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niumooniumoo
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code: add java arrays demo
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‎core-java-modules/core-java-collect/src/main/java/com/wdbyte/collection/EnumMapTest.java‎

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package com.wdbyte.collection;
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import java.util.EnumMap;
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/**
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* @author niulang
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* @date 2023/10/20
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*/
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public class EnumMapTest {
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public static void main(String[] args) {
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EnumMap<Week, String> enumMap = new EnumMap(Week.class);
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enumMap.put(Week.a,null);
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System.out.println(enumMap.get(Week.a));
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}
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}
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enum Week {
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a,
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b,
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c,
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d,
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e,
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f,
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g
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}

‎core-java-modules/core-java-collect/src/main/java/com/wdbyte/collection/JavaArrays.java‎

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package com.wdbyte.collection;
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import java.util.Arrays;
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import java.util.List;
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import java.util.Random;
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import java.util.function.IntFunction;
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import java.util.function.ToIntFunction;
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import org.junit.jupiter.api.Test;
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/**
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* @author niulang
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* @date 2024/03/04
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*/
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public class JavaArrays {
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@Test
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public void print() {
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String[] arr = new String[] {"a", "b", "c", "d"};
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System.out.println(Arrays.toString(arr));
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}
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@Test
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public void init() {
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String[] arr = new String[] {"a", "b", "c", "d"};
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String[] copyOf2 = Arrays.copyOf(arr, 2);
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String[] copyOfRange = Arrays.copyOfRange(arr, 1, 3);
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System.out.println(Arrays.toString(arr));
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System.out.println(Arrays.toString(copyOf2));
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System.out.println(Arrays.toString(copyOfRange));
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// 目标大小大于原始大小，则copyOf 会用null填充数组 。
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String[] copyOf10 = Arrays.copyOf(arr, 10);
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System.out.println(Arrays.toString(copyOf10));
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}
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@Test
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public void fill() {
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String[] arr = new String[5];
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Arrays.fill(arr, "java");
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System.out.println(Arrays.toString(arr));
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// 生成 100以内的 随机数
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IntFunction<Integer> intFunction = i -> new Random().nextInt(100);
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Integer[] intArr = new Integer[5];
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Arrays.setAll(intArr, intFunction);
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System.out.println(Arrays.toString(intArr));
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}
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@Test
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public void diff() {
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String[] arr = new String[] {"a", "b", "c", "d"};
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Object[] arr1 = new Object[] {arr, new String[] {"a", "b", "c", "d"}};
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Object[] arr2 = new Object[] {arr, arr};
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System.out.println(Arrays.equals(arr1, arr2));
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System.out.println(Arrays.deepEquals(arr1, arr2));
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// hashCode
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System.out.println(Arrays.hashCode(arr2));
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System.out.println(Arrays.deepHashCode(arr2));
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arr[0] = null;
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System.out.println(Arrays.hashCode(arr2));
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System.out.println(Arrays.deepHashCode(arr2));
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}
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@Test
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public void sort() {
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// 生成 100以内的 随机数
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IntFunction<Integer> intFunction = i -> new Random().nextInt(100);
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Integer[] intArr = new Integer[5];
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Arrays.setAll(intArr, intFunction);
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System.out.println(Arrays.toString(intArr));
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Arrays.sort(intArr);
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System.out.println(Arrays.toString(intArr));
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//long cost = 0;
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//for (int ii = 0; ii < 10; ii++) {
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// IntFunction<Integer> intFunction = i -> new Random().nextInt(100000);
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// Integer[] intArr = new Integer[100000];
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// Arrays.setAll(intArr, intFunction);
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//
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// long start = System.currentTimeMillis();
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// Arrays.sort(intArr);
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// long end = System.currentTimeMillis();
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// cost = cost + (end - start);
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//}
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//System.out.println(cost / 10);
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//
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//cost = 0;
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//for (int ii = 0; ii < 10; ii++) {
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// IntFunction<Integer> intFunction = i -> new Random().nextInt(100000);
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// Integer[] intArr = new Integer[100000];
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// Arrays.setAll(intArr, intFunction);
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//
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// long start = System.currentTimeMillis();
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// Arrays.parallelSort(intArr);
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// long end = System.currentTimeMillis();
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// cost = cost + (end - start);
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//}
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//System.out.println(cost / 10);
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}
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@Test
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public void search() {
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Integer[] intArr = new Integer[] {2, 3, 4, 5, 6, 7, 8, 9};
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int index = Arrays.binarySearch(intArr, 3);
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System.out.println("index:"+index);
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System.out.println(intArr[index]);
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}
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@Test
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public void stream() {
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Integer[] intArr = new Integer[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
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System.out.println(Arrays.stream(intArr).count());
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ToIntFunction toIntFunction = i -> (int)i;
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System.out.println(Arrays.stream(intArr).mapToInt(toIntFunction).sum());
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}
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@Test
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public void cast() {
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String[] arr = new String[] {"a", "b", "c", "d"};
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Object[] arr2 = new Object[] {arr, arr};
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System.out.println(Arrays.toString(arr));
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System.out.println(Arrays.toString(arr2));
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System.out.println(Arrays.deepToString(arr));
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System.out.println(Arrays.deepToString(arr2));
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List<String> list = Arrays.asList(arr);
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System.out.println(list);
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System.out.println(list.getClass());
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// list.add("e"); 报错
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}
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@Test
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public void prefix() {
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Integer[] intArr = new Integer[] {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
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Arrays.parallelPrefix(intArr, (left, right) -> left + right);
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System.out.println(Arrays.toString(intArr));
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}
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}

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