Generate Combination in java

public class Combination {

     void doCombination(char []str,int start,int end){   

     char []data=new char[3];

     int i=0;

     int r=3;// how many element combine in one selection

     doCombinationUntil(str,data,start,end,i,r);

     }

    

   void   doCombinationUntil(char []str,char []data,int start,int end,int index,int r){

        if (index==r) {

          for (int j = 0; j < r; j++) {

              System.out.print(data[j]);

          }

          System.out.println("");

          return;

     }

        for (int i=start; i<=end && end-i+1 >= r-index; i++)

         {

             data[index] = str[i];

             doCombinationUntil(str, data, i+1, end, index+1, r);

         }

     }

    

     public static void main(String[] args) {  

          String str1="abcd";

          char []str=str1.toCharArray();

          Combination obj=new Combination();

          obj.doCombination(str,0,(str.length-1));

     }

    

}

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