- IntStream
- IntStream的使用介紹
前言
建立初始陣列的方便性，傳統做法需要在一開始宣告陣列長度，再利用for loop 將值塞入。若再複雜一點要取1-10中的偶數值建立陣列，又更為複雜，
使用IntStream則可經過一連串操作後，再用forEach進行印出，也能很容易地將操作完畢的IntStream轉成array，請參考以下範例。
public static void main(String[] args) throws Exception {
//建立陣列長度5 初始值10
//tradition
int [] arr1 = new int[5];;
for(int i = 0; i<arr1.length;i++)
arr1[i] = 10;
//use IntStream
arr1 = IntStream.rangeClosed(1, 5).map(s->10).toArray();
//建立1-10中偶數的陣列
//tradition
int [] arr2 = new int[10/2];
for(int i =1;i<=10;i++) {
if(i% 2 ==0)
arr2[i/2-1] = i ;
}
for(int i = 0 ;i<arr2.length;i++)
System.out.println(arr2[i]);
System.out.println("----------------------");
//use IntStream
arr2 = IntStream.rangeClosed(1, 10).filter(s->s%2==0).toArray();
Arrays.stream(arr2).forEach(System.out::println);
}
產生數列
public static void main(String[] args) throws Exception {
//iterate 需初始值，配合limit限制取出個數
IntStream.iterate(0,s->s+2).limit(5).forEach(System.out::println);;
//generate 不需初始值，配合limit限制取出個數
IntStream.generate(()->(int) (Math.random()*100)).limit(5).forEach(System.out::println);
IntStream.generate(()->5).limit(5).toArray();
}
reduce
reduce可以對IntStream做自訂運算需求生成為一 個值，比如以下例子
public static void main(String[] args) throws Exception {
OptionalInt total1 = IntStream.rangeClosed(0, 5).reduce((u,v)->u+v);
int total2 = IntStream.rangeClosed(1, 5).reduce(1,(u,v)->u*v);
System.out.println(total1);//OptionalInt[15]
System.out.println(total2);//120
}
更多用法
- forEach
- range、rangeClosed
- filter
- map
- anyMatch
- noneMatch
- allMatch
- distinct
- sorted
public static void main(String[] args) throws Exception {
//range rangeClosed
IntStream.range(0, 10);//exclude 10
IntStream.rangeClosed(0, 10);//include 10
//filter map
IntStream.range(0, 10).filter(s->s%2==0).forEach(System.out::println);//filter 0 2 4 6 8
IntStream.range(0, 10).map(s->s/2).forEach(System.out::println); //mapping 0 1 1 2 2 3 3 4 4
//anyMatch allMatch noneMatch
boolean isAnyBiggerThanTen = IntStream.rangeClosed(0, 10).anyMatch(s->s>10);//false
boolean isAllSmallThanTen = IntStream.rangeClosed(0, 10).allMatch(s->s<10);//false
boolean isAllNoneBiggerThanTen = IntStream.rangeClosed(0, 10).noneMatch(s->s>10);//true
System.out.println(isAnyBiggerThanTen+" "+isAllSmallThanTen+" "+isAllNoneBiggerThanTen);
//distinct
IntStream.range(0, 10).map(s->s/2).distinct().forEach(System.out::println); //0 1 2 3 4
}
基本統計運算
IntStream有基本的統計運算max、min、sum、count、average，以及包含以上的物件方法summaryStatistics
public static void main(String[] args) throws Exception {
IntStream.rangeClosed(0, 10);//include 10
System.out.println(IntStream.rangeClosed(0, 10).max());//OptionalInt[10]
System.out.println(IntStream.rangeClosed(0, 10).min());//OprionalInt[0]
System.out.println(IntStream.rangeClosed(0, 10).sum());//55
System.out.println(IntStream.rangeClosed(0, 10).average());//OptionalDouble[5.5]
System.out.println(IntStream.rangeClosed(0,10).count());//11
//summaryStatistic
IntSummaryStatistics statistics = IntStream.rangeClosed(0, 10).summaryStatistics();
System.out.println(statistics.getMax());//10
System.out.println(statistics.getMin());//0
System.out.println(statistics.getSum());//55
System.out.println(statistics.getAverage());//5.0
System.out.println(statistics.getCount());//11
}
Reference