集合框架学习题

ArrayList集合存储HashMap元素并遍历

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Set;

public class ArrayListIncludeHashMapDemo {
    public static void main(String[] args) {
        //创建ArrayList集合
        ArrayList> array = new ArrayList>();
        HashMap hm1 = new HashMap<>();
        hm1.put("张1","张艺");
        hm1.put("张2","张尔");
        array.add(hm1);

        HashMap hm2 = new HashMap<>();
        hm2.put("赵1","赵一");
        hm2.put("赵2","赵二");
        array.add(hm2);

        HashMap hm3 = new HashMap<>();
        hm3.put("钱1","钱意");
        hm3.put("钱2","钱而");
        array.add(hm3);

        for (HashMap hm:array){
            Set keySet = hm.keySet();
            for (String key:keySet){
                String value = hm.get(key);
                System.out.println(key+" ,"+value);
            }
        }
    }
}

HashMap集合存储ArrayList 元素并遍历

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Set;

public class HashMapIncludeArrayListDemo1 {
    public static void main(String[] args) {
        HashMap> hm = new HashMap>();
        ArrayList sgyy = new ArrayList<>();
        sgyy.add("诸葛亮");
        sgyy.add("周瑜");
        hm.put("三国演义",sgyy);

        ArrayList xyj = new ArrayList<>();
        xyj.add("孙悟空");
        xyj.add("猪八戒");
        hm.put("西游记",xyj);

        ArrayList hlm = new ArrayList<>();
        hlm.add("贾宝玉");
        hlm.add("林黛玉");
        hm.put("红楼梦",hlm);

        Set keySet = hm.keySet();
        for (String key:keySet){
            System.out.println(key);
            ArrayList value = hm.get(key);
            for (String s:value){
                System.out.println("\t"+s);
            }
        }
    }

}

  
需求
  键盘录入一个字符串，要求统计字符中每个字符串出现的次数
  举例：键盘录入“aababcabcdabcde”在控制台输出“a(5)b(4)c(3)d(2)e(1)”

思路
1.键盘录入一个字符串
2.创建HashMap集合，键是Character,值是Integer
3.遍历字符串得到每一个字符
4.得到的每一个字符作为键到HashMap集合中去找对应的值，看其返回值
     如果返回值是null：说明该字符在HashMap集合中不存在，就把该字符作为键，1作为值存储
     如果返回值不是null，说明该字符在HashMap集合中不存在，把该值加1，然后重新存储该字符和对应的值
5.遍历HashMap集合，得到键和值，按照要求进行拼接
6.输出结果
 

public class HashMapDemo1 {
     public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入一个字符串");
        String line = sc.nextLine();

        //创建HashMap集合
        HashMap hm = new HashMap();

        for (int i = 0; i < line.length(); i++) {
            char key = line.charAt(i);
            Integer value = hm.get(key);
            if(value==null){
                hm.put(key,1);
            }else{
                value++;
                hm.put(key,value);
            }
        }
        StringBuilder sb = new StringBuilder();
        Set keySet = hm.keySet();
        for(Character key:keySet){
            Integer value = hm.get(key);
            sb.append(key).append("(").append(value).append(")");
        }
        String s = sb.toString();
        System.out.println(s);
    }
}

需求：ArrayList存储学生对象，使用Collections对ArrayList进行排序

要求：按照年龄从小到大排序，年龄相同时，按照姓名的字母顺序排序

package heimaStudy.arrayList;

import heimaStudy.Test2.Student;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;

public class CollectionsDemo2 {
        public static void main(String[] args) {
            ArrayList stu = new ArrayList<>();
            Student s1 = new Student("张三", 18);
            Student s2 = new Student("李四", 17);
            Student s3 = new Student("王二", 19);
            stu.add(s1);
            stu.add(s2);
            stu.add(s3);

            Collections.sort(stu, new Comparator() {
                @Override
                public int compare(Student s1, Student s2) {
                    int num=s1.getAge()-s2.getAge();
                   int num1=num==0?s1.getName().compareTo(s2.getName()):num;
                    return num1;
                }
            });
            for (Student s:stu){
                System.out.println(s.getName()+" ,"+s.getAge());
            }
        }
}

//学生类
public class Student {
    private String name;
    private Integer age;

    public Student() {
    }

    public Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

需求：编写一个程序，获取10个1-20之间的随机数，要求随机数不能重复，并在控制台输出
思路：
1.创建Set集合对象
2.创建随机数对象
3.判断集合长度是不是小于10
  是：产生一个随机数添加到集合
  回到3继续
4.遍历集合

import java.util.Random;
import java.util.Set;
import java.util.TreeSet;

public class SetDemo1 {
  
    public static void main(String[] args) {
        //HashSet不排序
      //  Set set = new HashSet();
        //TreeSet可排序
        Set set = new TreeSet<>();
        Random r = new Random();
        while (set.size()<10){
            int num = r.nextInt(20)+1;
            set.add(num);
        }
        for (Integer i:set){
            System.out.println(i);
        }
    }
}

比较器排序comparator的使用

存储学生对象并遍历，创建TreeSet集合使用带参构造方法

要求：按照年龄从小到大排序，年龄相同时按照姓名的字母顺序排序

package heimaStudy.Test4;

import java.util.Comparator;
import java.util.TreeSet;

public class TreeSetDemo2 {
    public static void main(String[] args) {
        TreeSet ts = new TreeSet<>(new Comparator() {
            @Override
            public int compare(Student s1, Student s2) {
                int num=s1.getAge()-s2.getAge();
                int num2=num==0?s1.getName().compareTo(s2.getName()):num;
                return num2;
            }
        });
        Student stu1 = new Student("张三",18);
        Student stu2 = new Student("李四",16);
        Student stu3 = new Student("王五",19);
        Student stu4 = new Student("赵二",21);
        Student stu5 = new Student("孙达",16);
        Student stu6 = new Student("赵二",21);
        ts.add(stu1);
        ts.add(stu2);
        ts.add(stu3);
        ts.add(stu4);
        ts.add(stu5);
        ts.add(stu6);

        //遍历
        for(Student s:ts){
            System.out.println(s.getName()+" ,"+s.getAge());
        }
    }
}

//学生类
public class Student {
    private String name;
    private Integer age;

    public Student() {
    }

    public Student(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}