常用stream函数

本篇记录了常用的stream函数

聚合求和

public class demo {
    public void test() { 
        List<Map<String, Object>> items = new ArrayList<Map<String, Object>>();
        Map<String, Object> item1 = new HashMap<String, Object>();
        item1.put("key", "wo");
        item1.put("price", 1);
        items.add(item1);
        Map<String, Object> item2 = new HashMap<String, Object>();
        item2.put("key", "yuq");
        item2.put("price", 2);
        items.add(item2);
        Map<String, Object> item3 = new HashMap<String, Object>();
        item3.put("key", "wo");
        item3.put("price", 100);
        items.add(item3);
        Map<Object, Double> result = items.parallelStream().collect(Collectors.groupingBy(mapper -> { 
            return mapper.get("key"); 
            }, Collectors.summingDouble(mapper -> {
                return ((Integer) mapper.get("price")).doubleValue(); 
            })));
        System.out.println(result); 
    }
}

map

public class demo {
    public void test() { 
        List<Map<String, Object>> items = new ArrayList<Map<String, Object>>();
        //省略值
        System.out.println(items.stream().map(mapper ->mapper.get("key")).collect(Collectors.toList()));
    }
}

reduce

public class demo {
    public void test() { 
        List<Map<String, String>> items = new ArrayList<Map<String, Object>>();
        //省略值
        System.out.println(items.stream().map(mapper -> {
            return mapper.get("key"); 
        }).reduce((accumulator, row) -> accumulator + "," + row).orElse("错误")); 
    }
}

排序

public class demo {
    public void test() { 
        List<Map<String, String>> items = new ArrayList<Map<String, Object>>();
        //省略值
        items.stream().sorted(Comparator.comparing(demo::comparingByPX)).collect(Collectors.toList());
    }
    
    public static Integer comparingByPX(Map<String, Object> map) {
        return Integer.parseInt(map.get("key").toString());
    }
}

过滤

public class demo {
    public void test() { 
        List<Map<String, String>> items = new ArrayList<Map<String, Object>>();
        //省略值
        items.stream().filter(mapper -> "目标值".equals(mapper.get("key"))).collect(Collectors.toList());
    }
}