Complex number multiplication (leetcode), Java

Complex number multiplication (leetcode)

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

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

class Rextester
{  
    //1+-2i
    public static String complexNumberMultiply(String a, String b) {
        int num1,num2;
        int coe1,coe2;
        if(a.substring(0,1).equals("-")){
            int index = a.indexOf('+');
            num1 = Integer.parseInt(a.substring(1,index))*-1;
            if(a.substring(index+1,index+2).equals("-"))
                coe1 = Integer.parseInt(a.substring(index+2,a.indexOf('i')))* -1;
            else
                coe1 = Integer.parseInt(a.substring(index+1,a.indexOf('i')));
        }
        else{
            int index = a.indexOf('+');
            num1 = Integer.parseInt(a.substring(0,index));
            if(a.substring(index+1,index+2).equals("-"))
                coe1 = Integer.parseInt(a.substring(index+2,a.indexOf('i')))* -1;
            else
                coe1 = Integer.parseInt(a.substring(index+1,a.indexOf('i')));//78+-76i
        }
        
        if(b.substring(0,1).equals("-")){
            int index = b.indexOf('+');
            num2 = Integer.parseInt(b.substring(1,index))*-1;
            if(b.substring(index+1,index+2).equals("-"))
                coe2 = Integer.parseInt(b.substring(index+2,b.indexOf('i')))* -1;
            else
                coe2 = Integer.parseInt(b.substring(index+1,b.indexOf('i')));
        }
        else{
            int index = b.indexOf('+');
            num2 = Integer.parseInt(b.substring(0,index));
            if(b.substring(index+1,index+2).equals("-"))
                coe2 = Integer.parseInt(b.substring(index+2,b.indexOf('i')))* -1;
            else
                coe2 = Integer.parseInt(b.substring(index+1,b.indexOf('i')));//78+-76i
        }
        
        int resNum = (num1*num2)-(coe1*coe2);
        int resCoe = (num1*coe2)+(num2*coe1);
        
        return new String(resNum+"+"+resCoe+"i");       
    }
    public static void main(String args[])
    {
        System.out.println("Hello, World!");
        System.out.println(complexNumberMultiply("20+22i","-18+-10i"));
    }
}


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