能帮我们看下为啥输出是空的不？

Question

public class StringExer {
public static void main(String[] args) {
StringExer exer = new StringExer();
System.out.println(exer.test1(" he ll o "));

}

/*
1.模拟一个trim方法，去除字符串两端的空格。
*/

public String test1(String str){

    int startNum = 0;
    int endNum = 0;
    char[] arr = new char[str.length()];
    for (int i = 0; i < arr.length; i++) {
            if (arr[i] != ' '){
                startNum = i;
                break;
            }
    }
        for (int j = arr.length; j < arr.length; j--) {
            if (arr[j] != ' '){
                endNum = j + 1;
                break;
            }
        }



        return str.substring(startNum,endNum);

}