第 8 节 数据表与简单Java类映射

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在之前已经给出了简单Java类一定要和数据表的结构对应，同时外键需要依靠引用来进行关联，于是，本次将针对简单Java类做一个功能上的扩充。

要求通过Java程序描述出dept-emp关系，使用字段:

• dept：deptno、dname、loc；

• emp：empno、ename、job、sal、deptno、mgr。

在dept-emp表的关系里面存在有如下的关系：

• 一个部门有多个雇员；

• 一个雇员有一个或零个领导。

**第一步：**实现基本字段的转换

class Dept {
    private int deptno;
    private String dname;
    private String loc;
    public Dept(int deptno, String dname, String loc) {
        this.deptno = deptno;
        this.dname = dname;
        this.loc = loc;
    }
    public String getInfo() {
        return "deptno=" + deptno + ", dname=" + dname + ", loc=" + loc;
    }
}
class Emp {
    private int demo;
    private String ename;
    private String job;
    private double sal;
    private double comm;
    public Emp(int demo, String ename, String job, double sal, double comm) {
        this.demo = demo;
        this.ename = ename;
        this.job = job;
        this.sal = sal;
        this.comm = comm;
    }
    public String getInfo() {
        return "demo=" + demo + ", ename=" + ename + ", job=" + job + ", sal=" + sal + ", comm=" 
+ comm;
    }
}

**第二步：**解决外键关系

一个雇员属于一个部门，应该在雇员里面保存部门信息

• Emp类：

private Dept dept;    // 表示对应的部门信息
public void setDept(Dept dept) {
    this.dept = dept;
}
public Dept getDept() {
    return this.dept;
}

一个部门有多个雇员

• Dept类

private Emp[] emps;    // 多个雇员
public void setEmps(Emp[] emps) {
    this.emps = emps;
}
public Emp[] getEmps() {
    return this.emps;
}

一个雇员有一个领导

private Emp mgr;    // 表示雇员对应的领导
public void setMgr(Emp mgr) {
    this.mgr = mgr;
}
public Emp getMgr() {
    return this.mgr;
}

所有匹配的映射关系都已经可以成功的描述出来了。

**第三步：**设置并取得数据

对于这种映射的方式一定要分两步完成，第一步根据结构设置数据，第二步根据结构取出数据。

• 设置数据

public class MainClass {
    public static void main(String[] args) {
        // 第一步：设置数据
        // 1、产生各自的独立对象
        Dept dept = new Dept(10, "ACCOUNTING", "New York"); // 部门信息
        Emp ea = new Emp(7369, "SMITH", "CLERK", 800.0, 0.0); // 雇员信息
        Emp eb = new Emp(7902, "FORD", "MANAGER", 2450.0, 0.0); // 雇员信息
        Emp ec = new Emp(7839, "KING", "PRESIDENT", 5000.0, 0.0); // 雇员信息
        // 2、设置雇员和领导关系
        ea.setMgr(eb);
        eb.setMgr(ec);
        // 3、设置雇员和部门关系
        ea.setDept(dept); // 雇员与部门
        eb.setDept(dept); // 雇员与部门
        ec.setDept(dept); // 雇员与部门
        dept.setEmps(new Emp[] {ea, eb, ec});
    }
}

• 取出数据

根据给定结构取出数据，要求如下：

• 可以根据一个雇员查询他对应的领导信息和部门信息；

• 可以根据一个部门取出所有的雇员以及每个雇员的领导信息；

System.out.println(ea.getInfo());
System.out.println("\t|-" + ea.getMgr().getInfo());
System.out.println("\t|-" + ea.getDept().getInfo());

System.out.println(dept.getInfo());
for (int i = 0; i < dept.getEmps().length; i ++) {
    System.out.println("\t|- " + dept.getEmps()[i].getInfo());
    if (dept.getEmps()[i].getMgr() != null) {
        System.out.println("\t\t|- " + dept.getEmps()[i].getMgr().getInfo());
    }
}

整个代码之中都是依靠代码链进行数据的取出的。

题目一：一对多

1、要求可以通过一个城市找到它对应的省份信息；

2、可以通过一个省份找到所有城市的信息；

**第一步：**先写出基本字段

class Province { // 类名称就是表名称
    private int pid;
    private String name;
    public Province(int pid, String name) {
        this.pid = pid;
        this.name = name;
    }
    public String getInfo() {
        return "pid=" + pid + ", name=" + name;
    }
}
class City {
    private int cid;
    private String name;
    public City(int cid, String name) {
        this.cid = cid;
        this.name = name;
    }
    public String getInfo() {
        return "cid=" + cid + ", name=" + name;
    }
}

**第二步：**设置关联字段(引用)

class Province { // 类名称就是表名称
    private int pid;
    private String name;
    private City[] cities;
    public Province(int pid, String name) {
        this.pid = pid;
        this.name = name;
    }
    public void setCities(City[] cities) {
        this.cities = cities;
    }
    public City[] getCities() {
        return this.cities;
    }
    public String getInfo() {
        return "pid=" + pid + ", name=" + name;
    }
}
class City {
    private int cid;
    private String name;
    private Province province;
    public City(int cid, String name) {
        this.cid = cid;
        this.name = name;
    }
    public void setProvince(Province province) {
        this.province = province;
    }
    public Province getProvince() {
        return this.province;
    }
    public String getInfo() {
        return "cid=" + cid + ", name=" + name;
    }
}

**第三步：**进行代码测试

按照两步完成，第一步是设置数据，第二步是根据结构取出数据。

public class MainClass {
    public static void main(String[] args) {
        // 第一步：设置关系数据
        // 1、先准备好各自独立的对象
        Province pro = new Province(1, "Shaan xi");
        City c1 = new City(1001, "Xi'an");
        City c2 = new City(1002, "Baoji");
        City c3 = new City(1003, "Xianyang");
        // 2、设置关系
        c1.setProvince(pro); // 一个城市属于一个省份
        c2.setProvince(pro);
        c3.setProvince(pro);
        pro.setCities(new City[] {c1, c2, c3}); // 一个省份有多个城市
        // 第二步：取出关系数据
        System.out.println(c2.getProvince().getInfo());
        for (int i = 0; i < pro.getCities().length; i ++) {
            System.out.println("\t|- " + pro.getCities()[i].getInfo());
        }
    }
}

题目二：一对多

1、可以通过一个类型找到它所对应的全部子类型；

2、可以通过一个类型找到他所对应的全部商品，以及每个商品对应的子类型；

3、可以通过一个子类型找到所有对应的全部商品。

**第一步：**设计基本字段

class Item { // 父栏目
    private int iid;
    private String name;
    private String note;
    public Item(int iid, String name, String note) {
        this.iid = iid;
        this.name = name;
        this.note = note;
    }
    public String getInfo() {
        return "iid=" + iid + ", name=" + name + ", note=" + note;
    }
}
class Subitem { // 子栏目
    private int sid;
    private String name;
    private String note;
    public Subitem(int sid, String name, String note) {
        this.sid = sid;
        this.name = name;
        this.note = note;
    }
    public String getInfo() {
        return "sid=" + sid + ", name=" + name + ", note=" + note;
    }
}
class Product { // 商品
    private int pid;
    private String name;
    private double price;
    public Product(int pid, String name, double price) {
        this.pid = pid;
        this.name = name;
        this.price = price;
    }
    public String getInfo() {
        return "pid=" + pid + ", name=" + name + ", price=" + price;
    }
}

**第二步：**设置关系

class Item { // 父栏目
    private int iid;
    private String name;
    private String note;
    private Subitem[] subitems;
    private Product[] products;
    public Item(int iid, String name, String note) {
        this.iid = iid;
        this.name = name;
        this.note = note;
    }
    public void setSubitems(Subitem[] subitems) {
        this.subitems = subitems;
    }
    public Subitem[] getSubitems() {
        return this.subitems;
    }
    public void setProducts(Product[] products) {
        this.products = products;
    }
    public Product[] getProducts() {
        return this.products;
    }
    public String getInfo() {
        return "iid=" + iid + ", name=" + name + ", note=" + note;
    }
}
class Subitem { // 子栏目
    private int sid;
    private String name;
    private String note;
    private Item item;
    private Product[] products;
    public Subitem(int sid, String name, String note) {
        this.sid = sid;
        this.name = name;
        this.note = note;
    }
    public void setItem(Item item) {
        this.item = item;
    }
    public Item getItem() {
        return this.item;
    }
    public void setProducts(Product[] products) {
        this.products = products;
    }
    public Product[] getProducts() {
        return this.products;
    }
    public String getInfo() {
        return "sid=" + sid + ", name=" + name + ", note=" + note;
    }
}
class Product { // 商品
    private int pid;
    private String name;
    private double price;
    private Item item;
    private Subitem subitem;
    public Product(int pid, String name, double price) {
        this.pid = pid;
        this.name = name;
        this.price = price;
    }
    public void setItem(Item item) {
        this.item = item;
    }
    public Item getItem() {
        return this.item;
    }
    public void setSubitem(Subitem subitem) {
        this.subitem = subitem;
    }
    public Subitem getSubitem() {
        return this.subitem;
    }
    public String getInfo() {
        return "pid=" + pid + ", name=" + name + ", price=" + price;
    }
}

**第三步：**设置关系以及数据的取出

public class MainClass {
    public static void main(String[] args) {
        // 第一步：设置数据关系
        // 1、准备出单独的类对象
        Item item = new Item(1, "PC", "-");
        Subitem suba = new Subitem(1001, "CPU", "-");
        Subitem subb = new Subitem(1002, "Hard disk", "-");
        Subitem subc = new Subitem(1003, "Graphics", "-");
        Product proa = new Product(90001, "Intel CORE i7", 998);
        Product prob = new Product(90002, "Intel CORE i5", 798);
        Product proc = new Product(90003, "Seagate 1TB", 398);
        Product prod = new Product(90004, "Seagate 500MB", 358);
        Product proe = new Product(90005, "NVIDIA 1080Ti", 1080);
        Product prof = new Product(90006, "NVIDIA TITAN", 9998);
        // 2、设置对象间的引用关系
        suba.setItem(item);
        subb.setItem(item);
        subc.setItem(item);
        item.setSubitems(new Subitem[] {suba, subb, subc});
        proa.setSubitem(suba);
        proa.setItem(item);
        prob.setSubitem(suba);
        prob.setItem(item);
        proc.setSubitem(subb);
        proc.setItem(item);
        prod.setSubitem(subb);
        prod.setItem(item);
        proe.setSubitem(subc);
        proe.setItem(item);
        prof.setSubitem(subc);
        prof.setItem(item);
        suba.setProducts(new Product[] {proa, prob});
        subb.setProducts(new Product[] {proc, prod});
        subc.setProducts(new Product[] {proe, prof});
        item.setProducts(new Product[] {proa, prob, proc, prod, proe, prof});
        // 第二步：取出数据
        // 1、通过一个类型找到对应的全部子类型
        System.out.println(item.getInfo());
        for (int i = 0; i < item.getSubitems().length; i ++) {
            System.out.println("\t|- " + item.getSubitems()[i].getInfo());
        }
        System.out.println("----------------------------------------------------------");
        // 2、通过一个类型找到他所对应的全部商品，以及每个商品对应的子类型
        System.out.println(item.getInfo());
        for (int i = 0; i < item.getProducts().length; i ++) {
            System.out.println("\t|- " + item.getProducts()[i].getInfo());
            System.out.println("\t\t|- " + item.getProducts()[i].getSubitem().getInfo());
        }
        System.out.println("----------------------------------------------------------");
        // 3、通过一个子类型找到所有对应的全部商品
        System.out.println(subb.getInfo());
        for (int i = 0; i < subb.getProducts().length; i ++) {
            System.out.println("\t|- " + subb.getProducts()[i].getInfo());
        }
    }
}

题目三：多对多

1、可以根据一个管理员找到它所对应的角色，以及每个角色包含的所有权限组的信息，以及每个权限组所包含的所有权限的内容；

2、可以根据一个权限组找到所有具备此权限组的角色以及每个角色所拥有的管理员信息；

在本次的设计之中给出的“角色_权限组”表实际上属于一张关系表，它所保存的只是两个外键的关联关系，这样的关系表不需要为其生成映射类。

生成的只能是实体表的转换操作，而多对多的中间转换表只需要通过类属性的关系配置即可。

**第一步：**生成基本的字段映射

class Admin {
    private String aid;
    private String password;
    public Admin(String aid, String password) {
        this.aid = aid;
        this.password = password;
    }
    public String getInfo() {
        return "aid=" + aid + ", password=" + password;
    }
}
class Role {
    private int rid;
    private String title;
    public Role(int rid, String title) {
        this.rid = rid;
        this.title = title;
    }
    public String getInfo() {
        return "rid=" + rid + ", title=" + title;
    }
}
class Group {
    private int gid;
    private String title;
    public Group(int gid, String title) {
        this.gid = gid;
        this.title = title;
    }
    public String getInfo() {
        return "gid=" + gid + ", title=" + title;
    }
}
class Action {
    private int aid;
    private String title;
    private String url;
    public Action(int aid, String title, String url) {
        this.aid = aid;
        this.title = title;
        this.url = url;
    }
    public String getInfo() {
        return "aid=" + aid + ", title=" + title + ", url=" + url;
    }
}

**第二步：**设置引用关系

class Admin {
    private String aid;
    private String password;
    private Role role;
    public Admin(String aid, String password) {
        this.aid = aid;
        this.password = password;
    }
    public void setRole(Role role) {
        this.role = role;
    }
    public Role getRole() {
        return this.role;
    }
    public String getInfo() {
        return "aid=" + aid + ", password=" + password;
    }
}
class Role {
    private int rid;
    private String title;
    private Admin[] admins;
    private Group[] groups;
    public Role(int rid, String title) {
        this.rid = rid;
        this.title = title;
    }
    public void setAdmins(Admin[] admins) {
        this.admins = admins;
    }
    public Admin[] getAdmins() {
        return this.admins;
    }
    public void setGroups(Group[] groups) {
        this.groups = groups;
    }
    public Group[] getGroups() {
        return this.groups;
    }
    public String getInfo() {
        return "rid=" + rid + ", title=" + title;
    }
}
class Group {
    private int gid;
    private String title;
    private Role[] roles;
    private Action[] actions;
    public Group(int gid, String title) {
        this.gid = gid;
        this.title = title;
    }
    public void setRoles(Role[] roles) {
        this.roles = roles;
    }
    public Role[] getRoles() {
        return this.roles;
    }
    public void setActions(Action[] actions) {
        this.actions = actions;
    }
    public Action[] getActions() {
        return this.actions;
    }
    public String getInfo() {
        return "gid=" + gid + ", title=" + title;
    }
}
class Action {
    private int aid;
    private String title;
    private String url;
    private Group group;
    public Action(int aid, String title, String url) {
        this.aid = aid;
        this.title = title;
        this.url = url;
    }
    public void setGroup(Group group) {
        this.group = group;
    }
    public Group getGroup() {
        return this.group;
    }
    public String getInfo() {
        return "aid=" + aid + ", title=" + title + ", url=" + url;
    }
}

实际上多对多和一对多从类的定义上来讲区别不是很大。

**第三步：**设置并取得数据

public class MainClass {
    public static void main(String[] args) {
        // 第一步：设置完整关系
        // 1、准备出若干个对象
        Admin a1 = new Admin("admin", "hello");
        Admin a2 = new Admin("guest", "hello");
        Admin a3 = new Admin("scott", "tiger");
        Role r1 = new Role(1, "System administrator");
        Role r2 = new Role(2, "Information administrator");
        Group g1 = new Group(10, "Information manager");
        Group g2 = new Group(20, "User manager");
        Group g3 = new Group(30, "Data manager");
        Group g4 = new Group(40, "Interface manager");
        Group g5 = new Group(50, "Backups manager");
        Action at1 = new Action(1001, "get info", "-");
        Action at2 = new Action(1002, "del info", "-");
        Action at3 = new Action(1003, "add info", "-");
        Action at4 = new Action(1004, "add user", "-");
        Action at5 = new Action(1005, "del user", "-");
        Action at6 = new Action(1006, "update user", "-");
        Action at7 = new Action(1007, "user data", "-");
        Action at8 = new Action(1008, "dept data", "-");
        Action at9 = new Action(1009, "co. data", "-");
        Action at10 = new Action(1010, "service", "-");
        Action at11 = new Action(1011, "shrot message", "-");
        Action at12 = new Action(1012, "all backups", "-");
        Action at13 = new Action(1013, "scope backups", "-");
        // 2、要设置这些对象之间的基本关系
        // 设置管理员与角色
        a1.setRole(r1);
        a2.setRole(r2);
        a3.setRole(r2);
        r1.setAdmins(new Admin[] {a1});
        r2.setAdmins(new Admin[] {a2, a3});
        // 设置角色与管理员组
        r1.setGroups(new Group[] {g1, g2, g3, g4, g5});
        r2.setGroups(new Group[] {g1, g2});
        g1.setRoles(new Role[] {r1, r2});
        g2.setRoles(new Role[] {r1, r2});
        g3.setRoles(new Role[] {r1});
        g4.setRoles(new Role[] {r1});
        g5.setRoles(new Role[] {r1});
        // 设置管理员组与权限
        g1.setActions(new Action[] {at1, at2, at3});
        g2.setActions(new Action[] {at4, at5, at6});
        g3.setActions(new Action[] {at7, at8, at9});
        g4.setActions(new Action[] {at10, at11});
        g5.setActions(new Action[] {at12, at13});
        at1.setGroup(g1);
        at2.setGroup(g1);
        at3.setGroup(g1);
        at4.setGroup(g2);
        at5.setGroup(g2);
        at6.setGroup(g2);
        at7.setGroup(g3);
        at8.setGroup(g3);
        at9.setGroup(g3);
        at10.setGroup(g4);
        at11.setGroup(g4);
        at12.setGroup(g5);
        at13.setGroup(g5);
        // 第二步：取出数据内容
        System.out.println(a1.getInfo());
        System.out.println("\t|- " + a1.getRole().getInfo());
        for (int i = 0; i < a1.getRole().getGroups().length; i ++) {
            System.out.println("\t\t|- " + a1.getRole().getGroups()[i].getInfo());
            for (int j = 0; j < a1.getRole().getGroups()[i].getActions().length; j ++) {
                System.out.println("\t\t\t|- " + a1.getRole().getGroups()[i].getActions()[j].getInfo());
            }
        }
        System.out.println("-------------------------------------------------------------");
        System.out.println(g2.getInfo());
        for (int i = 0; i < g2.getRoles().length; i ++) {
            System.out.println("\t|- " + g2.getRoles()[i].getInfo());
            for (int j = 0; j < g2.getRoles()[i].getAdmins().length; j ++) {
                System.out.println("\t\t|- " + g2.getRoles()[i].getAdmins()[j].getInfo());
            }
        }
    }
}