java常用代码

 

// ArrayList转int数组
int[] arr = list.stream().mapToInt(Integer::valueOf).toArray();

// ArrayList初始化
ArrayList<Type> obj = new ArrayList<Type>(Arrays.asList(Object o1, Object o2, Object o3, ....so on));
ArrayList<T> obj = new ArrayList<T>(Collections.nCopies(count,element));//把element复制count次填入A

// arrayCopy
public static native void arraycopy(Object src,  int  srcPos, Object dest, int destPos,int length);

 

//  对二维数组按照先按第一个值再按第二个值排序
        int[][] arr = new int[][] {new int[] {1, 4}, new int[] {4, 2}, new int[] {3, 7}, new int[] {1, 2}};
        Collections.sort(arr, Comparator.comparingInt(t -> ((int[]) t)[0]).thenComparingInt(t -> ((int[]) t)[1]));