史上最全的 Java 新手问题汇总(3)_H5之家 - 中国HTML5教程资源分享第一站

下面是编码成JSON串的一个简单的例子。

import org.json.simple.JSONObject; import org.json.simple.JSONArray; public class JsonEncodeDemo { public static void main(String[] args) { JSONObject obj = new JSONObject(); obj.put("Novel Name", "Godaan"); obj.put("Author", "Munshi Premchand"); JSONArray novelDetails = new JSONArray(); novelDetails.add("Language: Hindi"); novelDetails.add("Year of Publication: 1936"); novelDetails.add("Publisher: Lokmanya Press"); obj.put("Novel Details", novelDetails); System.out.print(obj); } }

输出：

{"Novel Name":"Godaan","Novel Details":["Language: Hindi","Year of Publication: 1936","Publisher: Lokmanya Press"],"Author":"Munshi Premchand"}JSON解析

开发人员要想解析JSON串，首先你得知道它的格式。下面例子有助于你来理解这一点：

import java.io.FileNotFoundException; import java.io.FileReader; import java.io.IOException; import java.util.Iterator; import org.json.simple.JSONArray; import org.json.simple.JSONObject; import org.json.simple.parser.JSONParser; import org.json.simple.parser.ParseException; public class JsonParseTest { private static final String filePath = "//home//user//Documents//jsonDemoFile.json"; public static void main(String[] args) { try { // read the json file FileReader reader = new FileReader(filePath); JSONParser jsonParser = new JSONParser(); JSONObject jsonObject = (JSONObject)jsonParser.parse(reader); // get a number from the JSON object Long id = (Long) jsonObject.get("id"); System.out.println("The id is: " + id); // get a String from the JSON object String type = (String) jsonObject.get("type"); System.out.println("The type is: " + type); // get a String from the JSON object String name = (String) jsonObject.get("name"); System.out.println("The name is: " + name); // get a number from the JSON object Double ppu = (Double) jsonObject.get("ppu"); System.out.println("The PPU is: " + ppu); // get an array from the JSON object System.out.println("Batters:"); JSONArray batterArray= (JSONArray) jsonObject.get("batters"); Iterator i = batterArray.iterator(); // take each value from the json array separately while (i.hasNext()) { JSONObject innerObj = (JSONObject) i.next(); System.out.println("ID "+ innerObj.get("id") + " type " + innerObj.get("type")); } // get an array from the JSON object System.out.println("Topping:"); JSONArray toppingArray= (JSONArray) jsonObject.get("topping"); Iterator j = toppingArray.iterator(); // take each value from the json array separately while (j.hasNext()) { JSONObject innerObj = (JSONObject) j.next(); System.out.println("ID "+ innerObj.get("id") + " type " + innerObj.get("type")); } } catch (FileNotFoundException ex) { ex.printStackTrace(); } catch (IOException ex) { ex.printStackTrace(); } catch (ParseException ex) { ex.printStackTrace(); } catch (NullPointerException ex) { ex.printStackTrace(); } } }

jsonDemoFile.json

{ "id": 0001, "type": "donut", "name": "Cake", "ppu": 0.55, "batters": [ { "id": 1001, "type": "Regular" }, { "id": 1002, "type": "Chocolate" }, { "id": 1003, "type": "Blueberry" }, { "id": 1004, "type": "Devil's Food" } ], "topping": [ { "id": 5001, "type": "None" }, { "id": 5002, "type": "Glazed" }, { "id": 5005, "type": "Sugar" }, { "id": 5007, "type": "Powdered Sugar" }, { "id": 5006, "type": "Chocolate with Sprinkles" }, { "id": 5003, "type": "Chocolate" }, { "id": 5004, "type": "Maple" } ] }The id is: 1 The type is: donut The name is: Cake The PPU is: 0.55 Batters: ID 1001 type Regular ID 1002 type Chocolate ID 1003 type Blueberry ID 1004 type Devil's Food Topping: ID 5001 type None ID 5002 type Glazed ID 5005 type Sugar ID 5007 type Powdered Sugar ID 5006 type Chocolate with Sprinkles ID 5003 type Chocolate ID 5004 type Maple简单字符串查找

Java提供了一个库函数叫做indexOf()。这个方法可以用在String对象上，它返回的是要查找的字符串所在的位置序号。如果查找不到则会返回-1。

列出目录下的文件

你可以用下面的代码来列出目录下的文件。这个程序会遍历某个目录下的所有子目录及文件，并存储到一个数组里，然后通过遍历数组来列出所有文件。

import java.io.*; public class ListContents { public static void main(String[] args) { File file = new File("//home//user//Documents/"); String[] files = file.list(); System.out.println("Listing contents of " + file.getPath()); for(int i=0 ; i < files.length ; i++) { System.out.println(files[i]); } } }一个简单的IO程序