Using an array in a recursive algorithm to find combinations

Question

The idea is if i am at a certain stair i can either go one step down or two so if am at stair 3 i can go down 1 1 1 or 2 1 for example. My code should print all the possibilities. The error I get is that I can't convert the add function to an array (since the add method is a boolean). What is wrong with this algorithm?

public class Stairs {

public static void staircase (int height  ){

    ArrayList<Integer> Array = null;
    explore (height,Array);
}
public static void explore(int objheight,ArrayList<Integer>Array){
    int intialheight = 0; 
    if (intialheight == objheight){
        Array.toString();
    }
    else{ if (objheight > intialheight ){
        explore(objheight-2,Array.add(2));
        explore(objheight-1,Array.add(1));
    }
}

after your feedback I am getting an empty output import java.lang.reflect.Array; import java.util.ArrayList;

public class Stairs {

public static void staircase (int height  ){

    ArrayList<Integer> Array = new ArrayList<Integer>();
    explore (height,Array);
}
public static void explore(int objheight,ArrayList<Integer>Array){
    int intialheight = 0; 
    if (intialheight == objheight){
        Array.toString();
    }
    else{ if (objheight > intialheight ){
        Array.add(2);
        explore(objheight-2,Array);
        Array.add(1);
        explore(objheight-1,Array);
    }
}}
public static void main (String args[]){
staircase(3);

   }
 }

Answer 1

The method add(E e) in ArrayList returns true upon appending the element e passed as a parameter to the end of the ArrayList.

Your method, explore(int objHeight, ArrayList<Integer> Array) does not accept a boolean for its second parameter. Yet, in that same method, explore , you are recursively calling explore and passing in a boolean to the method.

The following code should be modified to first invoke the add method of Array and then pass Array to the explore method.

Before:

explore(objheight-2,Array.add(2)); This code is passing parameters int and boolean to the explore method, which is not the parameters it accepts. You should instead attempt the following.

After:

Array.add(2); explore(objheight-2,Array); This code first adds 2 to the Array and then passes the Array to the explore method without invoking any further methods on the Array object.

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You will also need to do this for the next line of code, where you have explore(objheight-1,Array.add(1)); .

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Edit: Upon further examination of the code, I discovered another (sooner) error that occurs. A NullPointerException will occur each time the program runs:

ArrayList<Integer> Array = null; explore (height,Array);

Then inside the explore method, different methods on Array are invoked, despite Array always being null :

Array.toString(); , Array.add(2) and Array.add(1) .

The Array object must be initialized inside of either the staircase or explore methods.

ArrayList<Integer> Array = new ArrayList<Integer>(); or ArrayList<Integer> Array = null; Array = new ArrayList<Integer>(); ArrayList<Integer> Array = null; Array = new ArrayList<Integer>();