Java basic knowledge points (array)

One, the array

1. Array assignment method:

1.String[] arr=new String[]; //[] write the length of the array in brackets

2.String[] arr2=new String[]{“a1”,”a2”,”a3”};

3.String[] arr3={“a1”,”a2”,”a3”,”a4”};

2. The definition of the array:

String[] arr;

The development of the array is in the stack space, and the addresses of the values ​​in the array are stored in the heap space

arr.length=The length of the array.

Traverse the array-enhanced for-original version:
for(int i=0;i<arr.length;i++){ int a=arr[i]; sout(a); }

Traverse the array-enhanced for-simplified version:
for(int a:arr){ sout(a); }

3. The sorting methods in the array are:

Bubble Sort

Select sort

Quick sort

4. Common methods of the Arrays class:

equals(); Compare whether the values ​​of the elements in the two arrays are exactly the same, and return true/false.
Usage: array name 1.equals (array name 2)

sort(); Sort the array in ascending order.
Usage: Arrays.sort (array name);

to String(); Convert an array to a string type and protect it with [].
Usage: Arrays.to String (array name)

fill(); Replace all elements in the array with a new value.
Usage: Arrays.fill (array name, element value);

copyOf(); Copy a copy
Usage: int[] newArr=Arrays.copyOf(array name, new array length);

binarySearch(); Find the subscript corresponding to the element, provided that the array is already in ascending order.
Usage: int i=Arrays.binarySearch (array name, element to be searched);

5. Array search method (premise: the array is already in ascending order):

Binary search method (recursive and loop two methods)

Two, two-dimensional array:

1. The definition of a two-dimensional array:

//Declaration-take the name
int[][] arr, arr2;

//Specify length-allocate space
arr=new int[3][];

//Integrate the above two steps-the length of the two-dimensional array must be specified, and the length of the one-dimensional array may not be specified first
int[][] arr3=new int[3][5];

2. The method of storing data in a two-dimensional array:

1. One element to
int[][] arr3=new int[1][2];
arr3[0][0]=1;
arr3[0][1]=2;

2. A one-dimensional array to
int[][] arr2=new int[3][];
arr2[0]=new int[2];
arr2[1]=new int[]{1,2,3} ;

2.
Int[][] arr3=new int[][]{ {1},{1,2},{1,2,3}};
int[][] arr4={ {1} {2}{1,2}};

3. Take out the value:

int[][] arr={ {1},{1,2},{1,2,3}};
//Traverse a two-dimensional array
for(int i=0;i<arr.length;i++){ / / Traverse one-dimensional array for(int j=0;j<arr[i].length;j++){ sout(arr[i][j]); } }