往JFrame直接添加Jlabel不能显示，为什么

[FindTheRight]

import java.awt.Toolkit;

import javax.swing.JComboBox;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextArea;
import javax.swing.JTextField;

public class mainFrame extends JFrame {
public mainFrame(String title) {
// TODO Auto-generated constructor stub
setTitle(title);
setSize(200, 300);
// 窗口居中
// ------------- 方法一 ----------------
int screenWidth = Toolkit.getDefaultToolkit().getScreenSize().width;
int screenHeight = Toolkit.getDefaultToolkit().getScreenSize().height;
setLocation((screenWidth - 200) / 2, (screenHeight - 200) / 2);
// --------- 方法二 ----------------
// setLocationRelativeTo(null);
// ------------- end -----------------

//创建组件
JLabel name,type,details;
JTextField inputName;
JComboBox choicType;
JTextArea inputDetails;

//定义组件
name=new JLabel("Name:");
type=new JLabel("Type:");
details=new JLabel("Details:");

inputName=new JTextField(15);

String[] items={"Movie","Animation","Novel","Note","others"};
choicType=new JComboBox(items);

inputDetails=new JTextArea(5, 10);

//添加组件
add(name);
add(inputName);
add(type);
add(choicType);
add(details);
add(inputDetails);

setVisible(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}

public static void main(String[] args) {
new mainFrame("newFrame");
}
}

[AAlAAzCCj]

没有设计布局，所以所有组件默认放在一个位置，被其他组件覆盖了