索引字符串，忽略大小写

public class youyi {
public static void main(String[] args) {
System.out.println(StringAt("APP", "abcd apple"));
    
}

public static int StringAt(String s,String sub){
    String a=s.toLowerCase();
    String b=sub.toLowerCase();
    
    if(b.contains(a))    {
        return b.indexOf(a);
    }else{
        return -1;

    }
}}