问程序中输入错误的原因

EN

import javax.swing.JOptionPane;
import java.awt.Color;
import java.awt.Graphics;
import javax.swing.JPanel;
import java.util.Scanner;

public class abc extends JPanel {

    public void paintComponent(Graphics g) {
        Scanner input = new Scanner(System.in);
        super.paintComponent(g);

        this.setBackground(Color.WHITE);
        int a, b;
        System.out.print("input a: ");
        a = input.nextInt();
        a = b;
        g.setColor(Color.BLUE);
        g.fillOval(150, 40, a, b);
    }
}

import java.awt.Color;
import javax.swing.JFrame;

public class abcd {

    public static void main(String args[]) {
        JFrame frame = new JFrame("Draw");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        abc panel = new abc();
        panel.setBackground(Color.WHITE);
        frame.add(panel);
        frame.setSize(400, 200);
        frame.setVisible(true);
    }
}