Avoid Nulls 避免Nulls

Try to avoid using nulls when you can. Do not return null collections when you should have instead returned an empty collection. If you're going to use null, consider the @Nullable annotation. IntelliJ IDEA has built-in support for the @Nullable annotation.

尽可能尝试避免使用Nulls。当你可以返回空集合的时候就不要返回null的集合对象。如果你要使用null的话，考虑一下@Nullable注解。IntelliJ IDEA已经内置支持了@Nullable注解。

If you're using Java 8, you can use the excellent new Optional type. If a value may or may not be present, wrap it in an Optional class like this:

如果你正在使用Java 8，你可以使用这个非常棒的Optional新类型。如果一个值不确定是否需要显示，就可以把它放到一个Optional类里边：

public class FooWidget {
    private final String data;
    private final Optional<Bar> bar;

    public FooWidget(String data) {
        this(data, Optional.empty());
    }

    public FooWidget(String data, Optional<Bar> bar) {
        this.data = data;
        this.bar = bar;
    }

    public Optional<Bar> getBar() {
        return bar;
    }
}

So now it's clear that data will never be null, but bar may or may not be present. Optional has methods like isPresent, which may make it feel like not a lot is different from just checking null. But it allows you to write statements like:

所以现在就很明确了，data永远不可能是null，但是bar可能显示出来也可能不显示。Optional有一些像isPresent的方法，可以让它感觉很大不同，就不只是检查null了。但是它也允许你写一些这样的语句：

final Optional<FooWidget> fooWidget = maybeGetFooWidget();
final Baz baz = fooWidget.flatMap(FooWidget::getBar)
                         .flatMap(BarWidget::getBaz)
                         .orElse(defaultBaz);

Which is much better than chained if null checks. The only downside of using Optional is that the standard library doesn't have good Optional support, so dealing with nulls is still required there.

这比链式的null检查好多了。唯一的缺点是标准库还没有很好地支持Optional的使用，所以 处理空值依然是有必要的。