stream的学习
应用场景:当你需要从数据库查询出某些数据的时候,不应该是在for循环里一遍一遍去查询数据库,
而是提取出for循环中的所有条件组成一个集合去一次性把数据库中满足条件的数据全部查出来,放到内存中来做比对,
可以利用下面的知识点得出你想要的结果,这样数据库的性能得到优化,并且查询的速度更快。
1、从一个数组中筛选出符合条件的对象
DpQuotationUnitPrice dpQuotationUnitPrice = dpUnitPrices. stream ( ) . filter ( dpPrice - >
dpPrice. getCustomerNo ( ) . equals ( "" ) &&
dpPrice. getDpTypeNo ( ) . equals ( dpOrder. getDpTypeNo ( ) ) &&
dpPrice. getDpHandleNo ( ) . equals ( "" ) &&
dpPrice. getDpColorNo ( ) . equals ( "" ) ) . findFirst ( ) . orElse ( null) ;
if ( dpQuotationUnitPrice != null) {
return dpQuotationUnitPrice. getUnitPrice ( ) ;
}
2、从一个数组中筛选出符合条件的数组
List< DpOrderResult> dpSplitResultOfHandles = dpOrder. getDpOrderElements ( ) . stream ( ) . flatMap ( ele - > ele. getDpOrderResults ( ) . stream ( ) ) . filter ( result - > result. getDpSplitResultType ( ) == DpSplitResultType. HANDLE. getIndex ( ) ) . collect ( Collectors. toList ( ) ) ;
3、从一个对象数组中抽取该对象的某一个字段重新生成一个新的集合
Set< String> allHandleItemNos = dpOrder. getDpOrderElements ( ) . stream ( ) . map ( DpOrderElement: : getDpHandleItemNo) . collect ( Collectors. toSet ( ) ) ;
4、将一个List转成一个list<对象>
应用场景:在sql语句只查询一个对象的某几个字段时使用
SQL语句: 只查询itemNo, UnitPrice两个字段
@Query ( "select i.itemNo,i.quotationUnitPrice from ItemMaster i where i.itemNo in :itemNos" )
List< Object[ ] > getAllGeneralDpHandleUnitPrices ( @Param ( value = "itemNos" ) Set< String> itemNos) ;
Service层:
private List< ItemMaster> genItemMastersWithDpHandleUnitPrice ( Set< String> allDPHandleItemNos) {
List< Object[ ] > allGeneralDpHandleUnitPrices = itemMasterRepository. getAllGeneralDpHandleUnitPrices ( allDPHandleItemNos) ;
return allGeneralDpHandleUnitPrices. stream ( ) . map ( item - > genItemMasterWithDpHandleUnitPrice ( item) ) . collect ( Collectors. toList ( ) ) ;
}
private ItemMaster genItemMasterWithDpHandleUnitPrice ( Object[ ] itemObject) {
ItemMaster itemMaster = new ItemMaster ( ) ;
if ( itemObject != null) {
itemMaster. setItemNo ( itemObject[ 0 ] . toString ( ) ) ;
itemMaster. setQuotationUnitPrice ( ( float ) itemObject[ 1 ] ) ;
}
return itemMaster;
}
5、遍历某一个对象集合,将该集合中每一个对象的字段内容赋值到一个用来构建新对象的函数中,最终结果返回新对象的集合
private List< DpQuotationOrderElement> createDpQuotationOrderElements ( DpOrder dpOrder, QuotationRemarkParameter remarkParameterConfig) {
List< DpQuotationOrderElement> dpQuotationOrderElements = dpOrder. getDpOrderElements ( ) . stream ( ) . map ( orderEle - > createDpQuotationOrderElement ( dpOrder, orderEle, remarkParameterConfig) ) . collect ( Collectors. toList ( ) ) ;
return dpQuotationOrderElements;
}
private DpQuotationOrderElement createDpQuotationOrderElement ( DpOrder dpOrder, DpOrderElement orderEle, QuotationRemarkParameter remarkParameterConfig) {
DpQuotationOrderElement dpQuotationOrderElement = new DpQuotationOrderElement ( ) ;
return dpQuotationOrderElement;
}
6、遍历DpOrder对象中的dpOrderElements集合,使用flatMap来使dpOrderElements对象中的dpOrderResults集合扁平化,变成一个DpOrderResult集合输出,可以再对该DpOrderResult集合进行筛选,得到想要的数据
public class DpOrder {
@ElementCollection ( fetch = FetchType. LAZY)
private List< DpOrderElement> dpOrderElements;
}
public class DpOrderElement {
@ElementCollection ( fetch = FetchType. LAZY)
private List< DpOrderResult> dpOrderResults;
}
public class DpOrderResult {
private int dpSplitResultType;
}
List< DpOrderResult> handleDpOrderResults = dpOrder. getDpOrderElements ( ) . stream ( ) . flatMap ( ele - > ele. getDpOrderResults ( ) . stream ( ) ) . filter ( result - > result. getDpSplitResultType ( ) == DpSplitResultType. HANDLE. getIndex ( ) ) . collect ( Collectors. toList ( ) ) ;
7、对某一个对象集合中的数字字段进行求和
float subDpArea = dpQuotationOrderElements. stream ( ) . collect ( Collectors. summingDouble ( DpQuotationOrderElement: : getDpArea) ) . floatValue ( ) ;
8、使用groupingBy可以进行分组求和,用来合并某些相同内容的对象
public class GeneralDpPpOrderDatailGroup {
public GeneralDpPpOrderDatailGroup ( GeneralDpPpOrderDetail generalDpPpOrderDetail) {
this . dPNo = generalDpPpOrderDetail. getdPNo ( ) ;
this . dpColorNo = generalDpPpOrderDetail. getDpColorNo ( ) ;
this . dpWidth = generalDpPpOrderDetail. getDpWidth ( ) ;
this . dpHeight = generalDpPpOrderDetail. getDpHeight ( ) ;
this . openDoorWayNo = generalDpPpOrderDetail. getOpenDoorWayNo ( ) ;
this . dpHandleItemNo = generalDpPpOrderDetail. getDpHandleItemNo ( ) ;
this . handleFixWay = generalDpPpOrderDetail. getHandleFixWay ( ) ;
this . hingePosition = generalDpPpOrderDetail. getHingePosition ( ) ;
this . dpBorderItemNo = generalDpPpOrderDetail. getDpBorderItemNo ( ) ;
this . backPanelItemNo = generalDpPpOrderDetail. getBackPanelItemNo ( ) ;
this . remark = generalDpPpOrderDetail. getRemark ( ) ;
this . dpColorWidth = generalDpPpOrderDetail. getDpColorWidth ( ) ;
this . dpColorHeight = generalDpPpOrderDetail. getDpColorHeight ( ) ;
this . borderWidth = generalDpPpOrderDetail. getBorderWidth ( ) ;
this . borderHeight = generalDpPpOrderDetail. getBorderHeight ( ) ;
this . backPanelWidth = generalDpPpOrderDetail. getBackPanelWidth ( ) ;
this . backPanelHeight = generalDpPpOrderDetail. getBackPanelHeight ( ) ;
this . handleLength = generalDpPpOrderDetail. getHandleLength ( ) ;
}
}
private void setGeneralDpPpOrderDetail ( DpOrder dpOrder, GeneralDpPpOrder generalDpPpOrder) {
List< GeneralDpPpOrderDetail> groupedDpPpDetails = dpOrder. getDpOrderElements ( ) . stream ( ) . map ( element - > genDpPpDetailOfNoGroup ( element, dpOrder. getBackPanelItemNo ( ) ) ) . collect ( Collectors. groupingBy ( GeneralDpPpOrderDatailGroup: : new ) ) . entrySet ( ) . stream ( )
. map ( entry - > genDpPpOrderDetail ( entry) ) . collect ( Collectors. toList ( ) ) ;
generalDpPpOrder. setGeneralDpPpOrderDetails ( groupedDpPpDetails) ;
}
private GeneralDpPpOrderDetail genDpPpOrderDetail (
Entry< GeneralDpPpOrderDatailGroup, List< GeneralDpPpOrderDetail> > entry) {
GeneralDpPpOrderDetail detail = new GeneralDpPpOrderDetail ( ) ;
if ( entry. getValue ( ) != null && entry. getValue ( ) . size ( ) > 0 ) {
setGeneralDpPpOrderSumField ( entry. getValue ( ) , detail) ;
}
return detail;
}
private void setGeneralDpPpOrderSumField ( List< GeneralDpPpOrderDetail> generalDpPpOrderDetails,
GeneralDpPpOrderDetail generalDpPpOrderDetail) {
float totalArea = 0 ;
int totalNumber = 0 ;
for ( GeneralDpPpOrderDetail generalDetail : generalDpPpOrderDetails) {
totalArea += generalDetail. getDpWidth ( ) * generalDetail. getDpHeight ( ) * generalDetail. getDpQty ( ) / 1000000 ;
totalNumber += generalDetail. getDpQty ( ) ;
}
GeneralDpPpOrder generalDpPpOrder = new GeneralDpPpOrder ( ) ;
generalDpPpOrder. setTotalArea ( totalArea) ;
generalDpPpOrder. setTotalNumber ( totalNumber) ;
}
9、根据对象中的某个字段对该对象集合进行排序
List< DpQuotationOrderElement> dpQuotationOrderElements = dpQuotationOrderElements. stream ( ) . sorted ( Comparator. comparing ( DpQuotationOrderElement: : getDpQty) ) . collect ( Collectors. toList ( ) ) ;
List< DpQuotationOrderElement> dpQuotationOrderElements = dpQuotationOrderElements. stream ( ) . sorted ( Comparator. comparing ( DpQuotationOrderElement: : getDpQty) . reversed ( ) ) . collect ( Collectors. toList ( ) ) ;
10、遍历某个集合
dpQuotationOrderElements. forEach ( ele - > System. out. println ( ele. getDpQty ( ) ) ) ;
11、过滤集合中相同元素
List< DpQuotationOrderElement> distinctElements = dpQuotationOrderElements. stream ( ) . distinct ( ) . collect ( Collectors. toList ( ) ) ;
12、比较对象的某个字段从集合中找出最大值/最小值的对象
DpQuotationOrderElement dpQuotationOrderElement = dpQuotationOrderElements. stream ( ) . max ( Comparator. comparing ( DpQuotationOrderElement: : getDpQty) ) . get ( ) ;
DpQuotationOrderElement dpQuotationOrderElement = dpQuotationOrderElements. stream ( ) . min ( Comparator. comparing ( DpQuotationOrderElement: : getDpQty) ) . get ( ) ;
13、对某一个对象集合中的数字字段取平均值
float avgQty = dpQuotationOrderElements. stream ( ) . collect ( Collectors. averagingInt ( DpQuotationOrderElement: : getDpQty) ) . floatValue ( ) ;