Find if a String2 is substring of String1, Java

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//'main' method must be in a class 'Rextester'.
//Compiler version 1.8.0_111

import java.util.*;
import java.lang.*;
/*

Find if a String2 is substring of String1. If it is, return the index of the first occurrence. else return -1.
Eg 1:Input:
        String 1: test123string
         String 2: 123
        Output: 4
Eg 2: Input:
        String 1: testing12
        String 2: 1234 
        Output: -1

*/
class Rextester
{  
    public static void main(String args[])
    {
        int index=-1;
        Scanner input=new Scanner(System.in);
        System.out.println("Enter the main string:");
        String mainString=input.next();
        char mainStringArray[]=mainString.toCharArray();
        System.out.println("Enter the substring:");
        String subString=input.next();                
        for(int i=0;i<mainStringArray.length;i++)
        {
            if(mainStringArray[i]==subString.charAt(0)) 
            {
                System.out.println("i :"+i);
                String temp="";
                int tempit=i;
                if((tempit+(subString.length()-1))<mainStringArray.length) 
                {
                    for(int j=tempit;j<(tempit+subString.length());j++ )
                    {
                        temp=temp+mainStringArray[j];
                    }  
                }                                                
                if(temp.equals(subString))
                {
                    index=i;                     
                    break;
                }
            }
        }
        System.out.println("Starting index: "+index);
    }
}

[ - ] Show input
Compilation time: 0.97 sec, absolute running time: 0.26 sec, cpu time: 0.3 sec, memory peak: 20 Mb, absolute service time: 1,24 sec