import javax.swing.*;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;

public class JDimensionTest1 extends JFrame{

  public static void main(String[] args){
    JDimensionTest1 frame = new JDimensionTest1();

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setBounds(10, 10, 300, 200);
    frame.setTitle("タイトル");
    frame.setVisible(true);
  }

  JDimensionTest1(){
    JLabel label = new JLabel("Dimensionテスト");
    label.setPreferredSize(new Dimension(150, 50));
    label.setOpaque(true);
    label.setBackground(Color.WHITE);

    JPanel p = new JPanel();
    p.add(label);

    getContentPane().add(p, BorderLayout.CENTER);
  }
}