Immutable-by-default 默认不可变

Unless you have a good reason to make them otherwise, variables, classes, and collections should be immutable.

除非你理由充分，不然的话就保持变量、类和集合数据不可变。

Variable references can be made immutable with final:

变量引用可以被final修饰成不可变的。

final FooWidget fooWidget;
if (condition()) {
    fooWidget = getWidget();
} else {
    try {
        fooWidget = cachedFooWidget.get();
    } catch (CachingException e) {
        log.error("Couldn't get cached value", e);
        throw e;
    }
}
// fooWidget is guaranteed to be set here

Now you can be sure that fooWidget won't be accidentally reassigned. The final keyword works with if/else blocks and with try/catch blocks. Of course, if the fooWidget itself isn't immutable you could easily mutate it.

现在你可以确定的是fooWidget不会被意外的重新赋值。final关键字将会和if/else以及try/catch代码块共同作用。当然，如果fooWidget不是不可变的话，你可以轻松得改变它。

Collections should, whenever possible, use the Guava ImmutableMap, ImmutableList, or ImmutableSet classes. These have builders so that you can build them up dynamically and then mark them immutable by calling the build method.

集合类都应该尽一切可能地使用Guava的ImmutableMap、 ImmutableList或者ImmutableSet。他们都有builder模式以便于你可以动态得构建他们，然后通过调用build方法来使之保持不可变。

Classes should be made immutable by declaring fields immutable (via final) and by using immutable collections. Optionally, you can make the class itself final so that it can't be extended and made mutable.

类都应该通过声明不可变字段的方式（使用 final）被构造成不可变的，以及使用不可变集合。可选的是，你也可以将这个类声明为final，所以它就不可能被继承和被改变。