In Java, you can use the Jackson library to convert JSON into a string. First, you need to add the dependency of the Jackson library to your project. Then, you can use the following code to convert JSON into a string:

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonToStringConverter {

    public static void main(String[] args) {
        // 定义一个JSON字符串
        String json = "{\"name\":\"John\", \"age\":30, \"city\":\"New York\"}";

        try {
            // 创建ObjectMapper对象
            ObjectMapper objectMapper = new ObjectMapper();

            // 将JSON字符串转换为字符串
            String jsonString = objectMapper.writeValueAsString(json);

            // 打印转换结果
            System.out.println(jsonString);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
    }
}

In the above code, a JSON string is first defined and then an ObjectMapper object is created. By calling the writeValueAsString() method, the JSON string is converted to a string. Finally, the conversion result is printed.