千锋逆战班，Day24

在千锋逆战学习的第24天
假如我不能，我一定要；假如我一定要，我就一定能！加油！
今天学习了泛型和Collections工具类。

作业题：
11.有如下代码

public class TestMyClass {

	public static void main(String[] args) {

		IA ia = new MyClass();
		System.out.println(ia);
		
	}
}
interface IA{
	void ma();
}
class MyClass implements IA{
	public void ma(){}
	public String toString(){
		return "MyClass toString()";
	}
}

答案C

import java.util.Scanner;
public class TestRandom {

	public static void main(String[] args) {

		int a = 0;
		StringBuilder sb = new StringBuilder();
		java.util.Random r = new java.util.Random();
		Scanner sc = new Scanner(System.in);
		
		System.out.println("请输入一个数字：");
		int length = sc.nextInt();
		
		for (int j = 0; j < length; j++) {
			// 大写65-90，小写97-122，数字48-57

			int choice = r.nextInt(3);
			switch (choice) {
			case 0:
				a = r.nextInt(26) + 65;// 大写字母对应的ASCII码
				break;
			case 1:
				a = r.nextInt(26) + 97;// 小写字母
				break;
			case 2:
				a = r.nextInt(10) + 48;// 数字
			}

			// 转成字符
			char c = (char) a;
			// 存入
			sb.append(c);
		}

		String str = sb.toString();
		System.out.println(str);
	}

}

运行结果

import java.util.*;
public class TestCheckString {

	public static void main(String[] args) {

		Scanner input = new Scanner(System.in);
		System.out.println("请输入字符串：");
		String s1 = input.next();
		
		System.out.println("请输入子串：");
		String s2 = input.next();
		
		int b = 0;
		for(int i = 0 ; i<s1.length(); i++){
			int a = s1.indexOf(s2,i);
			if(a != -1){
				i = a;
				b++;
				System.out.println("该子串第"+b+"次出现的下标"+a);
			}
		}
	}
}

运行结果

import java.util.*;
public class TestStudent {

	public static void main(String[] args) {
		
		List<Student> list = new ArrayList<Student>();
		list.add(new Student("Tom",18,100,"class05"));
		list.add(new Student("Jerry",22,70,"class04"));
		list.add(new Student("Owen",25,90,"class05"));
		list.add(new Student("Jim",30,80,"class05"));
		list.add(new Student("Steve",28,66,"class06"));
		list.add(new Student("Kevin",24,100,"class04"));
		
		double scores = 0;
		for (int i = 0; i < list.size(); i++) {
			double score = list.get(i).score;
			scores = scores + score;
		}
		System.out.println("所有学生的平均成绩为：" + scores/list.size());
		
		double[][]  classScore = new double[7][2];
		for (int i = 0; i < list.size(); i++) {;
			Character ch = list.get(i).classNum.charAt(6);
			int ci = Integer.valueOf(ch.toString());
			classScore[ci - 1][0]++;
			classScore[ci - 1][1] = (classScore[ci - 1][1] +list.get(i).score);
		}

		for (int i = 0; i < classScore.length; i++) {
			if(classScore[i][0] != 0) {
				classScore[i][1] = classScore[i][1] / classScore[i][0];
				System.out.println("class0" + (i + 1) + "的平均成绩：" + classScore[i][1]);
			}else {
				System.out.println("class0" + (i + 1) + "没有成绩");
			}
		}
	}

}

class Student{
	String name;
	int age;
	double score;
	String classNum;
	public Student(String name, int age, double score, String classNum) {
		this.name = name;
		this.age = age;
		this.score = score;
		this.classNum = classNum;
	}
}

运行结果

欢迎指正