部分Java集合练习题及答案

1.创建一个HashMap对象，并在其中添加一些学员的姓名和他们的分数：张三,90分，李四，88分。然后从HashMap对象获取这两个人的成绩并显示出来，接着把张三的成绩改为99分，再把他们的成绩显示出来。

import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
public class First {
	public static void main(String[] args) {
		HashMap<String, String> hp =new HashMap<String, String>();
		hp.put("张三", "90分");
		hp.put("李四", "88分");		
		Collection<String> con = hp.values();
		Iterator<String> it1 = con.iterator();
		while(it1.hasNext()) {
			System.out.println(it1.next());
		}		
		hp.replace("张三", "99分");		
		Collection<String> con1 = hp.values();
		Iterator<String> it2 = con.iterator();
		while(it2.hasNext()) {
			System.out.println(it2.next());
		}
	}
}

2.定义一个ArrayList集合类，然后放入多个字符串，排序，反序(Collections 集合工具类中包含了排序等功能)

用Iterator遍历集合

import java.util.*;
public class Second {
	public static void main(String[] args) {
		System.out.println("请随意输入一堆字符串：(输入0则结束)");
		Scanner sc = new Scanner(System.in);
		ArrayList<String> list = new ArrayList<String>();
		for(;;) {
			String s = sc.next();
			if(s.equals("0")) {
				break;
			}
			list.add(s);
		}		
		Collections.sort(list);		
		Iterator iterator = list.iterator();
		System.out.println("排序后");
		while(iterator.hasNext()) {
			System.out.println(iterator.next());
		}		
		iterator = list.iterator();
		System.out.println("反序后");
		Collections.reverse(list);
		while(iterator.hasNext()) {
			System.out.println(iterator.next());
		}		
		sc.close();
	}
}

3.定义一个Map集合,key是Integer类型,value是Cat类型,然后使用values()方法遍历每个元素，调用每个元素的叫的方法(定义Cat类,起码包含一个Cry()方法，建议结合泛型操作)

Cat类

public class Cat {
	public void Cry() {
		System.out.println("猫喵喵~~");
	}
}

测试方法

import java.util.*;
public class Third {
	public static void main(String[] args) {
		Map<Integer, Cat> map = new HashMap<Integer, Cat>();
		map.put(new Integer(1), new Cat());
		map.put(new Integer(2), new Cat());
		map.put(new Integer(3), new Cat());
		map.put(new Integer(4), new Cat());
		map.put(new Integer(5), new Cat());		
		Collection coll = map.values();
		Iterator iterator = coll.iterator();
		while (iterator.hasNext()) {
			Cat cat = (Cat) iterator.next();
			cat.Cry();
		}
	}
}

4.定义一个HashSet集合,只能添加Student类型的对象。往集合内添加多个元素，分别使用foreach语句，Iterator方式循环

Student类

public class Student {
	String name;
	int age;	
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + age;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}	
	@Override
	public String toString() {
		return "Student [name=" + name + ", age=" + age + "]";
	}
	@Override
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Student other = (Student) obj;
		if (age != other.age)
			return false;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
	public Student(String name, int age) {
		super();
		this.name = name;
		this.age = age;
	}	
}

测试类

import java.util.HashSet;
import java.util.Iterator;
public class Fourth {
	public static void main(String[] args) {
		HashSet<Student> hs = new HashSet<Student>();
		hs.add(new Student("张三",20));
		hs.add(new Student("李四",19));
		hs.add(new Student("王五",16));
		hs.add(new Student("张大",18));
		hs.add(new Student("李二",20));		
		Iterator<Student> iterator = hs.iterator();
		System.out.println("Iterator输出");
		while(iterator.hasNext()) {
			System.out.println(iterator.next());
		}
		System.out.println("*************************************");
		System.out.println("for-each输出");
		for(Student s : hs) {
			System.out.println(s);
		}
	}
}

5.**随机生成不重复的26个小写字母，生成26个后就停止生成。

import java.util.*;
public class Fifth {
	/**  
	 * @Description: 
	 * @author LYL
	 * @date 2021-01-10 12:48:38
	 */
	public static void main(String[] args) {
		HashSet hs = new HashSet();
		Random r = new Random();
		for(;;) {
			int i =  (r.nextInt(122-97+1)+97);
			char ch =  (char)i;
			if(hs.size()>=26) {
				break;
			}
			hs.add(ch);
		}		
		Iterator iterator = hs.iterator();
		while(iterator.hasNext()) {
			System.out.println(iterator.next());
		}
	}
}

6.**随机生成100个小写字母，同时统计每个字母生成了多少个

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map.Entry;
public class Sixth {
	public static void main(String[] args) {
		ArrayList list = new ArrayList();
		for(int i=0;i<100;i++) {
			int x = 97+(int)(Math.random()*26);
			list.add((char)x);
		}		
		HashMap map = new HashMap();
		for(int i=0;i<list.size();i++) {
			if(map.containsKey(list.get(i))) {
				int times = (int)(map.get(list.get(i)))+1;	
				map.put(list.get(i), times);
			}else {
				map.put(list.get(i),1);
			}
		}		
		Iterator it = map.entrySet().iterator();
		while(it.hasNext()){
			Entry e = (Entry) it.next();
			System.out.println("字母："+e.getKey()+"次数："+e.getValue());
		}
	}
}