JTextField added to JPanel does not display properly.
(There is a problem analysis in the back, after reading will definitely help you solve the problem oh!!)
Code examples:

package CSDN;

import javax.swing.*;

public class JtextjoinJpanel {
    public static void main(String[] args) {
        JFrame f=new JFrame("Text Box Add Panel");
        JPanel jp = new JPanel();
        JTextField jt = new JTextField(). f.setBounds(400,400,500,500); // Set the window position size;

        f.setBounds(400,400,500,500); // set window position size
        jp.setSize(200,200); //set panel size
        jt.setSize(100,100); //set textbox size

        jp.add(jt); //add the text box to the panel
        f.add(jp);

        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
    }
}

Results:

solution:

sets textbox parameters.

JTextField jt = new JTextField();
Change to：JTextField jt = new JTextField(10);

set the panel layout to null.

Add jp.setLayout(null);

Results:


Note :10 means the number of characters that can be entered in a text box.
Reasons for the problem (personal opinion):
Due to the panel USES is flow layout, create a text box, if no input default arguments (string), then the text box default is empty, will cover its flow way, leaving only border (so see like a vertical bar, click can still input characters), if 10 to its input parameters, is equivalent to 10 characters have been input, so will not cover; Not setting it to flow layout also solves the problem.
Therefore, if

TextField jt = new JTextField();
to TextField jt = new JTextField("Hello");

The result is