[英]Initializing 2D array with streams in Java
I have the following class:我有以下课程:
public class MyClass {
//...
public MyClass(int x, int y) {
//...
}
}
Now, I need to initialize 2D array with the items:现在,我需要使用以下项目初始化二维数组:
int rows;
int cols;
//initializing rows and cols
MyClass[][] arr = new MyClass[rows][cols];
//how to initialize arr[x][y] with
//new MyClass(x, y) with streams API
I looked at this example, but it doesn't work in my case: Java 8 Stream and operation on arrays .我看了这个例子,但它在我的情况下不起作用: Java 8 Stream and operation on arrays 。 They use a single IntStream
.他们使用单个IntStream
。
Question: Of course I can use nested for loops, but I think it's now old-style and is considering bad.问题:当然我可以使用嵌套的 for 循环,但我认为它现在是旧式的并且正在考虑不好。 So how to apply streams api and initilize it in Java 8 way?那么如何应用流 api 并以Java 8 的方式初始化它呢?
Streams are not very good at keeping track of index, which you need here. Streams 不太擅长跟踪索引,而您在此处需要用到它。 So you can abuse them like @NicolasFilotto proposes, or in a simpler way:所以你可以像@NicolasFilotto 建议的那样滥用它们,或者以更简单的方式:
MyClass[][] array = new MyClass[rows][cols];
IntStream.range(0, rows)
.forEach(r -> IntStream.range(0, cols)
.forEach(c -> array[r][c] = new MyClass(r, c)));
You could even make it look more functional and get rid of the forEach
and the mutation part:你甚至可以让它看起来更实用,去掉forEach
和 mutation 部分:
MyClass[][] array = IntStream.range(0, rows)
.mapToObj(r -> IntStream.range(0, cols)
.mapToObj(c -> new MyClass(r, c))
.toArray(MyClass[]::new))
.toArray(MyClass[][]::new);
But honestly, for loops are not obsolete:但老实说, for 循环并没有过时:
for (int r = 0; r < rows; r++) {
for (int c = 0; c < cols; c++) {
array[r][c] = new MyClass(r, c);
}
}
Here is a way to do it:这是一种方法:
int rows = 5;
int cols = 10;
MyClass[][] arr = new MyClass[rows][cols];
Stream.generate(new Supplier<MyClass>() {
int currentValue = 0;
@Override
public MyClass get() {
MyClass myClass = new MyClass(currentValue / cols, currentValue % cols);
currentValue++;
return arr[myClass.x][myClass.y] = myClass;
}
}).limit(rows * cols).forEach(System.out::println);
Output:输出:
MyClass{x=0, y=0}
MyClass{x=0, y=1}
MyClass{x=0, y=2}
...
MyClass{x=4, y=9}
You can generate a 2D array using two nested IntStream
s as follows:您可以使用两个嵌套的IntStream
生成二维数组,如下所示:
public static void main(String[] args) {
int rows = 2, cols = 3;
// generate an array
MyClass[][] arr = IntStream.range(0, rows)
.mapToObj(x -> IntStream.range(0, cols)
.mapToObj(y -> new MyClass(x, y))
.toArray(MyClass[]::new))
.toArray(MyClass[][]::new);
// output
Arrays.stream(arr).map(Arrays::toString).forEach(System.out::println);
//[{x=0, y=0}, {x=0, y=1}, {x=0, y=2}]
//[{x=1, y=0}, {x=1, y=1}, {x=1, y=2}]
}
public static class MyClass {
int x, y;
public MyClass(int x, int y) {
this.x = x;
this.y = y;
}
@Override
public String toString() {
return "{x=" + x + ", y=" + y + "}";
}
}
Adaptation for Integer[][]
for solution: answered assylias :适应Integer[][]
解决方案:回答 assylias :
package com.gmail.jackkobec.java.core;
import java.util.Arrays;
import java.util.Random;
import java.util.stream.IntStream;
/**
* @Author Jack <jackkobec>
*/
public class InitMatrixByStreamApi {
public static final int ROWS_COUNT = 5; // Matrix rows number
public static final int COLUMN_COUNT = 2; // Matrix columns number
public static void main(String[] args) {
Integer[][] matrixFromStreamApi = initMatrixInStream(ROWS_COUNT, COLUMN_COUNT);
printMatrix(matrixFromStreamApi);
}
/**
* Init matrix by random int values with border 777
*
* @param rowsCount - Matrix rows number
* @param columnsCount - Matrix columns number
* @return Integer[][]
*/
public static Integer[][] initMatrixInStream(int rowsCount, int columnsCount) {
return IntStream.range(0, rowsCount)
.mapToObj(rowArray -> IntStream.range(0, columnsCount)
.mapToObj(columnArrayElement -> new Random().nextInt(777))
.toArray(Integer[]::new))
.toArray(Integer[][]::new);
}
/**
* Prints matrix.
*
* @param matrix
*/
public static void printMatrix(Integer[][] matrix) {
Arrays.stream(matrix)
.map(array -> Arrays.toString(array) + "\n")
.forEach(System.out::println);
}
}
Output:输出:
[395, 58]
[361, 652]
[291, 76]
[112, 77]
[186, 282]
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