https://chikuwa-tech-study.blogspot.com/2021/06/java-binary-tree.html

TreeNode.java

public class TreeNode<E> {
    private E value;
    private TreeNode<E> leftNode;
    private TreeNode<E> rightNode;

    public TreeNode(E value) {
        this.value = value;
    }

    public E getValue() {
        return value;
    }

    public TreeNode<E> getLeftNode() {
        return leftNode;
    }

    public void setLeftNode(TreeNode<E> leftNode) {
        this.leftNode = leftNode;
    }

    public TreeNode<E> getRightNode() {
        return rightNode;
    }

    public void setRightNode(TreeNode<E> rightNode) {
        this.rightNode = rightNode;
    }

    public void preOrder() {
        System.out.print(value);

        if (leftNode != null) {
            leftNode.preOrder();
        }

        if (rightNode != null) {
            rightNode.preOrder();
        }
    }

    public void inOrder() {
        if (leftNode != null) {
            leftNode.inOrder();
        }

        System.out.print(value);

        if (rightNode != null) {
            rightNode.inOrder();
        }
    }

    public void postOrder() {
        if (leftNode != null) {
            leftNode.postOrder();
        }

        if (rightNode != null) {
            rightNode.postOrder();
        }

        System.out.print(value);
    }

    @Override
    public String toString() {
        return value.toString();
    }

    public static void main(String[] args) {
        TreeNode<Integer> node1 = new TreeNode<>(1);
        TreeNode<Integer> node3 = new TreeNode<>(3);
        TreeNode<Integer> node2 = new TreeNode<>(2);
        node2.setLeftNode(node1);
        node2.setRightNode(node3);

        TreeNode<Integer> node5 = new TreeNode<>(5);
        TreeNode<Integer> node7 = new TreeNode<>(7);
        TreeNode<Integer> node6 = new TreeNode<>(6);
        node6.setLeftNode(node5);
        node6.setRightNode(node7);

        TreeNode<Integer> node4 = new TreeNode<>(4);
        node4.setLeftNode(node2);
        node4.setRightNode(node6);

        node4.preOrder();
        System.out.println();
        node4.inOrder();
        System.out.println();
        node4.postOrder();
    }
}