空指针异常总结

           空指针异常：NullPointerException
	//情况一：
	int[] arr1 = new int[]{1,2,3};
	arr1 = null;
	System.out.println(arr1[0]);
	
	//情况二：
	int[][] arr2 = new int[4][];
	System.out.println(arr2[0][0]);
	
	//情况三：
	String[] arr3 = new String[]{"AA","BB","CC"};
	arr3[0] = null;
	System.out.println(arr3[0].toString());