这可能是项目中最实用的java8示例了，还不来看？

​一、组成及特点

• 数据源、中间操作、终端操作
• 流只能使用一次
• 并行流 和 串行流
  
  二、可以产生流的数据源
• 集合、文件
  
  三、中间操作
• 字符串操作拼接joining(""),底层实现是StringBuilder.append();

//例1
private static String appendStr(){
        List<String> stringList = Arrays.asList("h","e","l","l","o");
        String str = stringList.stream().collect(Collectors.joining("-"));
        //结果： h-e-l-l-o
        return str;
    }
//例2
public class ShangPin {
    private String id;
    private String mc;

    //getter ..,setter
    public static void main(String[] args) {
        List<ShangPin> list = new ArrayList<>();
        for (int i = 0; i < 5 ; i++) {
            ShangPin shangPin = new ShangPin();
            shangPin.setId(i+"");
            shangPin.setMc("名称"+i);
            list.add(shangPin);
        }
        //名称逗号分割
        String mcs = list.stream().map( shangPin -> shangPin.getMc()).collect(Collectors.joining(","));
        System.out.println(mcs);
    }
}

• for循环

public static void test17(){
 List<String> list = Arrays.asList("j","a","k","l");
 list.forEach(x->{
        System.out.println(x);
    });
}

• 筛选

@Test
public void test7(){
    List<String> stringList = new ArrayList<>();
    JSONArray jsonArray = new JSONArray();
    //数据源
    for (int i = 0; i < 4; i++) {
        stringList.add(i+"");
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("num",i+"");
        jsonArray.add(jsonObject);
    }
    //中间-筛选
    Stream<String> stream = stringList.stream().filter(x -> "1".equals(x) || "3".equals(x));
    Stream<JSONObject> _stream = jsonArray.stream().filter( o ->{
        String num = ((JSONObject)o).getString("num");
        return  "1".equals(num) || "3".equals(num);
    } );
    //终端-收集 单个(类似 for循环里break )/集合
    //String str = stream.findFirst().get();
    //JSONObject json = _stream.findFirst().get();
    List<String> str = stream.collect(Collectors.toList());
    JSONArray _str = _stream.collect(Collectors.toCollection(JSONArray::new));
}

• for循环+break 被替换
• filter过滤，for循环里的 break 可以用 findFirst

public static void test11(){
    String[] str = {"1","2","3","4","5","6","7","8","12","10","11","14"};
    List<String> list = Arrays.asList(str);
   String s = list.stream().filter( x -> (Integer.parseInt(x))%2 == 0).findAny().get();
    System.out.println(s);
    /*for (int i = 0; i < str.length; i++) {
        if((Integer.parseInt(str[i]))%2 == 0){
            System.out.println(str[i]);
            break;
        }
    }*/
}

• 数值比较/求和/排序

public static void test12(){
    String[] str = {"1","1","15","4","5","6","7","8","12","10","11","14"};
    //转为int后求和
    int sum = Arrays.stream(str).mapToInt(x -> Integer.parseInt(x)).sum();
   //求和2，自定义
   int sum2 =  Arrays.stream(str).mapToInt(x -> Integer.parseInt(x)).reduce(2,(m,n)->m+n);
   System.out.println(sum+"+2="+sum2);
   //最大值max(),最小值min 比较器实现
    String max = Arrays.stream(str).max(DemoApplicationTests::compator).get();
    //数组长度
    long length = Arrays.stream(str).count();
    //排序 当前从小到大
    List list = Arrays.stream(str).sorted(DemoApplicationTests::compator).collect(Collectors.toList());
    System.out.println(list);
    System.out.println(length);
    System.out.println(sum+"--"+max);
}
private static int compator(String x1, String x2) {
        if(Integer.parseInt(x1) < Integer.parseInt(x2)){
            return -1;
        }else if(Integer.parseInt(x1) == Integer.parseInt(x2)){
            return 0;
        }else{
            return 1;
        }
}

• 去重/截取

public static void test13(){
    String[] str = {"1","1","15","4","11","6","7","8","12","10","11","14"};
    //去重 distinct
    List strList = Arrays.stream(str).distinct().collect(Collectors.toList());
    //前四(长度)个
    List strList1 = Arrays.stream(str).limit(4).collect(Collectors.toList());
    //跳过4个
    List strList2 = Arrays.stream(str).skip(4).collect(Collectors.toList());
    System.out.println(strList2);
}

• limit+skip 实现截取集合
• 分割集合

public static void test14(){
    int dayNum = 5;
    List<String> strList = Arrays.asList("2","2","2","2","2",
            "4","4","4","4","4",
            "8","8","8","8","8");
    //总天数
    int sumDay = strList.size();
    //分3周
    int week = sumDay%dayNum == 0 ? sumDay/dayNum:sumDay/dayNum+1;
​
    List list = Stream.iterate(0,n->n+1).
            //3周
                    limit(week).
            //并行流
                    parallel().
                    map(
                            //循环，并返回值
                            num -> {
                                System.out.println(num);
                                List<String> list1 =  strList.stream().
                                        //跳过0*5个，取5个,即 0-5,
                                        //跳过1*5；取5个，5-10,。。。
                                                skip(num * dayNum).
                                                limit(dayNum).
                                                collect(Collectors.toList());
                                //求和
                                Double d = list1.stream().mapToDouble(x -> Double.valueOf(x) ).sum();
                                return d+"";
                            }
                    ).
                    collect(ArrayList::new,ArrayList::add,List::addAll);
    System.out.println(list);
}

• 数据处理 map,结果收集

public static void test15(){
 //构建数据源
 JSONArray jsonArray = new JSONArray();
    for (int i = 0; i < 6 ; i++) {
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("mc","mc"+i);
        jsonObject.put("xm","word"+i);
        jsonArray.add(jsonObject);
    }
    //创建流
    Stream<JSONObject> stream = jsonArray.stream();
    //中间操作filter(条件过滤)/map(数据处理返回)/peek(调试时无返回)
    Stream<JSONObject> streams = stream.
            filter(obj -> !"mc0".equals(obj.getString("mc"))).
            map(ele -> {
                ele.put("xm","fyhh");
        return ele;
    });
    //终端操作 收集/循环
    JSONArray newArr = streams.collect(Collectors.toCollection(JSONArray::new));
    //JSONArray newArr = streams.collect(JSONArray::new,JSONArray::add,JSONArray::addAll);
    //JSONArray newArr = streams.collect(()-> new JSONArray(),(arrt,json)->{arrt.add(json);},(parr,json) ->{parr.addAll(json);});
    //List list = streams.collect(Collectors.toCollection(ArrayList::new));
    System.out.println(newArr);
}

• 逻辑与/或

public static void test16(){
 List lsi = Arrays.asList("false","true","true");
 //逻辑或,只要有一个为"true"，即返回true,否则else; false||true||false  结果：false、true、--true（短路）
 boolean flg = lsi.stream().anyMatch(x -> {
        System.out.print(x+"、");
        return "true".equals(x);
    });
    System.out.println("flg--"+flg);
 //逻辑与 false&&true&&false 都为"true",才返回true,否则返回false
    boolean flg2 = lsi.stream().allMatch(x -> {
        System.out.print(x+"、");
        return "true".equals(x);
    });
    System.out.println("flg2--"+flg2);
    //anyMatch 取反,都不是"true"，才返回true;否则else,
    boolean flg3 = lsi.stream().noneMatch(x -> {
        System.out.print(x+"、");
        return "true".equals(x);
    });
    System.out.println("flg3--"+flg3);
}

四、数据收集collect

• List 转 Map

public static void test18(){
    List<String> list = Arrays.asList("j","a","k","a","l");
    //推荐
    Map<String,String> mp = list.stream().collect(HashMap::new,(theMap,ele) -> {
        theMap.put(ele,ele+"==");
    },Map::putAll);
    //不推荐，key重复会有异常 java.lang.IllegalStateException: Duplicate key a==
   /* Map<String,String> collect2 = list.stream()
            .collect(Collectors.toMap(ele ->ele,
                    ele ->ele+"=="));*/
    System.out.println(mp);
}

• List 转JSONarray

public static void test20(){
    List<JSONObject> list = new ArrayList<>();
    for (int i = 0; i <4; i++) {
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("a"+i,22.5*i);
        list.add(jsonObject);
    }
    JSONArray jsonArray = list.stream().collect(JSONArray::new,(theArr,json) -> {
        theArr.add(json);
    },JSONArray::addAll);
    System.out.println(jsonArray);
}

五、文件

public class FileUtils {
​
    public static void main(String[] args) throws IOException, URISyntaxException {
        FileUtils fileUtils = new FileUtils();
        JSONObject json = fileUtils.getJsonFromCfg("/config/fyjing.json");
        System.out.println(json);
    }
    /**
     * json配置文职转为json
     */
    public JSONObject getJsonFromCfg(String path) throws URISyntaxException, IOException {
        //获取项目的绝对路径
        URL allPath = FileUtils.class.getResource(path);
        //URI 格式为 f:/F:/......
        URI uri = allPath.toURI();
        //Paths.get(uri) 为 F:/.....
        String jsonStr = Files.lines(Paths.get(uri)).map(x->x.trim()).collect(Collectors.joining());
        //字符串转json
        JSONObject json = JSONObject.fromObject(jsonStr);
        return json;
    }
}

六、判断空指针

@Test
    public void testw(){
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("name", "2");
        String name = getNameJson(jsonObject);
    }
    private String getNameJson(JSONObject jsonObject) {
        return Optional.ofNullable(jsonObject).map(x -> {
            String name = x.getString("name");
            return name;
        }).orElseThrow(()->new RessException("",500));
    }