将原始数组值收集到集合中

Question

如何将原始值数组 int、long 或 double 转换为类型为 Map 的集合？

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class PrimitiveCollection {
    private static final String[] words = {
        "zero", "one", "two", "three", "four",
        "five", "six", "seven", "eight", "nine"
    };
    
    private static final int[] numbers = { 8, 6, 7, 5, 3, 0, 9 };

    public static Map<Integer, String> collect(int[] values) {
        return // Collect the array of values into a Map<Integer, String>
    }
    
    public static void main(String[] args) {
        Map<Integer, String> map = collect(numbers);
        System.out.println(map); // {0=zero, 3=three, 5=five, 6=six, 7=seven, 8=eight, 9=nine}
    }
}

Answer 1

为了将原始值数组转换为集合；这些值需要是 boxed 到相应的对象类型，或者需要调用原始流的 collect 方法。

拳击

以下是 8 种原始类型及其对应的包装类：

• bool→Boolean
• byte→Byte
• char→Character
• double→Double
• float → Float
• int→Integer
• long→Long
• short→Short

也可以看看: https://docs.oracle.com/javase/tutorial/java/data/autoboxing.html

在下面的代码中，int[] 被转换为 IntStream，然后装箱到 Stream<Integer> 中，并使用 Collectors.toMap 收集器进行收集。

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class PrimitiveCollection {
    private static final String[] words = {
        "zero", "one", "two", "three", "four",
        "five", "six", "seven", "eight", "nine"
    };

    private static final int[] numbers = { 8, 6, 7, 5, 3, 0, 9 };

    public static Map<Integer, String> collectBoxed(int[] values) {
        return Arrays.stream(values)
            .boxed()
            .collect(
                Collectors.toMap(
                    Function.identity(),
                    value -> words[value]));
    }

    public static void main(String[] args) {
        Map<Integer, String> boxedMap = collectBoxed(numbers);
        System.out.println(boxedMap);
    }
}

原始流

Arrays.stream 方法已被覆盖，仅接受double、int 和long 的数组。

• double[]→DoubleStream
• int[] → IntStream
• long[]→LongStream

在下面的代码中，直接在原始流对象上调用了collect方法。它需要三个参数：

• supplier - 在本例中为HashMap<Integer, String>
• accumulator - 如何将 iteree 应用于供应商
• combiner - 如何组合多个值

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class PrimitiveCollection {
    private static final String[] words = {
        "zero", "one", "two", "three", "four",
        "five", "six", "seven", "eight", "nine"
    };

    private static final int[] numbers = { 8, 6, 7, 5, 3, 0, 9 };

    public static Map<Integer, String> collectUnboxed(int[] values) {
        return Arrays.stream(values)
            .collect(
                HashMap::new,
                (acc, value) -> acc.put(value, words[value]),
                HashMap::putAll);
    }

    public static void main(String[] args) {
        Map<Integer, String> unboxedMap = collectUnboxed(numbers);
        System.out.println(unboxedMap);
    }
}