Heap sort (Implementation in Java)

Test.java

class MinHeap{

    int[] heap_array;
    int heap_size;
    int capacity = 100;

    MinHeap(){
        heap_array = new int[capacity];
        heap_size = 0;
    }

    int parent(int i){
        return (i-1)/2;
    }

    int left(int i){
        return (2*i + 1);
    }

    int right(int i){
        return (2*i + 2);
    }

    public void insertKey(int k){
        if(heap_size == capacity){
            System.out.println("Overflow");
            return;
        }
        heap_size += 1;
        int i = heap_size - 1;
        heap_array[i] = k;
        while(i > 0 && heap_array[parent(i)] > heap_array[i]){
            int temp = heap_array[parent(i)];
            heap_array[parent(i)] = heap_array[i];
            heap_array[i] = temp;
            i = parent(i);
        }
    }

    public void minHeapify(int i){
        int smallest = i;
        int l = left(i);
        int r = right(i);

        if(l < heap_size && heap_array[l] < heap_array[smallest]){
            smallest = l;
        }

        if(r < heap_size && heap_array[r] < heap_array[smallest]){
            smallest = r;
        }

        if(smallest != i){
            int temp = heap_array[i];
            heap_array[i] = heap_array[smallest];
            heap_array[smallest] = temp;
            minHeapify(smallest);
        }
    }

    public int extractMin(){
        if(heap_size == 0){
            System.out.println("No elements");
            return -1;
        }
        if(heap_size == 1){
            heap_size -= 1;
            return heap_array[0];
        }
        int root = heap_array[0];
        heap_array[0] = heap_array[heap_size - 1];
        heap_size -= 1;
        minHeapify(0);
        return root;
    }

    public void printTree(){
        for(int i = 0; i < heap_size; i++){
            System.out.print(heap_array[i] + " ");
        }
        System.out.println();
    }

    public boolean isEmpty(){
        if(heap_size == 0){
            return true;
        }
        else{
            return false;
        }
    }
}

public class Test{
    public static void main(String[] args){
        MinHeap ob = new MinHeap();
        ob.insertKey(5);
        ob.insertKey(2);
        ob.insertKey(3);
        ob.insertKey(4);
        ob.insertKey(1);
        while(!ob.isEmpty()){
            System.out.print(ob.extractMin() + " ");
        }
        System.out.println();
    }
}