Rojer.2017.10.05

先前寫的 文章 lambda example 是針對 String 來處理的，以下繼續補充對於 class 要如何處理。

測試格式

public class Item {
    public String ItemName;
    public String ItemNo;
    public String ItemSource;
    public Integer Total;
}

初始化

static private List<Item> init(){
    List<Item> list = new ArrayList<Item>();
    Item it = new Item("Name2","No2","E",25); list.add(it);
    it = new Item("Name2","No1","D",20); list.add(it);
    it = new Item("Name1","No2","C",15); list.add(it);
    it = new Item("Name1","No1","B",5); list.add(it);
    it = new Item("Name1","No1","A",10); list.add(it);
    return list;
}

Count

static private void SelectCount(){
    List<Item> list = init();
    long count = list.stream().count();
    System.out.println("count=" + count);
}

Sum 

static private void Sum(){
    List<Item> list = init();
    int sum = list.stream().filter(x->x.getItemName().equals("Name1")).mapToInt(x->x.getTotal()).sum();
    System.out.println("sum=" + sum);
}

Order by Then By

static private void orderByThenBy(){
    List<Item> list = init();
    list.sort(Comparator.comparing(Item::getItemName).thenComparing(Item::getItemNo).thenComparing(Item::getTotal));
    for (Item it : list) {
        System.out.println(it.getItemName() + "," + it.getItemNo() + "," + it.getItemSource() + "," + it.getTotal());
    }
}

Order by Desc

static private void orderByDesc(){
    List<Item> list = init();
    list.sort(Comparator.comparing(Item::getTotal).reversed());
    for (Item it : list) {
        System.out.println(it.getItemName() + "," + it.getItemNo() + "," + it.getItemSource() + "," + it.getTotal());
    }
}

只查詢單一欄位

static private void selectSingleColumn(){
    List<Item> list = init();
    List<String> itemSourceList = list.stream().map(x->x.getItemName()).collect(Collectors.toList());    
    for (String it : itemSourceList) {
        System.out.println(it);
    }
}

查詢單一欄位並過濾重複欄位

static private void selectDistinct(){
    List<Item> list = init();
    List<String> itemSourceList = list.stream().map(x->x.getItemName()).distinct().collect(Collectors.toList());    
    for (String it : itemSourceList) {
        System.out.println(it);
    }
}

Select New Object

static private void selectNew(){
    List<Item> list = init();
    List<String[]> newlist = list.stream()
    .map(x-> new String[] { x.ItemName , x.ItemNo }).collect(Collectors.toList());
    for(String[] it: newlist){
        System.out.println(it[0] + "," + it[1]);
    }
}