用Stream找到List裡面第一個符合條件的物件，用Optional來接查找後的結果。
下表是不同條件的商品規則，這邊要來用Stream來取得搜尋條件中符合的第一個項目。
製作商品規則List
ProductRule ruleA = new ProductRule("001", "cake BANANA", "TWD", "Y");
ProductRule ruleB = new ProductRule("002", "cake APPLE", "TWD", "Y");
ProductRule ruleC = new ProductRule("003", "cake GRAPE", "JPY", "N");
ProductRule ruleD = new ProductRule("004", "cake ORANGE", "JPY", "N");
ProductRule ruleE = new ProductRule("005", "cake KIWI", "HKD", "N");
List<ProductRule> ruleList = new ArrayList<ProductRule>();
ruleList.add(ruleA);
ruleList.add(ruleB);
ruleList.add(ruleC);
ruleList.add(ruleD);
ruleList.add(ruleE);
沒找到符合條件的情況
用filter過濾條件，用peek印出進行每個項目時的情況，最後用findFirst()取得結果。
Optional<ProductRule> ruleNotFound = ruleList.stream()
.peek( kx -> System.out.println("Currency: " + kx.getCurrency() + ", hasVoucher: " + kx.getHasVoucher() ))
.filter(kx -> StringUtils.equals("USD", kx.getCurrency() ) && StringUtils.equals("Y", kx.getHasVoucher() ) )
.findFirst();
System.out.println("ruleNotFound : " + ruleNotFound);
System.out.println("rule.isPresent(): " + ruleNotFound.isPresent());
/* 輸出結果
Currency: TWD, hasVoucher: Y
Currency: TWD, hasVoucher: Y
Currency: JPY, hasVoucher: N
Currency: JPY, hasVoucher: N
Currency: HKD, hasVoucher: N
ruleNotFound : Optional.empty
rule.isPresent(): false
*/
查找結果沒有符合條件的物件，印出ruleNotFound時不是顯示null，而是顯示Optional.empty，在判斷是否有結果時用.isPresent()。
找到符合條件的情況
Optional<ProductRule> ruleFound = ruleList.stream()
.peek( kx -> System.out.println("Currency: " + kx.getCurrency() + ", hasVoucher: " + kx.getHasVoucher() ))
.filter(kx -> StringUtils.equals("JPY", kx.getCurrency() ) && StringUtils.equals("N", kx.getHasVoucher() ) )
.findFirst();
System.out.println("ruleFound : " + ruleFound);
System.out.println("ruleFound.isPresent(): " + ruleFound.isPresent());
if( ruleFound.isPresent() ){
System.out.println("Product Name: " + ruleFound.get().getName());
}
/*
Currency: TWD, hasVoucher: Y
Currency: TWD, hasVoucher: Y
Currency: JPY, hasVoucher: N
ruleFound : Optional[hatoto.test.PlayStreamFindFirst$ProductRule@161cd475]
ruleFound.isPresent(): true
Product Name: cake GRAPE
*/
有符合條件的項目時，先用.isPresent判斷，再用.get()來取得原本類別的物件。
gist：streamFindFirst.java