Q12.java

// 12. Design a java program to implement interfaces.
interface IntStack {
    void push(int item); //Store an item.
    int pop(); //Retrieve an item.
}

class DynStack implements IntStack {
    private int stck[];
    private int tos;
    //Allocate and initialize stack.
    DynStack(int size) {
        stck = new int[size];
        tos = -1;
    }

    //Push an item onto the stack.
    public void push(int item) {
        //If stack is full, allocate a larger stack.
        if(tos==stck.length-1) {
            int temp[] = new int[stck.length * 2]; 
            //Making a double size array.
            for(int i=0; i<stck.length; i++) 
                temp[i] = stck[i];
            stck = temp;
            stck[++tos] = item;
        }
        else 
            stck[++tos] = item;
    }

    //Pop an item from the stack.
    public int pop() {
        if(tos < 0) 
        {
            System.out.println("Stack underflow.");
            return 0;
        }
        else 
            return stck[tos--];
    }
}

class Main {
    public static void main(String args[]) {

        DynStack mystack1 = new DynStack(5);
        DynStack mystack2 = new DynStack(8);

        //These loops cause each stack to grow.
        for(int i=0; i<12; i++) 
            mystack1.push(i);
        for(int i=0; i<20; i++) 
            mystack2.push(i);
        System.out.println("Stack in mystack1:");
        for(int i=0; i<12; i++) 
            System.out.println(mystack1.pop());
        System.out.println("Stack in mystack2:");
        for(int i=0; i<20; i++) 
            System.out.println(mystack2.pop());
    }
}