Let’s first understand the concept using newspaper subscription. I subscribe to a monthly newspaper. I signed a contract with the company and received a newspaper every month. The contract records what I have subscribed to. The company which produces the newspapers will look at the contract and send the product to me each month. I am also able to unsubscribe through their website. In this example, we have three objects, the company producing the products, the receiver(me), and the contract. The important and subtle object is the contract. It’s the contract that connects me and the company together.

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Another example, I have subscribed to two copies of the same products. Each of them is monthly. However, I actually don’t get a monthly delivery. Instead, the company sends the goods to me every two months because they want to save delivery fees. …

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Ryan Zheng

I am a software developer who is keen to know how things work

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