问题1

frame.getContentPane().setLayout(null);

避免使用null布局，像素完美布局是现代ui设计中的一种幻觉。有太多因素会影响组件的单个大小，您无法控制。Swing旨在与布局经理为核心一起工作，舍弃这些问题不会导致问题和问题的终结，您将花费越来越多的时间来进行纠正

有关更多详细信息，请参见在容器中布置组件

问题二

transactionPane.getViewport().add(tblTransactions);

请勿ad与JScrollPane或一起JViewport使用

transactionPane.getViewport().setView(tblTransactions);   

或者

transactionPane.setViewportView(tblTransactions);   

反而

有关更多详细信息，请参见如何使用滚动窗格。

import java.awt.Color;
import java.awt.EventQueue;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.table.DefaultTableModel;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JTable table = new JTable(new DefaultTableModel(100, 100));
                table.setGridColor(Color.LIGHT_GRAY);
                JScrollPane scrollPane = new JScrollPane();
                scrollPane.setViewportView(table);

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(scrollPane);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

}

问题三

使用多个窗口，请参见使用多个JFrame，良好/不良做法？进行更深入的讨论

我认为您真正想要的是某种模式对话框。有关更多详细信息，请参见如何制作对话框

您的代码无需修改

（除了删除数据库代码）

这就是您的代码在我的PC上的显示方式，请仔细看一下按钮...

您的代码已修改为使用布局管理器...

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.FlowLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTable;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;
import javax.swing.table.DefaultTableModel;

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                TransactionUI ui = new TransactionUI();
                JFrame frame = ui.getFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TransactionUI {

        private JFrame frame;
        private JScrollPane transactionPane;
        private JTable tblTransactions;

        public TransactionUI() {
            frame = new JFrame();
            frame.setTitle("Account Transactions");
            frame.setBounds(100, 100, 450, 300);
            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            frame.getContentPane().setLayout(new BorderLayout());

            transactionPane = new JScrollPane();
            frame.getContentPane().add(transactionPane);
            String[] cols = {"Type", "Amount", "Date"};
            String sql = "SELECT type, amount, transactionDate FROM srp63_bank1017.transaction;";

            System.out.println("use getDataTable()");
            DefaultTableModel transactionList = new DefaultTableModel(100, 100);
            System.out.println("getDataTable() used");
            tblTransactions = new JTable(transactionList);
            tblTransactions.setFillsViewportHeight(true);
            tblTransactions.setShowGrid(true);
            tblTransactions.setGridColor(Color.BLACK);
            transactionPane.setViewportView(tblTransactions);

            JPanel buttons = new JPanel(new FlowLayout(FlowLayout.RIGHT));
            JButton btnClose = new JButton("Close");
            buttons.add(btnClose);
            frame.getContentPane().add(buttons, BorderLayout.SOUTH);
        }

        public JFrame getFrame() {
            return frame;
        }
    }

}