3.24作业

1、有如下map:
Map<String,String> map = new HashMap<>();
map.put(“aaa”,“111”);
map.put(“bbb”,“111”);
map.put(“ccc”,“111”);
map.put(“ddd”,“222”);
使用3种方式，遍历并打印

import java.util.*;

public class Demo01 {
    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        map.put("aaa","111");
        map.put("bbb","111");
        map.put("ccc","111");
        map.put("ddd","222");
        System.out.println(map);
        System.out.println("======第一种========");
        Set<String> strings = map.keySet();
        Iterator<String> iterator = strings.iterator();
        while (iterator.hasNext()){
            String next = iterator.next();
            String s = map.get(next);
            System.out.println(next+":"+s);
        }
        System.out.println("=====第二种======");
        Collection<String> values = map.values();
        Iterator<String> iterator1 = values.iterator();
        while (iterator1.hasNext()){
            String next = iterator1.next();
            System.out.println(next);
        }
        System.out.println("=======第三种========");
        Set<Map.Entry<String, String>> entries = map.entrySet();
        Iterator<Map.Entry<String, String>> iterator2 = entries.iterator();
        while (iterator2.hasNext()){
            Map.Entry<String, String> next = iterator2.next();
            String value = next.getValue();
            String key = next.getKey();
            System.out.println(key+":"+value);
        }
    }
}

2、定义集合List， 依次将1，2，3，4，5，6，7，8，9 添加到集合中
使用Collections中的方法：
打乱顺序
求最大值
求最小值
升序排列
交换位置
将该集合所有元素拷贝到另一个集合中并打印

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Demo02 {
    public static void main(String[] args) {
        int[] arr = new int[9];
        List<Integer> list = new ArrayList<Integer>();
        List<Integer> list1 = new ArrayList<Integer>();
        for (int i = 0; i < arr.length; i++) {
            arr[i] = i;
            list.add(arr[i]);
        }
        for (int i = 0; i < arr.length; i++) {
            list1.add(arr[i]);
        }
        System.out.println(list);//第一次输出
        Collections.reverse(list);//打乱顺序
        System.out.println(list);//第二次输出
        Integer max = Collections.max(list);//输出最大值
        System.out.println("最大值："+max);
        Integer min = Collections.min(list);//输出最小值
        System.out.println("最小值："+min);
        Collections.sort(list);//从小到大升序排列
        System.out.println(list);
        Collections.swap(list,3,6);//交换位置
        System.out.println(list);//第三次输出
        Collections.copy(list,list1);//拷贝
        System.out.println(list1);

    }
}

3、编写代码，模拟如下异常：
ArithmeticException类 - 算术异常
ArrayIndexOutOfBoundsException类 - 数组下标越界异常
NullPointerException - 空指针异常
ClassCastException - 类型转换异常
NumberFormatException - 数字格式异常
OutOfMemoryError - 内存溢出错误

import java.util.Scanner;

public class Demo03 {
    public static void main(String[] args) {
//        System.out.println(1/0);                            //-算术异常
//        int[] arr = {1,3,4};    System.out.println(arr[3]);//- 数组下标越界异常
//        int[] arr = {1,2,3}; arr = null;System.out.println(arr[4]);//空指针异常
//        A b = new B();//类型转换异常
//        A b1 = (C) b;
//        String str = "abc";//内存溢出错误
//        for (int i = 0; i < 10000; i++) {
//            str = str+str;
//        }
//        System.out.println(str);
//        int i= Integer.parseInt("ab3");//数字格式异常
    }
}
class A{}
class B extends A{}
class C extends B{}

4、分析以下需求，并用代码实现：
(1)统计每个单词出现的次数
(2)有如下字符串
“If you want to change your fate I think you must come to the school to learn java”(用空格间隔)
(3)打印格式：
to=3
think=1
you=2

import com.qiku.day02.ListDemo01;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Demo04 {
    public static void main(String[] args) {
        List<String> list = new ArrayList<String >();
       String[] strs = {"If" ,"you" ,"want" ,"to" ,"change","your", "fate", "I", "think" ,"you", "must" ,"come","to","the" ,"school", "to" ,"learn", "java"};
        for (int i = 0; i < strs.length; i++) {
            list.add(strs[i]);
        }
        System.out.println(list);
        for (int i = 0; i < list.size(); i++) {
            int index =1;
            for (int j = 1; j < list.size(); j++) {
                if(list.get(i)==list.get(j)){
                    index++;
                    list.remove(j);
                }
            }
            System.out.println(list.get(i)+"="+index);
        }
    }
}