参考：继承

1 Java 的三大特性

继承、封装、多态。
继承，is a，关系；
封装，隐藏内部实现；（经常我们修改内部实现或逻辑，对外的数据模型却不用调整）
多态，运行时候才知道具体的东西，由实际场景所决定。（override，overload，不同的人认为不同，有的说都是多态的体现，有的只认可override覆盖，overload重载不算。不纠结这个，使用即可）

2 什么是继承

从代码讲，使用了 extends 关键字

class ClassName extends SuperClassName {
    // ...
}

从理论上讲，is a 关系时，就是继承。

3 继承能干什么

解决 is a ，代码复用；

3.1 复用对象的子段：

包括字段公用（其实归根到底就是方法的继承使用，可见修辞符的使用——public、protected、default、private），比如开发中分页查询，继承 BasePageQuery 基类；

package com.demo.activiti7.inherit;
import org.junit.Test;
public class DemoTest {
    /**
     * 参考： https://blog.csdn.net/chenssy/article/details/12786385
     */
    @Test
    public void testInherit() {
        A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();
        // 子类可将父类的 public、protected 对象
        c.a = b.a = a1.a = a2.a = d.a;
        c.b = b.b = a1.b = a2.b = d.b;
//        System.out.println("1--" + a1.show(b));
//        System.out.println("2--" + a1.show(c));
//        System.out.println("3--" + a1.show(d));
        System.out.println("4--" + a2.show(b));
        System.out.println("4--" + a2.show(a1));
        System.out.println("-----------" + c.show(c));
        System.out.println("-----------" + b.show(a2));
//        System.out.println("5--" + a2.show(c));
//        System.out.println("6--" + a2.show(d));
//        System.out.println("7--" + b.show(b));
//        System.out.println("8--" + b.show(c));
//        System.out.println("9--" + b.show(d));
    }
    class A {
        // 基类中的public、protected被子类继承
        public String a = "1";
        protected String b = "b";
        public String show(D obj) {
            return ("A and D");
        }
        public String show(A obj) {
            return ("A and A");
        }
        public <T extends B> String show(T obj) {
            return ("A and A");
        }
    }
    class B extends A {
        public String show(B obj){
            return ("B and B");
        }
        @Override
        public String show(A obj){
            return ("B and A");
        }
    }
    class C extends B {
    }
    class D extends B {
    }
}

package com.hbte.sharp.bo;
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
/**
 * @ClassName PageQuery
 * @Description 分页参数
 * @Version 1.0.0
 * @Author Bruce
 * @Date 2020/9/23 10:23
 **/
@ApiModel(value = "分页参数", description = "query")
public class PageQuery extends BaseQuery {
    @ApiModelProperty(value = "第几页，默认第一页")
    private Integer pageNum = 1;
    @ApiModelProperty(value = "每页条数，默认10")
    private Integer pageSize= 10;
    @ApiModelProperty(value = "导出方式 0.导出当前页 1.导出全部 导出所选在前端做 默认为0")
    private Integer type = 0;
    public Integer getPageNum() {
        return pageNum;
    }
    public void setPageNum(Integer pageNum) {
        this.pageNum = pageNum;
    }
    public Integer getPageSize() {
        return pageSize;
    }
    public void setPageSize(Integer pageSize) {
        this.pageSize = pageSize;
    }
    public Integer getType() {
        return type;
    }
    public void setType(Integer type) {
        this.type = type;
    }
}
package com.hbte.sharp.bo.flow;
import com.hbte.sharp.bo.PageQuery;
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
/**
 * @author wb
 * @date 2020-11-26-18:37
 */
@ApiModel(value = "流程基本信息关键字查询", description = "query")
public class FlowBaseInfoKeyWordsQuery extends PageQuery {
    @ApiModelProperty(value = "关键字")
    private String keyWords;
    public String getKeyWords() {
        return keyWords;
    }
    public void setKeyWords(String keyWords) {
        this.keyWords = keyWords;
    }
}

3.2 复用对象的方法

package com.hbte.sharp.po.data.listener;
import com.alibaba.excel.event.AnalysisEventListener;
import com.hbte.sharp.util.GenericSuperclassUtil;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.util.CollectionUtils;
import java.lang.reflect.Field;
import java.util.Date;
import java.util.List;
/**
 * 抽象listener
 *
 * @author Bruce
 * @date 2020/12/2 14:32
 */
public abstract class AbstractDataImportListener<T, R> extends AnalysisEventListener<T> {
    private final Logger log = LoggerFactory.getLogger(AbstractDataImportListener.class);
    private String userName;
    private Long userId;
    private Date createTime;
    private List<R> resposeList;
    public AbstractDataImportListener(String userName, Long userId, Date createTime, List<R> resposeList) {
        this.userName = userName;
        this.userId = userId;
        this.createTime = createTime;
        this.resposeList = resposeList;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
    public Long getUserId() {
        return userId;
    }
    public void setUserId(Long userId) {
        this.userId = userId;
    }
    public Date getCreateTime() {
        return createTime;
    }
    public void setCreateTime(Date createTime) {
        this.createTime = createTime;
    }
    public List<R> getResposeList() {
        return resposeList;
    }
    public void setResposeList(List<R> resposeList) {
        this.resposeList = resposeList;
    }
    public Class getDataModel(Class<? extends AbstractDataImportListener> listener) {
        return GenericSuperclassUtil.getActualTypeArgument(listener, 0);
    }
    public Class getReturnModel(Class<? extends AbstractDataImportListener> listener) {
        return GenericSuperclassUtil.getActualTypeArgument(listener, 1);
    }
    /**
     * 继承该类的，都可以会用该方法来确认失败的数据条数，不需要具体的业务类来实现一套
     */
    public int failNum() {
        int count = 0;
        if (CollectionUtils.isEmpty(resposeList)) {
            return count;
        }
        try {
            for (R obj : resposeList) {
                Class<?> objClass = obj.getClass();
                Field successfulFlag = objClass.getDeclaredField("successfulFlag");
                successfulFlag.setAccessible(true);
                if ("否".equals(successfulFlag.get(obj).toString())) {
                    count += 1;
                }
            }
        } catch (NoSuchFieldException e) {
            log.error("不存在 successfulFlag 标志字段。", e);
        } catch (IllegalAccessException e) {
            log.error("successfulFlag 标志字段无法访问。", e);
        }
        return count;
    }
}

3.3 小结

直观体现，就是减少重复代码，使得代码看起来更为清爽。另外的作用就是Java 的多态，继承是多态实现的根本前提（只为人 override 覆盖才是多态的人员，这么认为）。

4 如何使用继承

需要将对象抽象为 is a 关系时候，或者说具有相同的属性和行为的时候。

注意：Java 继承是单继承，推荐使用组合，而非继承。组合是指 has a 包含关系，继承是 is a 关系。各有各自的优缺点： java 组合VS继承可以看看

5 扩展

继承中，多态调用
Java 多态机制，继承链中对象方法的调用存在一个优先级:
this.method(O) -> super.method(O) -> this.method((super)O) -> super.method((super)O)

当超类对象引用变量引用子类对象时，首先这个被调用的方法必须在超类中定义过 = 根据上面继承链对象方法调用优先级可以在超类中找到这个方法
被引用对象的类型而不是引用变量的类型决定了调用谁的成员方法 = 判断子类是否重写了该方法，如果重写了直接调用子类的方法

5.1 小例子一

5.1.1 代码
```java package com.demo.activiti7.inherit;

import org.junit.Test;

public class DemoTest {

/**
 * 参考： https://blog.csdn.net/chenssy/article/details/12786385
 */
@Test
public void testInherit() {
    A a1 = new A();
    A a2 = new B();
    B b = new B();
    C c = new C();
    D d = new D();
    c.a = b.a = a1.a = a2.a = d.a;
    c.b = b.b = a1.b = a2.b = d.b;
    System.out.println("1--" + a1.show(b));
    System.out.println("2--" + a1.show(c));
    System.out.println("3--" + a1.show(d));
    System.out.println("4--" + a2.show(b));
    System.out.println("5--" + a2.show(c));
    System.out.println("6--" + a2.show(d));
    System.out.println("7--" + b.show(b));
    System.out.println("8--" + b.show(c));
    System.out.println("9--" + b.show(d));
}
class A {
    public String a = "1";
    protected String b = "b";
    public String show(D obj) {
        return ("A and D");
    }
    public String show(A obj) {
        return ("A and A");
    }
    public <T extends B> String show(T obj) {
        return ("A and A");
    }
}
class B extends A {
    public String show(B obj) {
        return ("B and B");
    }
    @Override
    public String show(A obj) {
        return ("B and A");
    }
}
class C extends B {
}
class D extends B {
}

}

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### 5.1.2 运行结果

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com.intellij.rt.execution.junit.JUnitStarter -ideVersion5 -junit4 com.demo.activiti7.inherit.DemoTest,testInherit 1—A and A 2—A and A 3—A and D 4—B and B 5—B and B 6—A and D 7—B and B 8—B and B 9—A and D

Process finished with exit code 0

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### 5.1.3 分析
此处只对 4--B and B 分析。<br />A a2 = new B();<br />B b = new B();<br />System.out.println("4--" + a2.show(b));<br />a2 为 B 的父类，所以show的方法，A中得有，a2 才可调用；调用时，是由 B 为实际的类，发现当前存在show(B) 的方法，直接运行该方法，输出 4--B and B。
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## 5.2 小例子二
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### 5.2.1 代码
```java
package com.demo.activiti7.inherit;
import org.junit.Test;
public class DemoTest {
    /**
     * 参考： https://blog.csdn.net/chenssy/article/details/12786385
     */
    @Test
    public void testInherit() {
        A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();
        c.a = b.a = a1.a = a2.a = d.a;
        c.b = b.b = a1.b = a2.b = d.b;
        System.out.println("1--" + a1.show(b));
        System.out.println("2--" + a1.show(c));
        System.out.println("3--" + a1.show(d));
        System.out.println("4--" + a2.show(b));
        System.out.println("5--" + a2.show(c));
        System.out.println("6--" + a2.show(d));
        System.out.println("7--" + b.show(b));
        System.out.println("8--" + b.show(c));
        System.out.println("9--" + b.show(d));
    }
    class A {
        public String a = "1";
        protected String b = "b";
        public String show(D obj) {
            return ("A and D");
        }
        public String show(A obj) {
            return ("A and A");
        }
//        public <T extends B> String show(T obj) {
//            return ("A and A");
//        }
    }
    class B extends A {
        public String show(B obj) {
            return ("B and B");
        }
        @Override
        public String show(A obj) {
            return ("B and A");
        }
    }
    class C extends B {
    }
    class D extends B {
    }
}

与例子一相比，就只是屏蔽代码：
// public String show(T obj) {
// return (“A and A”);
// }

5.2.2 运行结果

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com.intellij.rt.execution.junit.JUnitStarter -ideVersion5 -junit4 com.demo.activiti7.inherit.DemoTest,testInherit
1--A and A
2--A and A
3--A and D
4--B and A
5--B and A
6--A and D
7--B and B
8--B and B
9--A and D
Process finished with exit code 0

5.2.3 分析

还是分析结果“4—B and A”
a2.show(b)，父类接收子类，B 中不存在 show(B)，找父类 show(B)，没有找本类的show(super(B))，找到 show(A)。

6 小结

6.1 继承的特性

继承是面向对象编程的一种强大的代码复用方式，子类拥有父类非 private 的属性、方法。（protected允许子类访问父类的字段和方法；）
子类可以拥有自己的属性和方法，即子类可以对父类进行扩展，即子类可以用自己的方式实现父类的方法。
子类的构造方法可以通过super()调用父类的构造方法；
子类和父类的关系是is，has关系不能用继承。Java 的继承是单继承，所有类最终的根类是Object，但是可以多重继承，单继承就是一个子类只能继承一个父类，多重继承就是，例如 B 类继承 A 类，C 类继承 B 类，所以按照关系就是 B 类是 C 类的父类，A 类是 B 类的父类，这是 Java 继承区别于 C++ 继承的一个特性。
提高了类之间的耦合性（继承的缺点，耦合度高就会造成代码之间的联系越紧密，代码独立性越差）。
向上转型：可以安全地向上转型为更抽象的类型；
向下转型：可以强制向下转型，最好借助instanceof判断；
```
if (obj instanceOf B) {
  B b = (B) obj;
}
```
注意：
final 关键字声明类可以把类定义为不能继承的，即最终类；或者用于修饰方法，该方法不能被子类重写：
构造器：子类是不继承父类的构造器（构造方法或者构造函数）的，它只是调用（隐式或显式）。如果父类的构造器带有参数，则必须在子类的构造器中显式地通过 super 关键字调用父类的构造器并配以适当的参数列表。如果父类构造器没有参数，则在子类的构造器中不需要使用 super 关键字调用父类构造器，系统会自动调用父类的无参构造器。
通过 spuer 调用父类的属性，方法。

开发笔记、心得

Java 继承特性