关于Map集合的练习

关于Map集合的练习

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 mapDemo {
    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");

        Set<String> keys = map.keySet();
        Iterator<String> itKey = keys.iterator();
        while (itKey.hasNext()) {
            String key = itKey.next();
            String value = map.get(key);
            System.out.println(key + ":" + value);
        }

        Collection<String> values = map.values();
        Iterator<String> valueIt = values.iterator();
        while (valueIt.hasNext()) {
            System.out.println(valueIt.next());

        }

        Set<Map.Entry<String, String>> entries = map.entrySet();
        Iterator<Map.Entry<String, String>> entryIt = entries.iterator();
        while (entryIt.hasNext()) {
            Map.Entry<String, String> entry = entryIt.next();
            String key = entry.getKey();
            String value = entry.getValue();
            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 ListDemo {
    public static void main(String[] args) {
        List<Integer> list = new ArrayList<>();
        int[] arr= new int[]{1,2,3,4,5,6,7,8,9};
        for (int i : arr) {
            list.add(i);
        }
        System.out.println(list);
        Collections.shuffle(list);//随机
        System.out.println(list);
        System.out.println(Collections.max(list));//取list集合的最大值
        System.out.println(Collections.min(list));//取list集合的最小值
        Collections.sort(list);//排序
        System.out.println(list);
        Collections.swap(list,0,3);//交换集合元素的位置
        System.out.println(list);
        
        List<Integer> list2 = new ArrayList<>();
        for (Integer in : list) {//拷贝集合1
            list2.add(in);
        }
        
        System.out.println(list2);
        
        Collections.copy(list2,list);//拷贝集合
        
        System.out.println(list2);

    }
}
  

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.ArrayList;
import java.util.LinkedList;
import java.util.List;

public class ExceptionDemo {
    public static void main(String[] args) {
//        System.out.println(3%0);
//        int[] arr = new int[1];
//        int a = arr[10000000];

//        String[] str = null;
//        System.out.println(str.length);

//        A b =new B();
//        C c = (C)b;

//        double a = Double.parseDouble("asdfasdasdf");

// 时间稍久 给JVM的内存大小 越大运行时间越久
//        List<String> list = new LinkedList<>();
//        list.add("a");
//        for (int i = 0; i < 10000; i++) {
//            list.addAll(list);
//        }



    }
}
class A{}
class B extends A{}
class C extends A{}

4. 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 java.util.*;

public class mapDemo02 {
    public static void main(String[] args) {
        String str = "If you want to change your fate I think you must come to the school to learn java";
        String[] strArr = str.split(" ");
        System.out.println(Arrays.toString(strArr));

        HashMap<String,Integer> map = new HashMap<>();
        for (int i = 0; i < strArr.length; i++) {
            if(!map.containsKey(strArr[i])){
                map.put(strArr[i],1);
            }else {
                Integer in = map.get(strArr[i])+1;
                map.put(strArr[i],in);
            }
        }

        Set<String> keys = map.keySet();
        Iterator<String> itKey = keys.iterator();
        while (itKey.hasNext()) {
            String key = itKey.next();
            Integer value = map.get(key);
            System.out.println(key + "=" + value);
        }

    }
}