Java基础题集

1.显示从2开始的20个回文素数 要求每十个换一行 间距一定

public class PrimeNumber {
//    //判断是否为素数
//    public static boolean isPrime(int n){
//        for(int divisor = 2; divisor*divisor<=n; divisor++){
//            if(n%divisor==0){
//                return false;
//            }
//        }
//        return true;
//    }
    //判断素数的另一种方法
    public static boolean isPrime(int n){
        for(int divisor=2; divisor <= n/2; divisor++){
            if(n%divisor==0){
                return false;
            }
        }
        return true;
    }

    //判断回文数
    public static boolean isPalindrome(int n){
        String s = String.valueOf(n);//数字转字符串
        int low = 0;
        int heigh = s.length() - 1;
        while(low
            if(s.charAt(low)!=s.charAt(heigh)){
                return false;
            }
            low++;
            heigh--;
        }
        return true;
    }

    public static void main(String[] args) {
        int count = 0;
        int n = 2;
        do{
            if(isPrime(n) && isPalindrome(n)){
                count++;
                if(count%10==0){
                    System.out.printf("%-5s%n", n);//%n:用在printf语句里的换行符 %5s:向前补齐5位
                }else {
                    System.out.printf("%-5s", n);
                }
            }
            n++;
        }while(count<20);
    }
}

2.递归方法求斐波那契数列的前20项

public class NumberUtil {
    public static long fibo(int n){
        if(n==1 || n==2){
            return 1;
        }else{
            return fibo(n-1)+fibo(n-2);
        }
    }

    public static void main(String[] args) {
        for (int i = 1; i <= 20; i++) {
            System.out.println("fibo("+i+")="+fibo(i));
        }
    }
}

3在该类中实现两个静态函数
parseInt(char[]):int 将字符数组转换为整数
pardeInt(String):int 将字符串数组转换为整数

public class MyInteger {

    public static int parseInt(char[] c){
        int result = 0;
        int scale = 1;
        for(int i = c.length-1; i>=0; i--){
            int d = c[i] -'0';
            result = d*scale+result;
            scale*=10;
        }
        return  result;
    }
    public static int parseInt(String s){
        int result = 0;
        int scale = 1;
        for (int i = s.length()-1; i >=0 ; i--) {
            int d = s.charAt(i)-'0';
            result = result+d*scale;
            scale*=10;
        }
        return result;
    }

    public static void main(String[] args) {
        char[] c = {'3','4','5'};//字符：'' char类型
        char c1 = c[0];
        int i = c[0] - '0'; //char - '0' 转变成int类型
        System.out.println(c[0]);
        System.out.println(MyInteger.parseInt(c));

        String s = "1234";
        System.out.println(MyInteger.parseInt(s));
    }
}

4.一元二次方程计算类

public class QuadraticEquation {
    private double a;
    private double b;
    private double c;

    public QuadraticEquation(double a, double b, double c) {
        this.a = a;
        this.b = b;
        this.c = c;
    }

    public double getA() {
        return a;
    }

    public double getB() {
        return b;
    }

    public double getC() {
        return c;
    }
    public double getDiscriminant(){
        double discriminant;
        discriminant = b*b - 4*a*c; //本类下可以直接访问私有对象
        return discriminant;
    }

    public double getRoot1(){
        return (-b+Math.sqrt(getDiscriminant()))/(2*a);
    }
    public double getRoot2(){
        return (-b-Math.sqrt(getDiscriminant()))/(2*a);
    }

    public static void main(String[] args) {
        double a, b, c;
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入方程的三个系数：");
        a = sc.nextDouble();
        b = sc.nextDouble();
        c = sc.nextDouble();
        QuadraticEquation q = new QuadraticEquation(a, b, c);
        double discriminant = q.getDiscriminant();
        System.out.println(discriminant);
        if(discriminant<0){
            System.out.println("该方程无解");
        }else if(discriminant==0){
            System.out.println("该方程有一个根："+q.getRoot1());
        }
        else{
            System.out.println("该方程有两个根："+q.getRoot1()+" 和 "+q.getRoot2());
        }
    }

}

5.创建一个三角形类，包括计算面积的方法

```java
public class Triangle {
    double a, b, c;//三角形三边边长
    public Triangle(){
        this.a = 0;
        this.b = 0;
        this.c = 0;
    }
    public Triangle(double a, double b, double c){
        this.a = a;
        this.b = b;
        this.c = c;
    }
    public double area(){
        double s = (a+b+c)/2.0;
        return Math.sqrt(s*(s-a)*(s-b)*(s-c));//海伦公式求解三角形面积
    }

    public static void main(String[] args) {
        Triangle t = new Triangle(3, 4, 5);
        System.out.println(t.area());


    }

}