Not displaying elements from stack

So I have written this JAVA-program for stack and the problem is I cant display the elements using the display() method which I used in the code.

here is my Stack class.

public class Stack {
//members
private int top;
private int size;
private int[] a;
private int i;

//constructor
public Stack() {
    this.top = -1;
    this.size = 5;
    this.a = new int[size];
}

private boolean isempty() {
    if(top == -1) {
        System.out.println("Stack Underflow");
        return true;
    }
    return false;
}

private boolean isfull() {
    if(top == size-1) {
        System.out.println("Stack Overflow");
        return true;
    }
    return false;
}

public void push(int n) {
if(isempty()) {
    top+=1;
    a[top] = n;
    }
}

public void pop() {
    if(isfull()) {
        System.out.println("popped : "+ a[top]);
        top-=1;         
    }
}

public void display() {
    for(i=0;i<top;i++) {
        System.out.println(a[i]);
    }

}
}

here is the main method class

public class Stackex {
public static void main(String[] args) {
    Stack s = new Stack();
    s.push(2);
    s.push(4);
    s.display();
}

}

when I try to execute what is get is “Stack underflow” from the isempty() and nothing is displayed after that. Please help me where I need to correct this code.

Answer

fix methods push & display:

public void push(int n) {
    if (!isfull()) {
        top += 1;
        a[top] = n;
    }
}

public void display() {
    for (int i = 0; i <= top; i++) {
        System.out.println(a[i]);
    }
}