小白提问，需求是用户输入1或者2后，得到1-100以内的偶数和或者奇数和

import java.util.Scanner;
public class Test13{
public static void main(String[] args){
Scanner sc = new Scanner(System.in);

        int r;
        System.out.println("“1. 求1—100的偶数和   2. 求1—100的奇数和：”");
         r = sc.nextInt();
    //while
    int a = 1;
    int c = 0;
    int d = 0;
    while(true){
    if(r==1){
        while(a<=100&&a>=1&&a%2==0){
            a++;
            c+=a;


        }
        System.out.println(c);
    }
    if(r==2){
        while(a<=100&&a>=1&&a%2 !=0){
            a++;
            d+=a;

        }
        System.out.println(d);
    }
    }
     }

}

 public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int r;
        System.out.println("“1. 求1—100的偶数和   2. 求1—100的奇数和：”");
        r = sc.nextInt();
        // while
        int a = 1;
        int c = 0;
        int d = 0;
//      while (true) {
            if (r == 1) {
                while (a <= 100 && a >= 1 ) {
                    if(a % 2 == 0){
                        c += a;
                    }
                    a++;
                }
                System.out.println(c);
            }
            if (r == 2) {
                while (a <= 100 && a >= 1 ) {
                    if(a % 2 != 0){
                        d += a;
                    }
                    a++;
                }
                System.out.println(d);
            }
//      }
    }

在你代码的基础上修改的，有以下几个错误：
1.最外面的while循环是多余的
2.a%2 == 0 判断应该放在循环里面
3.a++ 应该放在循环的最后面

 import java.util.Scanner;
public class Test {
    public static void main(String[] args) {
        int sum = 0;
        Scanner scanner = new Scanner(System.in);
        int input = scanner.nextInt();
        while (input == 1 || input == 2) {
            int i = 0;//默认是输入的1，当然是2的话，i也会改变
            if (input == 2) {
                i = 1;
            }
            for (;  i<= 100; i = i + 2) {
                sum = i + sum;
            }
            System.out.println(sum);
            sum = 0;//重置sum的值
            input = scanner.nextInt();//用户重新输入
        }
    }
}