BinaryTree | C language realizes the basic operation of binary tree

public class Diameter {
    
    // Klasse BinaryTree nicht modifizieren!
    public static class BinaryTree {
        int value;
        BinaryTree left;
        BinaryTree right;

        BinaryTree(int value) {
            this(value, null, null);
        }

        BinaryTree(int value, BinaryTree left, BinaryTree right) {
            this.value = value;
            this.left = left;
            this.right = right;
        }
    }

    public static int diameter(BinaryTree tree) {
        return 0;
    }

    public static void main(String[] args) {
        case1();
        case2();
        case3();
        case4();
        case5();
        case6();
        case7();
    }

    // Testfälle nicht modifizieren!
    public static void case1() {
        BinaryTree root = new BinaryTree(1,
                new BinaryTree(3,
                        new BinaryTree(7,
                                new BinaryTree(8,
                                        new BinaryTree(9),
                                        null),
                                null),
                        new BinaryTree(4,
                                null,
                                new BinaryTree(5,
                                        null,
                                        new BinaryTree(6)))),
                new BinaryTree(2)
        );
        System.out.println("Case 1: Solution should be 6 -- " + diameter(root));
    }
    
    public static void case2() {
        BinaryTree root = new BinaryTree(1);
        System.out.println("Case 2: Solution should be 0 -- " + diameter(root));
    }
    
    public static void case3() {
        BinaryTree root = new BinaryTree(1,
                new BinaryTree(2),
                null);
        System.out.println("Case 3: Solution should be 1 -- " + diameter(root));
    }
    
    public static void case4() {
        BinaryTree root = new BinaryTree(1,
                new BinaryTree(2),
                new BinaryTree(3));
        System.out.println("Case 4: Solution should be 2 -- " + diameter(root));
    }
    
    public static void case5() {
        BinaryTree root = new BinaryTree(1,
                new BinaryTree(2,
                        new BinaryTree(4),
                        null),
                new BinaryTree(3));
        System.out.println("Case 5: Solution should be 3 -- " + diameter(root));
    }

    public static void case6() {
        BinaryTree root = new BinaryTree(1,
                new BinaryTree(2,
                        new BinaryTree(3,
                                new BinaryTree(4,
                                        new BinaryTree(5,
                                                new BinaryTree(6,
                                                        null,
                                                        new BinaryTree(7)),
                                                null),
                                        null),
                                null),
                        null),
                null);
        System.out.println("Case 6: Solution should be 6 -- " + diameter(root));
    }

    public static void case7() {
        System.out.println("Case 7: Solution should be 0 -- " + diameter(null));
    }

}