Here's a Java program to perform postorder traversal of a binary tree:

class Node {
    int data;
    Node left, right;
    
    public Node(int item) {
        data = item;
        left = right = null;
    }
}

public class PostorderTraversal {
    
    Node root;
    
    public PostorderTraversal() {
        root = null;
    }
    
    void postorder(Node node) {
        if (node == null) {
            return;
        }
        postorder(node.left);
        postorder(node.right);
        System.out.print(node.data + " ");
    }
    
    public static void main(String[] args) {
        PostorderTraversal tree = new PostorderTraversal();
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
        
        System.out.println("Postorder traversal of binary tree is:");
        tree.postorder(tree.root);
    }
}

Explanation:

The program first defines a Node class to represent a node in the binary tree. Each Node object has a data field to store the integer value at the node, as well as left and right fields to represent the left and right subtrees.

The program then defines a PostorderTraversal class to represent the binary tree. The root field represents the root node of the tree.

The postorder method performs a recursive postorder traversal of the binary tree. The method first checks if the current node is null. If it is, the method simply returns. Otherwise, the method first recursively visits the left subtree, then the right subtree, and finally prints the value of the current node.

The main method creates a new PostorderTraversal object and initializes the root node and its left and right subtrees. It then calls the postorder method on the root node to perform the postorder traversal of the binary tree. The result is printed to the console.