基本类型(例如:int)数组和ArrayList之间的转化

ArrayList转化为基本类型int数组！

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);
参数mapper是一个函数式接口！

ArrayList转化为包装类Interger数组！

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]);
        //对比public Object[] toArray(){...}　public <T> T[] toArray(T[] a){...}源码
        //推荐方式
        Integer[] integers3 = (Integer[]) arrayList.toArray();
    }
}

基本类型数组转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
",(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
",(end - begin)*1.0/cycle);
    }
}

比较结果:

从结果可以看出:方式1明显快于方式２！

包装类型Interger数组转ArrayList!

Arrays.asList()!