boolean equals(Object o) {
if (o == this) {
return true;
} else if (!(o instanceof Girl3)) {
return false;
} else {
Girl3 other = (Girl3)o;
if (!other.canEqual(this)) {
return false;
} else if (!super.equals(o)) {
return false;
} else {
Object this$name = this.name;
Object other$name = other.name;
if (this$name == null) {
if (other$name != null) {
return false;
}
} else if (!this$name.equals(other$name)) {
return false;
}
if (this.age != other.age) {
return false;
} else if (this.weight != other.weight) {
return false;
} else if (this.height != other.height) {
return false;
} else {
return true;
}
}
}
}
注意：区别就是对于hashCode方法多了如下的代码：
int result = super.hashCode();
对于equals方法多了如下代码：
else if (!super.equals(o)) {
return false;
}
好了本文就介绍到这里了，博主也要洗洗睡了。