如何 Java Map Stream 多个条件从 int Array 到 String Array

Question

Hi guys can u help me i have a problem here,

i have list book:

1.Math Rp.15.000

2.Sains Rp.18.000

3.Geologi Rp.16.000

4.Biologi Rp.12.000

The user enters the book number he chooses, then he got the array

int[] arrayNumberBookTake = {1,2,4};

Then I want to change the value of the array to a new value and it depends on the price in the list

int[] listBooktoPrice= Arrays.stream(arrayNumberBookTake).map(i -> i == 1 ? 15000 : i == 2 ? 18000 : i == 3 ? 16000 : i == 4 ? 12000 : i).toArray();

it working and the output of array listBooktoPrice:

[15000, 18000, 12000]

now i want to make a new array it convert from arrayNumberBookTake to an String

String[] nameBookTake = Arrays.stream(arrayNumberBookTake).map(i -> i == 1 ? "Math" : i == 2 ? "Sains " : i == 3 ? "Geologi" : i == 4 ? "Biologi" : i).toArray();

but that code not work, i want array nameBookTake like this:

{"Math", "Sains", "Geologi"}

array nameBookTake should depends from the array arrayNumberBookTake and match with list book on top.

I hope u guys understand what i means, thx u guys:)

Answer 1

Your last conditional ends with the parameter i . This is an int , not a String . Therefor the array is not uniform; adding ints to an Array of String is not permitted.

Solution: delete the last conditional surrounding "Biologi" .

Furthermore, Java streams differ between primitives (like int ) and objects (like String ). To convert the IntStream into the Stream of String , one can use mapToObj , and collect with passing the constructor of the String[] to toArray() .

like so:

        String[] nameBookTake = Arrays.stream(arrayNumberBookTake)
                .mapToObj(i -> i == 1 ? "Math" : i == 2 ? "Sains " : i == 3 ? "Geologi" : "Biologi" )
                .toArray(String[]::new);

This answers your question.

However: as Andy Turner pointed out, using conditionals in this manner is a bad practice. Better practice is to use a switch, like so:

        String[] nameBookTake = Arrays.stream(arrayNumberBookTake)
                .mapToObj( i -> switch( i )
                        {
                            case 1 -> "Math";
                            case 2 -> "Sains";
                            case 3 -> "Geologi";
                            case 4 -> "Biologi";
                            default -> "Unknown book!";
                        } )
                .toArray(String[]::new);

Better still is to bundle the properties of a Book into its separate class, in this case an Enum , yielding a complete solution:

@Log4j2
public class Main
{
    public static void main( String[] args )
    {
        int[] arrayNumberBookTake = {1,2,4};
        int[] listBooktoPrice= Arrays.stream(arrayNumberBookTake)
                .map( i -> Books.values()[ i ].price )
                .toArray();

        String[] nameBookTake = Arrays.stream(arrayNumberBookTake)
                .mapToObj( i -> Books.values()[ i ].name  )
                .toArray(String[]::new);
    }
}

enum Books
{
    Math("Math", 15_000 ),
    Sains("Sains", 18_000 ),
    Geologi( "Geologi", 16_000 ),
    Biologi( "Biologi", 12_000 );

    String name;
    int price;

    Books( String name, int price )
    {
        this.name = name;
        this.price = price;
    }
}