“Immutable objects are simple. They can only be in one state, which is carefully controlled by the constructor. One of the most difficult elements of program design is reasoning about the possible states of complex objects. Reasoning about the state of immutable objects, on the other hand, is trivial. Immutable objects are also safer. Passing a mutable object to untrusted code, or otherwise publishing it where untrusted code could find it, is dangerous — the untrusted code might modify its state, or, worse, retain a reference to it and modify its state later from another thread. On the other hand, immutable objects cannot be subverted in this manner by malicious or buggy code, so they are safe to share and publish freely without the need to make defensive copies.”

“From the perspective of a class C, an alien method is one whose behavior is not fully specified by C. This includes methods in other classes as well as overrideable methods (neither private nor final) in C itself. Passing an object to an alien method must also be considered publishing that object. Since you can’t know what code will actually be invoked, you don’t know that the alien method won’t publish the object or retain a reference to it that might later be used from another thread.”

“The object isn't to make art, it's to be in that wonderful state which makes art inevitable.”

“Object-oriented programming offers a sustainable way to write spaghetti code. It lets you accrete programs as a series of patches.”

“Socialism, like the ancient ideas from which it springs, confuses the distinction between government and society. As a result of this, every time we object to a thing being done by government, the socialists conclude that we object to its being done at all. We disapprove of state education. Then the socialists say that we are opposed to any education. We object to a state religion. Then the socialists say that we want no religion at all. We object to a state-enforced equality. Then they say that we are against equality. And so on, and so on. It is as if the socialists were to accuse us of not wanting persons to eat because we do not want the state to raise grain.”

“Code without tests is bad code. It doesn't matter how well written it is; it doesn't matter how pretty or object-oriented or well-encapsulated it is. With tests, we can change the behavior of our code quickly and verifiably. Without them, we really don't know if our code is getting better or worse.”