Selecting all text in the JTextArea component can be done by using the selectAll() method. In the example below we use a button action to do the selection. After select the text we transfer focus back to the JTextArea so that the highlighted text is shown.

package org.kodejava.swing;

import javax.swing.*;
import java.awt.*;

public class TextAreaSelectAll extends JPanel {
    public TextAreaSelectAll() {
        initializeUI();
    }

    public static void showFrame() {
        JPanel panel = new TextAreaSelectAll();
        panel.setOpaque(true);

        JFrame frame = new JFrame("JTextArea Demo");
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setContentPane(panel);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(TextAreaSelectAll::showFrame);
    }

    private void initializeUI() {
        this.setLayout(new BorderLayout());
        this.setPreferredSize(new Dimension(500, 200));

        final JTextArea textArea = new JTextArea(
                "The quick brown fox jumps over the lazy dog.");
        textArea.setLineWrap(true);
        textArea.setWrapStyleWord(true);

        JScrollPane pane = new JScrollPane(textArea);
        pane.setPreferredSize(new Dimension(500, 200));
        pane.setVerticalScrollBarPolicy(
                JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);

        final JButton button1 = new JButton("Select All");
        button1.addActionListener(e -> {
            textArea.selectAll();

            // Transfer focus to JTextArea to show the selected
            // text.
            button1.transferFocusBackward();
        });
        final JButton button2 = new JButton("Get Selected Text");
        button2.addActionListener(e -> {
            String text = textArea.getSelectedText();
            System.out.println("Text = " + text);
        });

        JPanel buttonPanel = new JPanel(new FlowLayout());
        buttonPanel.add(button1);
        buttonPanel.add(button2);

        this.add(pane, BorderLayout.CENTER);
        this.add(buttonPanel, BorderLayout.SOUTH);
    }
}

And here the screenshot of the code snippet above:

To append text to the end of the JTextArea document we can use the append(String str) method. This method does nothing if the document is null or the string is null or empty.

package org.kodejava.swing;

import javax.swing.*;
import java.awt.*;

public class TextAreaAppendText extends JPanel {
    public TextAreaAppendText() {
        initializeUI();
    }

    public static void showFrame() {
        JPanel panel = new TextAreaAppendText();
        panel.setOpaque(true);

        JFrame frame = new JFrame("JTextArea Demo");
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setContentPane(panel);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(TextAreaAppendText::showFrame);
    }

    private void initializeUI() {
        String text = "The quick brown fox ";

        JTextArea textArea = new JTextArea(text);
        textArea.setLineWrap(true);
        textArea.setWrapStyleWord(true);
        JScrollPane scrollPane = new JScrollPane(textArea);

        String appendText = "jumps over the lazy dog.";
        textArea.append(appendText);

        this.setPreferredSize(new Dimension(500, 200));
        this.setLayout(new BorderLayout());
        this.add(scrollPane, BorderLayout.CENTER);
    }
}

The output of the code snippet above is:

The insert(String str, int pos) of the JTextArea allows us to insert a text at the specified position. In the example below we insert brown and over words into the current JTextArea text.

package org.kodejava.swing;

import javax.swing.*;
import java.awt.*;

public class TextAreaInsertText extends JPanel {
    public TextAreaInsertText() {
        initializeUI();
    }

    public static void showFrame() {
        JPanel panel = new TextAreaInsertText();
        panel.setOpaque(true);

        JFrame frame = new JFrame("JTextArea Demo");
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setContentPane(panel);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(TextAreaInsertText::showFrame);
    }

    private void initializeUI() {
        String text = "The quick fox jumps the lazy dog.";

        JTextArea textArea = new JTextArea(text);
        textArea.setLineWrap(true);
        textArea.setWrapStyleWord(true);
        JScrollPane scrollPane = new JScrollPane(textArea);

        textArea.insert("brown ", 10);
        textArea.insert("over ", 26);

        this.setPreferredSize(new Dimension(500, 200));
        this.setLayout(new BorderLayout());
        this.add(scrollPane, BorderLayout.CENTER);
    }
}

The screenshot of the result from the code snippet above is: