46、作业

古城微笑少年丶 2024-03-31 11:50 92阅读 0赞

1、![b734017375d84af2b0c95c56eca84843.png][]

//我的代码：

package Collection_;
    import java.lang.String;
    import java.util.ArrayList;
    import java.util.Comparator;
    import java.util.Iterator;
    import java.util.List;
    @SuppressWarnings({"all"})
    public class Homework01 {
        public static void main(String[] args) {
            List list=new ArrayList();
            list.add(new New("新冠确认病例超千万，数百万印度教信徒赴恒河“圣浴“引民众担忧"));
            list.add(new New("男子突然想起2个月前钓的鱼还在网兜里，捞起一看赶紧放生"));
            Iterator iterator=list.iterator();
            while (iterator.hasNext()) {
                Object obj =  iterator.next();
                New obj_=(New)obj;
                String title=obj_.getTitle();
                if(title.length()>15){
                    title=title.substring(0,14)+"...";
                }
                System.out.println(title);
            }
        }
    }
    class New{
        private String title;
        private String content;
    
        public New(String title) {
            this.title = title;
        }
    
        public String getTitle() {
            return title;
        }
    
        public void setTitle(String title) {
            this.title = title;
        }
    
        public String getContent() {
            return content;
        }
    
        public void setContent(String content) {
            this.content = content;
        }
    
        @Override
        public String toString() {
            return "New{" +
                    "title='" + title + '\'' +
                    '}';
        }
    }
    //新冠确认病例超千万，数百万印...
    //男子突然想起2个月前钓的鱼还...

//老师的代码：

package Collection_;
    import java.util.ArrayList;
    @SuppressWarnings({"all"})
    public class Homework01 {
        public static void main(String[] args) {
            ArrayList arrayList=new ArrayList();
            arrayList.add(new News("新冠确认病例超千万，数百万印度教信徒赴恒河\"圣浴\"引民众担忧"));
            arrayList.add(new News("男子突然想起2个月前钓的鱼还在网兜里，捞起一看赶紧放生"));
    
            int size=arrayList.size();
            for(int i=size-1;i>=0;i--){
                News news=(News)arrayList.get(i);
                System.out.println(processTitle(news.getTitle()));
            }
        }
        //专门写一个方法来处理新闻标题
        public static String processTitle(String title){
            if (title == null) {
                return "";
            }
            if(title.length()>15){
                return title.substring(0,15)+"...";//[0,15)
            }else{
                return title;
            }
        }
    }
    class News{
        private String title;
        private String content;
    
        public News(String title) {
            this.title = title;
        }
    
        public String getTitle() {
            return title;
        }
    
        public void setTitle(String title) {
            this.title = title;
        }
    
        public String getContent() {
            return content;
        }
    
        public void setContent(String content) {
            this.content = content;
        }
    
        @Override
        public String toString() {
            return "News{" +
                    "title='" + title + '\'' +
                    '}';
        }
    }
    //男子突然想起2个月前钓的鱼还在...
    //新冠确认病例超千万，数百万印度...

2、![d6aa3d87a93049bfb7967cb1588b041b.png][]

package Collection_;
    
    import java.util.ArrayList;
    @SuppressWarnings({"all"})
    public class Homework02 {
        public static void main(String[] args) {
            ArrayList arrayList=new ArrayList();
            ArrayList arrayList1=new ArrayList();
            Car car=new Car("宝马",400000);
            Car car1=new Car("宾利",5000000);
            arrayList.add(car);
            arrayList.add(car1);
            arrayList1.add(car1);
            arrayList1.add(car);
            System.out.println("arrayList="+arrayList);
    
            arrayList.remove(0);
            arrayList.contains(car);
            System.out.println(arrayList.size());
            System.out.println(arrayList.isEmpty());
            arrayList.clear();
            arrayList.addAll(arrayList1);
            System.out.println(arrayList.containsAll(arrayList1));
            arrayList.removeAll(arrayList1);
            System.out.println(arrayList.isEmpty());
            //arrayList=[Car{name='宝马', price=400000.0}, Car{name='宾利', price=5000000.0}]
            //1
            //false
            //true
            //true
        }
    }
    class Car{
        private String name;
        private double price;
    
        public Car(String name, double price) {
            this.name = name;
            this.price = price;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public double getPrice() {
            return price;
        }
    
        public void setPrice(double price) {
            this.price = price;
        }
    
        @Override
        public String toString() {
            return "Car{" +
                    "name='" + name + '\'' +
                    ", price=" + price +
                    '}';
        }
    }

3、![2011038feb1948e8ae0df8ce2bef62a7.png][]

//我的代码：

package Collection_;
    
    import java.util.HashMap;
    import java.util.Map;
    import java.util.Set;
    
    @SuppressWarnings({"all"})
    public class Homework03 {
        public static void main(String[] args) {
            Map m=new HashMap();
            m.put("jack",650);
            m.put("tom",1200);
            m.put("smith",2900);
            System.out.println(m);
            m.replace("jack",2600);
            System.out.println(m);
            Set keyset=m.keySet();//把键取出来
            for (Object key :keyset) {
                Integer values=(Integer) m.get(key);
                values+=100;
                System.out.println(key+"="+values);
            }
    
        }
    }
    //{tom=1200, smith=2900, jack=650}
    //{tom=1200, smith=2900, jack=2600}
    //tom=1300
    //smith=3000
    //jack=2700

//老师的代码

package Collection_;
    
    import java.util.*;
    
    @SuppressWarnings({"all"})
    public class Homework03 {
        public static void main(String[] args) {
            Map m=new HashMap();
            m.put("jack",650);//int-->Integer，自动封箱
            m.put("tom",1200);
            m.put("smith",2900);
            System.out.println(m);
            m.put("jack",2600);//替换，更新
            System.out.println(m);
            Set keyset=m.keySet();//把键取出来
            for (Object key :keyset) {
                m.put(key,(Integer) m.get(key)+100);
            }
            System.out.println(m);
    
            System.out.println("===========遍历所有的员工============");
            Set entrySet=m.entrySet();
            Iterator iterator=entrySet.iterator();
            while (iterator.hasNext()) {
                Map.Entry entry=(Map.Entry)iterator.next();
                System.out.println(entry.getKey()+"-"+entry.getValue());
            }
    
            System.out.println("===========遍历所有的工资============");
            Collection values=m.values();
            for(Object value:values){
                System.out.println("工资："+value);
            }
        }
    }
    //{tom=1200, smith=2900, jack=650}
    //{tom=1200, smith=2900, jack=2600}
    //{tom=1300, smith=3000, jack=2700}
    //===========遍历所有的员工============
    //tom-1300
    //smith-3000
    //jack-2700
    //===========遍历所有的工资============
    //工资：1300
    //工资：3000
    //工资：2700

4、分析HashSet和TreeSet分别如何实现去重的

（1）HashSet的去重机制：hashCode()+equals()，底层先通过存入对象，进行运算得到一个hash值，通过hash值得到对应的索引，如果发现table索引所在的位置，没有数据，就直接存放，如果有数据，就进行equals比较（遍历比较），如果比较后，不相同，就加入，否则就不加入

（2）TreeSet的去重机制：如果你传入了一个Comparator匿名对象，就使用实现的compare去重，如果方法返回0，就认为是相同的元素/数据，就不添加，如果你没有传入一个Comparator匿名对象，则以你添加的对象实现的Compareable接口的compareTo去重

5、![3b1f11cf3eb6403ea4449bac39ad419a.png][]

package Collection_;
    import java.util.TreeSet;
    @SuppressWarnings({"all"})
    public class Homework05 {
        public static void main(String[] args) {
            TreeSet treeSet=new TreeSet();
            //分析源码：
            //add方法，因为TreeSet()构造器没有传Comparator接口的匿名内部类
            //所以在底层Comparable<? super K> k = (Comparable<? super K>) key;
            //即把Person转成Comparable类型，但是Person没有重写Comparable的方法
            treeSet.add(new Person001());//所以会抛出一个ClassCastException.
    
            //treeSet.add(new Person());
            //treeSet.add(new Person());
            //treeSet.add(new Person());
            //无论加多少个treeSet.add(new Person());最终都是只有一个，因为class Person_ implements Comparable
            //返回的永远是0，判断永远相等，后面的永远加不进去
    
            System.out.println(treeSet);
            //输出：[Collection_.Person001@27d6c5e0]
        }
    }
    //class Person_{}
    //解决方案：
    class Person001 implements Comparable{
    
        @Override
        public int compareTo(Object o) {
            return 0;
        }
    }

6、![12b87767a39940f99d5c2923bf74fb18.png][]

package Collection_;
    
    import java.util.HashSet;
    import java.util.Objects;
    
    @SuppressWarnings({"all"})
    public class Homework06 {
        public static void main(String[] args) {
            HashSet set=new HashSet();
            Person02 p1=new Person02(1001,"AA");
            Person02 p2=new Person02(1002,"BB");
            set.add(p1);
            set.add(p2);
            p1.name="CC";
            set.remove(p1);//删除不成功，因为p1.name被修改后的存放的位置已经不再是原来的位置了，hash()方法被重写了
            System.out.println("更换p1.name后："+set);//输出2个对象
            set.add(new Person02(1001,"CC"));
            System.out.println("添加new Person02(1001,\"CC\")后："+set);//输出3个对象
            set.add(new Person02(1001,"AA"));
            System.out.println("添加new Person02(1001,\"AA\")后："+set);//输出4个对象
        }
    }
    class Person02{
        public String name;
        public int id;
    
        public Person02(int id,String name) {
            this.name = name;
            this.id = id;
        }
    
        @Override
        public String toString() {
            return "Person02{" +
                    "name='" + name + '\'' +
                    ", id=" + id +
                    '}';
        }
    
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Person02 person02 = (Person02) o;
            return id == person02.id && Objects.equals(name, person02.name);
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(name, id);
        }
    }
    //更换p1.name后：[Person02{name='BB', id=1002}, Person02{name='CC', id=1001}]
    //添加new Person02(1001,"CC")后：[Person02{name='BB', id=1002}, Person02{name='CC', id=1001}, Person02{name='CC', id=1001}]
    //添加new Person02(1001,"AA")后：[Person02{name='BB', id=1002}, Person02{name='CC', id=1001}, Person02{name='CC', id=1001}, Person02{name='AA', id=1001}]

[b734017375d84af2b0c95c56eca84843.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/89fb44f9ff794ae3a2bdf787be9d672f.png
[d6aa3d87a93049bfb7967cb1588b041b.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/b6984eb270904c2d82f7ab79cabf74ad.png
[2011038feb1948e8ae0df8ce2bef62a7.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/f0a82d5d221d4da5b2fa6bbb4d5f23e9.png
[3b1f11cf3eb6403ea4449bac39ad419a.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/6fc331e3134b4baea63d793fc2ece0a0.png
[12b87767a39940f99d5c2923bf74fb18.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/31/1b2035bb80b04de7872baac9457fe154.png