1Optional<User> user = ......
2if (user.isPresent()) {
3    return user.getOrders();
4} else {
5    return Collections.emptyList();
6}

1User user = .....
2if (user != null) {
3    return user.getOrders();
4} else {
5    return Collections.emptyList();
6}

1public class User {
2  public int id;
3  public String name;
4  public String email;
5  public String address;
6}

 1List<String> strings = new ArrayList<String>();  //刚引入泛型时我们是这样
 2List<String> strings = new ArrayList<>();        //后来变成这样了, 可以钻石符号推断, Java 7
 3
 4List<String> list = new ArrayList<>();
 5list.addAll(new ArrayList<>());                  //根据 list.addAll() 上下文推断要创建的类型是 new ArrayList<String>(), Java 8
 6
 7interface Foo {
 8    void dodo();
 9}
10Foo foo = () -> System.out.println("Foo:dodo()");   //Java 8 Lambda 可以根据 Lambda 表达式的签名推断出接口类型
11foo.dodo();

1"ab" == "ab"                                                //true
2"ab" == "new String("ab")                                   //false
3"ab" == String.value("ab")                                  //true
4new String("ab").equals(new String("ab"))                   //true
5new String("ab").intern() == new String("ab").intern()      //true