在Oracle数据库中，可以使用序列（Sequence）来生成唯一的主键。下面是一个示例代码，展示了如何保存两个相关实体，并使用序列生成主键：

1. 创建序列：

CREATE SEQUENCE entity_sequence
    START WITH 1
    INCREMENT BY 1
    NOCACHE
    NOCYCLE;

2. 创建实体表：

CREATE TABLE entity1 (
    id NUMBER,
    name VARCHAR2(50),
    CONSTRAINT entity1_pk PRIMARY KEY (id)
);

CREATE TABLE entity2 (
    id NUMBER,
    entity1_id NUMBER,
    name VARCHAR2(50),
    CONSTRAINT entity2_pk PRIMARY KEY (id),
    CONSTRAINT entity2_fk FOREIGN KEY (entity1_id) REFERENCES entity1(id)
);

3. Java代码示例：

import java.sql.*;

public class Main {
    public static void main(String[] args) {
        // 数据库连接信息
        String url = "jdbc:oracle:thin:@localhost:1521:xe";
        String username = "your_username";
        String password = "your_password";
        
        // 实体信息
        String entity1Name = "Entity1";
        String entity2Name = "Entity2";
        
        // 建立数据库连接
        try (Connection conn = DriverManager.getConnection(url, username, password)) {
            // 开启事务
            conn.setAutoCommit(false);
            
            // 保存实体1
            int entity1Id = saveEntity(conn, entity1Name);
            
            // 保存实体2
            saveEntity(conn, entity2Name, entity1Id);
            
            // 提交事务
            conn.commit();
            
            System.out.println("保存成功！");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
  
    // 保存实体1
    private static int saveEntity(Connection conn, String name) throws SQLException {
        String sql = "INSERT INTO entity1 (id, name) VALUES (entity_sequence.nextval, ?)";
        try (PreparedStatement stmt = conn.prepareStatement(sql, Statement.RETURN_GENERATED_KEYS)) {
            stmt.setString(1, name);
            stmt.executeUpdate();
            
            // 获取生成的主键值
            try (ResultSet rs = stmt.getGeneratedKeys()) {
                if (rs.next()) {
                    return rs.getInt(1);
                } else {
                    throw new SQLException("无法获取生成的主键值！");
                }
            }
        }
    }
  
    // 保存实体2
    private static void saveEntity(Connection conn, String name, int entity1Id) throws SQLException {
        String sql = "INSERT INTO entity2 (id, entity1_id, name) VALUES (entity_sequence.nextval, ?, ?)";
        try (PreparedStatement stmt = conn.prepareStatement(sql)) {
            stmt.setInt(1, entity1Id);
            stmt.setString(2, name);
            stmt.executeUpdate();
        }
    }
}

请注意替换代码中的数据库连接信息和实体信息，以适应您的环境。