编写增量整数供应商

Question

我正在努力掌握 Java 8 函数式编程。我尝试编写以下内容 IntSupplier "functionally" 但我总是遇到问题。

import java.util.function.IntSupplier;

    @Test public void test_nonFunctional() {
        IntSupplier supplier = new IntSupplier() {
            private int nextInt = 0;
            @Override public int getAsInt() {
                return nextInt++;
            }
        };
    }

这是我的尝试。这些问题在代码中标记为注释。

import org.junit.Test;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.IntSupplier;

public class IntSupplierTest {
    @Test public void test_nonFunctional() {
        IntSupplier supplier = new IntSupplier() {
            private int nextInt = 0;
            @Override public int getAsInt() { return nextInt++; }
        }; // Works but is not functional.
    }

    @Test public void test_naive() {
        int nextInt = 0;
        IntSupplier supplier = () -> nextInt++; // Doesn't compile: requires nextInt to be final.
    }

    @Test public void test_nextIntIsFinal() {
        final int nextInt = 0;
        IntSupplier supplier = () -> nextInt++; // Doesn't compile: nextInt can't be incremented because it's final.
    }

    @Test public void test_useWrapper() {
        final AtomicInteger nextInt = new AtomicInteger(0);
        IntSupplier supplier = () -> nextInt.getAndIncrement(); // It is not the same as my original question as this test uses an extra object.
    }
}

如果答案只是不使用额外的对象就无法完成，请直说。

Answer 1

您对问题的定义已经不起作用。在功能中，没有参数就不能有不同的输出。这就是定义。但是如何创建数字序列，您可以在 java 库中看到：java.util.function.IntUnaryOperator。它是这样使用的：

IntStream.iterate(0, i -> i+1).limit(10).foreach(System.out::printLn);

Answer 2

有一种解决此问题的标准方法：

int[] nextInt = { 0 }; // optionally mark as final
IntSupplier supplier = () -> nextInt[0]++;

但它实际上并不比 "Works but is not functional" 解决方案更实用。

Answer 3

你可以这样做：

IntSupplier supplier = new AtomicInteger(0)::incrementAndGet;

编写增量整数供应商

Writing an incremental integer supplier

java

functional-programming