HADOOP大数据处理技术9-JavaSe

心若有阳光你便会看见这个世界有那么多美好值得期待和向往

2024/4/9

13.static 用于修饰属性和方法 static属性通过类名访问

1）static实现内存共享

bean1和bean2的内存是互相独立的怎么实现内存共享呢？

共享内存public class Comm {    public static float money;}public class HomeMember {    private String name;    private String sex;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public String getSex() {        return sex;    }    public void setSex(String sex) {        this.sex = sex;    }    public void store(float c){        Comm.money=Comm.money+c;    }    public void consumer(float c){        Comm.money=Comm.money-c;    }}public class Test {    public static void main(String[] args) {        HomeMember homeMember=new HomeMember();        homeMember.setName("zhangsan");        homeMember.store(3000);        HomeMember homeMember1=new HomeMember();        homeMember1.setName("lisi");        homeMember1.consumer(1000);        System.out.println(Comm.money);    }}

练习：学校的资金 共享public class Expend {    public static double money;}public class School {    private String name;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public void paid(double d){        Expend.money=Expend.money-d;    }    public void income(double d){        Expend.money=Expend.money+d;    }}public class Test1 {    public static void main(String[] args) {        School school=new School();        school.setName("shandong");        school.income(234556);        School school1=new School();        school1.setName("guanli");        school1.paid(2763);        System.out.println(Expend.money);    }}

2）为静态属性初始化赋值需要用静态块

3）静态方法：写了static之后这个方法就不再属于某个对象了可以直接使用类名.来调用

import java.text.SimpleDateFormat;import java.util.Date;public class DateUtils {    //Date yyyy-mm-dd hh:mm:ss mi    //在开发当中 经常要用yyyy-mm-dd 2020-12-12    public static String toYYMMDD(Date date){//Alt+Enter导入类        SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");        String dt=sdf.format(date);        return dt;    }    //在开发当中 经常要用yyyy-mm-dd 2020-12-12    public String toYYMM(Date date){        SimpleDateFormat sdf=new SimpleDateFormat("YYYY-MM");        String dt=sdf.format(date);        return dt;    }}import java.util.Date;public class Test2 {    public static void main(String[] args) {//        DateUtils dateUtils=new DateUtils();//        String dt=dateUtils.toYYMMDD(new Date());//        System.out.println(dt);          String dt=DateUtils.toYYMMDD(new Date());        System.out.println(dt);           String dt1=DateUtils.toYYMM(new Date());        System.out.println(dt1);    }}

以前我们调用方法需要创建对象现在我们可以直接使用类名来调用（static）

14.final

final放到类上不能被继承

final放到方法上不能被重写

final放到属性上不能被更改

15.StringBuffer（String）

执行速度慢线程安全

执行速度快线程不安全

public class Test {    public static void main(String[] args) {        String str="xxx";        //方法有synchronized关键字        //同步 先来先调用 后来后调用        StringBuffer stringBuffer=new StringBuffer("aa");        stringBuffer.append("bb");        stringBuffer.append("cc");//动态添加        stringBuffer.append("dd");        System.out.println(stringBuffer);        //方法没有synchronized关键字        StringBuilder stringBuilder=new StringBuilder("aa");        stringBuilder.append("bb");        stringBuilder.append("cc");        stringBuilder.append("dd");    }}

16.接口（父类的思想接口相当于父类）

1）创建接口右键 Ixxxxx(如IStudentDAO)

2）接口中只放方法的声明

3）创建实现类实现接口

4）创建接口的原因（好处）：松耦合保证架构的稳定使我们在维护的时候改变的代码最少（父类的思想接口相当于父类）

为什么不选择父类而选择接口呢？

因为父类中的是通用方法子类只能调用父类的方法而接口的类中都有方法的实现可以调用所有的方法所以说开发当中接口用的比较多

//开发的时候：通过接口 implements就是为声明加{} 实现public class StudentDao implements IStudentDAO{    //几十个参数    public void addStu(Student stu){        System.out.println(stu.getStuName()+" "+stu.getStuSex()+" "+stu.getStuAge());    }}public interface IStudentDAO {    //接口中只放方法的声明    //先声明 在创建DAO类 实现方法    public void addStu(Student stu);}

2024/4/10

练习：课程 使用接口1.创建接口 放上方法的声明import java.util.ArrayList;public interface ICourseDAO {    public void addCou(Course course);    public void updateCou(Course course);    public ArrayList<Course> findallCous();}2.创建实体类 包括 属性 get set 方法public class Course {    private int cid;    private String cname;    public int getCid() {        return cid;    }    public void setCid(int cid) {        this.cid = cid;    }    public String getCname() {        return cname;    }    public void setCname(String cname) {        this.cname = cname;    }}3.写主方法import java.util.ArrayList;public class Test2 {    public static void main(String[] args) {        ICourseDAO courseDAO=new CourseDao();        Course c=new Course();        c.setCid(3);        c.setCname("lis");        courseDAO.addCou(c);        Course c1=new Course();        c1.setCid(4);        c1.setCname("maliu");        courseDAO.updateCou(c1);        ArrayList<Course> list=courseDAO.findallCous();        //增强for循环        for (Course course : list) {            System.out.println(course.getCid()+" "+course.getCname());        }    }}

练习：新闻 使用接口1.创建实体类public class New {    private String nsme;    private String content;    public String getNsme() {        return nsme;    }    public void setNsme(String nsme) {        this.nsme = nsme;    }    public String getContent() {        return content;    }    public void setContent(String content) {        this.content = content;    }}2.创建接口 放上方法的声明import java.util.ArrayList;public interface INewDAO {    public void addNew(New n);    public void updateNew(New n);    public ArrayList<New> findallNews();}import java.util.ArrayList;3.创建实现类 为声明添加实现public class NewDao implements INewDAO{    public void addNew(New n){        System.out.println(n.getNsme()+" "+n.getContent());    }    public void updateNew(New n){        System.out.println(n.getNsme()+" "+n.getContent());    }    public ArrayList<New> findallNews(){        ArrayList<New> list=new ArrayList<>();        New n1=new New();        n1.setNsme("社会");        n1.setContent("外交会见");        list.add(n1);        New n2=new New();        n2.setNsme("天气");        n2.setContent("炎热无比");        list.add(n2);        return list;    }}4.创建测试类import java.util.ArrayList;public class Test3 {    public static void main(String[] args) {        INewDAO newDAO=new NewDao(); //父类可以接收子类的对象        New n1=new New();        n1.setNsme("政策");        n1.setContent("贷款5000玩");        newDAO.addNew(n1);        New n2=new New();        n2.setNsme("心灵鸡汤");        n2.setContent("勇敢迈出脚步");        newDAO.updateNew(n2);        ArrayList<New> list=newDAO.findallNews();        for (New aNew : list) {            System.out.println(aNew.getNsme()+" "+aNew.getContent());        }    }}

练习： 教师 使用接口1.创建实体类public class Teacher {    private int tid;    private String tname;    public int getTid() {        return tid;    }    public void setTid(int tid) {        this.tid = tid;    }    public String getTname() {        return tname;    }    public void setTname(String tname) {        this.tname = tname;    }}import java.util.ArrayList;2.创建接口 放入方法的声明public interface ITeacherDAO {    public void addTeacher(Teacher t);    public void updateTeacher(Teacher t);    public ArrayList<Teacher> findallTeachers();}3.创建实现类 为声明添加实现import java.util.ArrayList;public class TeacherDao implements ITeacherDAO{    @Override    public void addTeacher(Teacher t) {        System.out.println(t.getTid()+" "+t.getTname());    }    @Override    public void updateTeacher(Teacher t) {        System.out.println(t.getTid()+" "+t.getTname());    }    @Override    public ArrayList<Teacher> findallTeachers() {        ArrayList<Teacher> list=new ArrayList<>();        Teacher t1=new Teacher();        t1.setTid(1);        t1.setTname("zhangsan");        list.add(t1);        Teacher t2=new Teacher();        t2.setTid(2);        t2.setTname("lisi");        list.add(t2);        return list;    }}4.测试类import java.util.ArrayList;public class Test4 {    public static void main(String[] args) {        ITeacherDAO teacherDAO=new TeacherDao();        Teacher t3=new Teacher();        t3.setTid(3);        t3.setTname("wangwu");        teacherDAO.addTeacher(t3);        Teacher t4=new Teacher();        t4.setTid(4);        t4.setTname("maliuu");        teacherDAO.updateTeacher(t4);        ArrayList<Teacher> list=teacherDAO.findallTeachers();        for(int i=0;i<list.size();i++){            Teacher t=list.get(i);            System.out.println(t.getTid()+" "+t.getTname());        }    }}