转换清单 <string> 字符串数组

Question

I wanted to split a json file and store its contents in an array and then print them out to the console, i have been successful in doing so except in the problem of converting List to a string array.

My code is :

package com.acme.datatypes;

public class User {

private List<String> authors;
private String publisher;
private String title;
private int year;

public List<String> getAuthors() {
    return this.authors;
}

public void setAuthors(List<String> authors) {
    this.authors = authors;
}

public String getPublisher() {
    return this.publisher;
}

public void setPublisher(String publisher) {
    this.publisher = publisher;
}

public String getTitle() {
    return this.title;
}

public void setTitle(String title) {
    this.title = title;
}

public int getYear() {
    return this.year;
}

public void setYear(int year) {
    this.year = year;
}

}

and another class :

package com.acme.datatypes;

public class UserTest {

public static void main(String[] args) throws JsonParseException,
        JsonMappingException, IOException {
    split("");

}

// Parsing or Reading the JSON file using external libraries
public static String split(String V) throws JsonParseException,
        JsonMappingException, IOException {
    File jsonFile = new File("library.json");

    ObjectMapper mapper = new ObjectMapper();
    List<User> userList = mapper.readValue(jsonFile,
            new TypeReference<List<User>>() {
            });

    // Store the titles in an array and then print them out to the
    // console
    for (User usert : userList) {
        String[] title = new String[1];
        for (int c = 0; c < 1; c++) {
            title[c] = usert.getTitle();
            System.out.println(title[c]);
        }
    }
    // Create blank line on console
    System.out.println();

    // Store the publishers in an array and then print them out to the
    // console
    for (User userp : userList) {
        String[] publisher = new String[1];
        for (int i = 0; i < 1; i++) {
            publisher[i] = userp.getPublisher();

            System.out.println(publisher[i]);
        }
    }
    // Create blank line on console
    System.out.println();

    // Store the year(for a book) in an array and then print them out to the
    // console
    for (User usery : userList) {
        int[] year = new int[1];
        for (int j = 0; j < 1; j++) {
            year[j] = usery.getYear();

            System.out.println(year[j]);
        }
    }
    // Create blank line on console
    System.out.println();

    ;

        return V;
    }
}

}

 My json file is : 

[
            {
                "title": "Principles of Compiler Design",
                "authors": [
                    "Aho",
                    "Ullman"
                ],
                "publisher": "Addison Wesley",
                "year": 1977
            },
            {
                "title": "Compilers: Principles Techniques and Tools",
                "authors": [
                    "Aho",
                    "Sethi",
                    "Ullman"
                ],
                "publisher": "Addison Wesley",
                "year": 1985
            }]

Answer 1

This is as easy as:

List<String> yourList = ...
String[] array = yourList.toArray(new String[yourList.size()]);

Can't find the part of code where you are trying to do this, or else I'd had used your variable names.

Answer 2

The following sample can be replicated for all the places in the code where you need to put a field from the list of objects into an array. It declares the array outside of the loop and uses the size of the list to set the size of the array.

It is important to instantiate and assign the array outside of the loop, otherwise your going to instantiate a new array during every iteration of the loop.

String[] titles = new String[userList.size()];
for (int x; x < userList.size(); x++) {
       titles[x] = usert.getTitle();
       System.out.println(titles[x]);
    }
}

Place this code here

// Parsing or Reading the JSON file using external libraries public static String split(String V) throws JsonParseException, JsonMappingException, IOException { File jsonFile = new File("library.json");

ObjectMapper mapper = new ObjectMapper();
List<User> userList = mapper.readValue(jsonFile,
        new TypeReference<List<User>>() {
        });

// Store the titles in an array and then print them out to the
// console
String[] titles = new String[userList.size()];
for (int x; x < userList.size(); x++) {
       titles[x] = usert.getTitle();
       System.out.println(titles[x]);
    }
}
// Create blank line on console
//Rest of class...