“类型不匹配：无法从 int 转换为布尔值”我不知道错误究竟是什么

Question

My teacher for the Java class went over an example with his students and I ended up with an error and I don't understand what is wrong. All I know is that the error is "Type mismatch: cannot convert from int to boolean". The issue is on the line that has "input.length" in it that entire section of code begins with "public boolean validate(String input)"

There was no suggestions to fix the issue so I have no idea what to try. I don't know enough about Java to fix the issue.

import java.util.Scanner;

public class Application {
public void start(){
    Scanner scan = new Scanner(System.in);

    while(true)//trap
    {
        System.out.println("Enter an integer: ");
        int x = scan.nextInt();//could cause exception
        String s = scan.next();

        if(validate(s)){
            System.out.println("You entered "+s);
            System.out.println("Input is valid");
        }else{
            System.out.println("You entered "+s);
            System.out.println("Input is NOT valid");
        }
    }//end while


}//end start

public boolean validate(String input){
    //loop through strings char array looking for #
    for(int i = 0;input.length();i++){
        char c = input.charAt(i);
        if(!Character.isDigit(c)){//if char not #
            return false;
        }
    }

    return true;
}



}//end class

Expected results are to figure out what the issue is.

Answer 1

Normally it is helpful to include the line number where the error is, but I think I've found the issue anyway. You have a for loop written:

for(int i = 0;input.length();i++){

The second condition in a for loop is supposed to be a boolean condition indicating if the loop should continue or not. input.length(); returns an int not a boolean .

I believe you want to loop through the whole of input, in which case you should use:

for (int i = 0; i < input.length(); i++) {