Java basics-day 4

I. Definition of arrays new operators generate container entities in memory 1. Concept: a set of data of the same type. In fact, an array is a container. 2. array benefits: the array elements can be automatically numbered from 0 to facilitate operations on these elements. 3. arrays are arrays and reference data types. The preceding format type is the data type in the container. 4. Format 1: element type [] array name = new element type [number of elements or array length]. Int [] arr = new int [5]; 5. Format 2: Element type [] array name = new element type [] {element, element, element} int [] arr = new int [] {3, 4, 5, 6} // static initialization int [] arr = {3, 4, 5, 6} int arr [] = {3, 4, 5, 6} // This is also true, but it is not recommended to write this statement 6, example (1), int [] x = new int [3]; int [] y = x; y [1] = 1; x [1] is also 1, reference type (2), int [] x = new int [3]; int [] y = new int [3]; y [1] = 1; then x [1] is still 0, two Memory Spaces (3), int [] x = new int [3]; System. out. print (x [5]); no error will be reported during compilation, because it is only compiled, but the memory space will be allocated during runtime. When x [5] is used, an exception jav will be reported. A. lang. when the ArrayIndexOutOfBoundsException array has an out-of-bounds exception operation, it accesses the non-existent badge (4), int [] x = new int [3]; x = null; // at this time, x is assigned null and no longer points to the array, not the array System. out. print (x [5]); java. lang. when the NullpointerException null pointer runs abnormally, a null pointer exception is reported: when the reference does not point to a null value, the reference is still used to operate the object. 7. An attribute in the array operation array can directly obtain the number of array elements :. length usage: array name. length (1), get the element int [] arr = new int [3]; arr [1]; (2) traverse int [] arr = new int [3]; for (int x = 0; x <3; x ++) {System. out. println (arr [x]);} (3), length traversal int [] arr = new int [3]; for (int x = 0; x <x. length; x ++) {System. out. println (arr [x]);} (4), direct print (array) will print the [1 @ hexadecimal [: indicates the array, 1: indicates the type ,@: followed by address (5), get the maximum value of public class Demo {public static void main (String args []) {int [] num = new Int [] {,}; int max = theFunction (num); System. out. println ("max number is:" max);} public static int theFunction (int [] num) {int max = num [0]; for (int x = 0; x <num. length; x ++) {max = max <num [x]? Num [x]: max;} return max;} (6), array selection sorting method public static void theFunction (int [] num) {int temp = 0; for (int x = 0; x <num. length-1; x ++) {for (int y = x + 1; y <num. length; y ++) {if (num [x]> num [y]) {temp = num [x]; num [x] = num [y]; num [y] = temp ;}}( 7), Bubble Sorting: compare two adjacent elements. If the conditions are met, the transposition is low // efficiency and the heap transposition is slow, hill sorting fast public static void theFunction (int [] num) {int temp = 0; for (int x = 0; x <num. length-1; x ++) {for (int y = 0; y <num. length-x-1; y ++) {If (num [y]> num [y + 1]) {temp = num [y + 1]; num [y + 1] = num [y]; num [y] = temp ;}}} (8), java comes with sorting import java. util. *; // introduce Arrays. sort (num); (9). Search for arrays // return the badge.-1 is not returned. The conventional half-lookup method is only applicable to ordered arrays (1) the first public static int halfSearch (int [] arr, int key) {int min, max, mid; min = 0; max = arr. length-1; mid = (min + max)/2; while (arr [mid]! = Key) {if (key> arr [mid]) {min = mid + 1;} else if (key <arr [mid]) {min = max-1 ;} if (min> max) {return-1;} mid = (min + max)/2;} return mid;} (2) public static int halfSearch (int [] arr, int key) {int min, max, mid; min = 0; max = arr. length-1; while (min <= max) {mid = (min + max)> 1; if (key> arr [mid]) {min = mid + 1 ;} else if (key <arr [mid]) {max = mid-1 ;}else {return mid ;}} return-1 ;}8. hexadecimal conversion (1), decimal --- "binary publi C static void toBin (int num) {StringBuffer sb = new StringBuffer (); while (num> 0) {sb. append (num % 2); num = num> 1;} System. out. print (sb. reverse ();} (2), decimal ----- hexadecimal public static void toBin (int num) {StringBuffer sb = new StringBuffer (); for (int x = 0; x <8; x ++) {int temp = num & 15; if (temp> 9) {sb. append (char) (temp-10 + 'A');} else {sb. append (temp);} num = num >>> 4;} System. out. print (sb. reverse ();} (3), conversion optimization, can Decimal to binary, decimal, octal public static void trans (int num, int base, int offset) {if (num = 0) {System. out. print ('0'); return;} char [] chs = {'0', '1', '2', '3', '4', '5 ', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E ', 'F'}; char [] arr = new char [32]; int pos = arr. length; while (num! = 0) {int temp = num & base; arr [-- pos] = chs [temp]; num = num >>> offset ;}for (int x = pos; x <arr. length; x ++) {System. out. print (arr [x]);} 8. Two-dimensional array (1). Format: int [] [] arr = new int [3] [2]; three two-dimensional arrays with the name of arr are defined. Each array has two elements. The names of one-dimensional arrays are given by arr [0] arr [1] arr [2] in the first one-dimensional array, the expression for assigning a value of 78 to the 1st position is as follows: arr [0] [1] = 78; (2), Format 2: int [] [] arr = new int [3] []; two-dimensional arrays have three one-dimensional arrays. Each one-dimensional array has the default initialization value null. You can initialize these three one-dimensional arrays respectively. arr [0] = new int [3]; arr [1] = new int [2]; arr [2] = new int [1]; (3), int [] [] arr = new int [3] [2]; 3 required, 2 not allowed to write (4), Format 3: int [] [] arr = {1, 2, 3}, {4}, {5}, {3, 4 },};