Java流减少数组泛型类型列表

Question

我有一个名为“目录”的ArrayList文章（泛型类型）。

文章有以下方法：

public int getUnitsInStore()
public long getUnitPrice()

为了获得目录中所有文章的总价值，我尝试使用Java流Api的Reduce方法。

我尝试了以下内容：

long value = 0;
    
value = catalog.stream().reduce((a,b) -> a.getUnitPrice()*b.getUnitsInStore());

但这给了我一个错误：

Type mismatch: cannot convert from Optional<Article> to long

我做错了什么？

Answer 1

我建议您使用流。reduce（）重载，需要两个参数：

• 标识：标识元素既是还原的初始值，也是流中没有元素时的默认结果

因此，您将使用以下方式获得您的价值：

value = catalog
        .stream()
        .reduce(0L, (partialSum, nextElement) -> partialSum + nextElement.getUnitPrice() * nextElement.getUnitsInStore());

Answer 2

有三种类型的减少方法。尝试这段代码以更好地理解它

    /**
     * T reduce(T identity, BinaryOperator<T> accumulator)
     * 
     * identity = initial value
     * accumulator = first process initial value to first element of the stream,
     * then process the result with the next element in the stream
     */
     
    String name = Stream.of("T", "O", "M", "M", "Y")
            .reduce("", (a,n) -> a + n);
    System.out.println(name);
    
    int sum = Stream.of(1,2,3,4,5,6,7,8,9,10)
            .reduce(0, (a,n) -> a + n);
    System.out.println(sum);

    int multi = Stream.of(1,2,3,4,5,6,7,8,9,10)
            .reduce(1, (a,n) -> a * n);
    System.out.println(multi);
    
    
    /**
     * Optional<T> reduce(BinaryOperator<T> accumulator)
     * 
     */
    
    Optional<String> optName = Stream.of("T", "O", "M", "M", "Y")
            .reduce((a,n) -> a + n);
    
    if(optName.isPresent()){
        System.out.println(" get from optional --> " + optName.get());
    } 
    
    /**
     * <U> U reduce​(U identity, BiFunction<U,​? super T,​U> accumulator, BinaryOperator<U> combiner)
     * 
     * This method signature is used when we are dealing with different types.
     * It allows Java to create intermediate reductions and then combine them at the end.
     */
    
    int total = Stream.of("R", "a", "z", "v", "a", "n")
        .reduce(0, (a,b) -> a + b.length(), (x,y) -> x + y);
    
            // 0 = initial value type int
            // a = int, must match the identity type
            // b.method() return type must match the a and the identity type
            // x,y from BinaryOperator 
    
    System.out.println(total);
    
    
    String names[] = {"Bobby", "Mark", "Anthony", "Danna"};
    
    int sumAll = Stream.of(names)
            .reduce(0, (a,b) -> a + b.length(), (x,y) -> x + y);
    System.out.println(sumAll);
    
    String csvNames = Stream.of(names)
            .reduce("", (a,b) -> a + b + ";");
    System.out.println(csvNames);
    

Answer 3

累加器lambda中的a和b分别是流中的部分结果和下一个元素。由于您正在减少文章列表，因此部分结果是文章，并且您不能将Long添加到文章中。

所以在你的情况下，你可能想做这样的事情。

value = catalog.stream()
            .map(a -> a.getUnitPrice() * a.getUnitsInStore())
            .reduce(0L, Long::sum);