基本類型(例如:int)數組和ArrayList之間的轉化

ArrayList轉化為基本類型int數組！

import java.util.ArrayList;
import java.util.Random;

public class Main {
    public static void main(String[] args) {
        ArrayList<Integer> arrayList = new ArrayList<>();
        Random random = new Random();
        int Len = 10000;
        for (int i = 0; i < Len; i++) {
            arrayList.add(random.nextInt());
        }
        int[] ints = arrayList.stream().mapToInt(value -> 2 * value).toArray();
　　　　　　
    }

}

IntStream mapToInt(ToIntFunction<? super T> mapper);
參數mapper是一個函數式接口！

ArrayList轉化為包裝類Interger數組！

import java.util.ArrayList;
import java.util.Random;

public class Main {

    public static void main(String[] args) {
        ArrayList<Integer> arrayList = new ArrayList<>();
        Random random = new Random();
        int Len = 10000;
        for (int i = 0; i < Len; i++) {
            arrayList.add(random.nextInt());
        }
        //方式一:public Object[] toArray(){...}
        Object[] objects0 = arrayList.toArray();//方式一
        //方式二:public <T> T[] toArray(T[] a){...}
        Integer[] integers1 = arrayList.toArray(new Integer[arrayList.size()]);
        Integer[] integers2 = arrayList.toArray(new Integer[0]);
        //對比public Object[] toArray(){...}　public <T> T[] toArray(T[] a){...}源碼
        //推薦方式
        Integer[] integers3 = (Integer[]) arrayList.toArray();
    }
}

基本類型數組轉ArrayList!

public class Main {

    public static void main(String[] args) {
        int[] array = new int[10000000];
        Random random = new Random();
        for (int  i = 0;  i < array.length;  i++) {
            array[i] = random.nextInt();
        }

        int cycle = 100;
        long  begin = 0;
        long  end   = 0;
　　　　　//方式１
        begin = System.currentTimeMillis();
        for (int i = 0; i < cycle; i++) {
            List<Integer> collect1 =new ArrayList<>(array.length);
            for (int k = 0; k < array.length; k++) {
                collect1.add(array[k]);
            }
            collect1 = null;
        }
        end = System.currentTimeMillis();
        System.out.printf("(end - begin)*1.0/cycle:%.6f\n",(end - begin)*1.0/cycle);
　　　　　//方式２　
        begin = System.currentTimeMillis();
        for (int i = 0; i < cycle; i++) {
            List<Integer> collect0 = Arrays.stream(array).boxed().collect(Collectors.toList());
            collect0 = null;
        }
        end = System.currentTimeMillis();
        System.out.printf("(end - begin)*1.0/cycle:%.6f\n",(end - begin)*1.0/cycle);
    }
}

比較結果:

從結果可以看出:方式1明顯快於方式２！

包裝類型Interger數組轉ArrayList!

Arrays.asList()!