Gson是Google開源的一個Java庫，可以將Java對象轉為JSON格式的字符串，也可以將JSON字符串轉為Java對象。在實際的開發中，有時需要生成比較復雜的JSON結構，而手動構造JSON字符串會比較繁瑣，這時就可以使用Gson來快速生成復雜的JSON字符串。

首先，需要定義一個Java對象來表示需要生成的JSON結構。例如，要生成如下的JSON結構：

{
"name": "John",
"age": 30,
"address": {
"street": "Main Street",
"city": "New York",
"state": "NY"
},
"phoneNumbers": [
{
"type": "home",
"number": "111-111-1111"
},
{
"type": "work",
"number": "222-222-2222"
}
]
}

此時，可以定義一個Java類來表示該JSON結構：

public class Person {
private String name;
private int age;
private Address address;
private List<PhoneNumber> phoneNumbers;
// getters and setters
}
public class Address {
private String street;
private String city;
private String state;
// getters and setters
}
public class PhoneNumber {
private String type;
private String number;
// getters and setters
}

然后，就可以使用Gson來將該Java對象轉為JSON字符串：

Gson gson = new Gson();
Person person = new Person();
person.setName("John");
person.setAge(30);
Address address = new Address();
address.setStreet("Main Street");
address.setCity("New York");
address.setState("NY");
person.setAddress(address);
List<PhoneNumber> phoneNumbers = new ArrayList<>();
PhoneNumber homePhoneNumber = new PhoneNumber();
homePhoneNumber.setType("home");
homePhoneNumber.setNumber("111-111-1111");
phoneNumbers.add(homePhoneNumber);
PhoneNumber workPhoneNumber = new PhoneNumber();
workPhoneNumber.setType("work");
workPhoneNumber.setNumber("222-222-2222");
phoneNumbers.add(workPhoneNumber);
person.setPhoneNumbers(phoneNumbers);
String json = gson.toJson(person);
System.out.println(json);

運行該代碼，就可以得到如下的JSON字符串：

{
"name": "John",
"age": 30,
"address": {
"street": "Main Street",
"city": "New York",
"state": "NY"
},
"phoneNumbers": [
{
"type": "home",
"number": "111-111-1111"
},
{
"type": "work",
"number": "222-222-2222"
}
]
}

可以看到，使用Gson可以快速生成復雜的JSON結構，并且代碼也很簡潔易懂。