PL
Java - przekonwertuj List <Double> na double[] - boxed Double list na tablicę prymitywów podwójnych
9 points
W tym poście omówimy, jak konwertować List <Double> na double[] na kilka różnych sposobów. Najprostrzy sposób:
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List<Double> list = new ArrayList<>(Arrays.asList(1.2, 2.7, 3.9));2
double[] arr = list.stream().mapToDouble(Double::doubleValue).toArray();3
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System.out.println(Arrays.toString(arr)); // [1.2, 2.7, 3.9]Stream.mapToDouble z odniesieniem do metody.
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import java.util.ArrayList;2
import java.util.Arrays;3
import java.util.List;4
5
public class Example1 {6
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public static void main(String[] args) {8
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List<Double> list = new ArrayList<>(Arrays.asList(1.2, 2.7, 3.9));10
double[] arr = list.stream().mapToDouble(Double::doubleValue).toArray();11
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System.out.println(Arrays.toString(arr)); // [1.2, 2.7, 3.9]13
}14
}Wynik:
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[1.2, 2.7, 3.9]Stream.mapToDouble z funkcją tożsamości
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import java.util.ArrayList;2
import java.util.Arrays;3
import java.util.List;4
5
public class Example2 {6
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public static void main(String[] args) {8
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List<Double> list = new ArrayList<>(Arrays.asList(1.2, 2.7, 3.9));10
double[] arr = list.stream().mapToDouble(d -> d).toArray();11
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System.out.println(Arrays.toString(arr)); // [1.2, 2.7, 3.9]13
}14
}Wynik:
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[1.2, 2.7, 3.9]xxxxxxxxxx1
import java.util.ArrayList;2
import java.util.Arrays;3
import java.util.List;4
5
public class Example3 {6
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public static void main(String[] args) {8
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List<Double> list = new ArrayList<>(Arrays.asList(1.2, 2.7, 3.9));10
double[] arr = new double[list.size()];11
for (int i = 0; i < arr.length; i++) {12
arr[i] = list.get(i).doubleValue();13
}14
System.out.println(Arrays.toString(arr)); // [1.2, 2.7, 3.9]15
}16
}Wynik:
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[1.2, 2.7, 3.9]xxxxxxxxxx1
import com.google.common.primitives.Doubles;2
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import java.util.ArrayList;4
import java.util.Arrays;5
import java.util.List;6
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public class Example4 {8
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public static void main(String[] args) {10
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// With Guava12
List<Double> list = new ArrayList<>(Arrays.asList(1.2, 2.7, 3.9));13
double[] arr = Doubles.toArray(list);14
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System.out.println(Arrays.toString(arr)); // [1.2, 2.7, 3.9]16
}17
}Wynik:
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[1.2, 2.7, 3.9]Zależność Maven - wersja Guava 28.0.0
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<!-- https://mvnrepository.com/artifact/com.google.guava/guava -->2
<dependency>3
<groupId>com.google.guava</groupId>4
<artifactId>guava</artifactId>5
<version>28.0-jre</version>6
</dependency>7
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import org.apache.commons.lang3.ArrayUtils;2
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import java.util.ArrayList;4
import java.util.Arrays;5
import java.util.List;6
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public class Example5 {8
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public static void main(String[] args) {10
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List<Double> list = new ArrayList<>(Arrays.asList(1.2, 2.7, 3.9));12
Double[] arrBoxed = list.toArray(new Double[list.size()]);13
double[] arr = ArrayUtils.toPrimitive(arrBoxed);14
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System.out.println(Arrays.toString(arr)); // [1.2, 2.7, 3.9]16
}17
}Wynik:
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[1.2, 2.7, 3.9]Zależność Maven - Apache commons lang3 w wersji 3.8.1
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<!-- https://mvnrepository.com/artifact/org.apache.commons/commons-lang3 -->2
<dependency>3
<groupId>org.apache.commons</groupId>4
<artifactId>commons-lang3</artifactId>5
<version>3.8.1</version>6
</dependency>