第五章:数组
1.数组概述
数组可以看成是多个相同类型数据组合,对这些数据的统一管理。
数组变量属引用类型,数组也可以看成是对象,数组中的每个元素相当于该对象的成员变量。
数组中的元素可以是任何数据类型,包括基本数据类型和引用数据类型。
一维数组的声明方式:
type var[] ; 或 type [] var ;
例如:
int a1[] ; int [] s2;
double b[];
person [] p1;
Sting s1[];
Java语言中声明数组时不能指定其长度(数组中元素的个数),例如:
int a[5];//非法
2.数组对象的创建
Java中使用关键字new创建数组对象,格式:
数组名 = new 数组元素的类型 [数组元素的个数]
例如:
public class Test {
public static void main(String[] srgs){
int [] s;
s = new int[5];
for (int i = 0;i<5;i++){
s[i]=i;
System.out.print(" "+s[i]);
}
}
}
// 0 1 2 3 4
3.数组的初始化
动态初始化:
public class Test {
public static void main(String[] srgs){
int a [];
a = new int[3];
a[0] = 3; a[1] = 9; a[2] = 8;
Date days[];
days = new Date[3];
days[0] = new Date(1,4,2004);
days[1] = new Date(2,4,2004);
days[2] = new Date(3,4,2004);
}
}
class Date{
int day,month,year;
Date(int d,int m, int y) {
day = d;
month = m;
year = y;
}
}
静态初始化:
public class Test {
public static void main(String[] srgs){
int a [] = {3,9,8}
Date days[]={
new Date(1,4,2004),
new Date(2,4,2004),
new Date(3,4,2004)
};
}
4.数组元素的引用
每个数组都有一个属性length指明它的长度,例如:
a.length的值为数组a的长度(元素个数)。
例子打印出数组:
public class TestArray {
public static void main(String[] args) {
int[] a = {2, 4, 6, 7, 3, 5, 1, 9, 8}; //静态初始化
for(int i=0; i<a.length; i++) {
System.out.print(a[i] + " ");
}
}
}
1)数组排序法例子如下:
public class NumSort {
public static void main(String[] args) {
int[] a = {2,4,6,7,3,5,1,9,8};
print(a);
selectionSort(a);
print(a);
}
private static void selectionSort(int[] a) { //排序法
int k, temp;
for(int i=0; i<a.length; i++) {
k = i;
for(int j=k+1; j<a.length; j++) {
if(a[j] < a[k]) {
k = j;
}
}
if(k != i) {
temp = a[i];
a[i] = a[k];
a[k] = temp;
}
}
}
private static void print(int[] a) {
for(int i=0; i<a.length; i++) {
System.out.print(a[i] + " ");
}
System.out.println();
}
}
//2 4 6 7 3 5 1 9 8
//1 2 3 4 5 6 7 8 9
2)冒泡排序法
public class TestDateSort {
public static void main(String[] args) {
Date[] days = new Date[5];
days[0] = new Date(2006, 5, 4);
days[1] = new Date(2006, 7, 4);
days[2] = new Date(2008, 5, 4);
days[3] = new Date(2004, 5, 9);
days[4] = new Date(2004, 5, 4);
bubbleSort(days);
for(int i=0; i<days.length; i++) {
System.out.println(days[i]);
}
}
public static Date[] bubbleSort(Date[] a){ //冒泡排序法
int len = a.length;
for(int i = len-1;i>=1;i--){
for(int j = 0;j<=i-1;j++){
if(a[j].compare(a[j+1]) > 0){
Date temp = a[j];
a[j]=a[j+1];
a[j+1]=temp;
}
}
}
return a;
}
}
class Date {
int year, month, day;
Date(int y, int m, int d) {
year = y; month = m; day = d;
}
public int compare(Date date) {
return year > date.year ? 1
: year < date.year ? -1
: month > date.month ? 1
: month < date.month ? -1
: day > date.day ? 1
: day < date.day ? -1 : 0;
}
public String toString() {
return "Year:Month:Day -- " + year + "-" + month + "-" + day;
}
}
//Year:Month:Day -- 2004-5-4
//Year:Month:Day -- 2004-5-9
//Year:Month:Day -- 2006-5-4
//Year:Month:Day -- 2006-7-4
//Year:Month:Day -- 2008-5-4
3)查找法
public class TestSearch {
public static void main(String[] args) {
int a[] = { 1, 3, 6, 8, 9, 10, 12, 18, 20, 34 };
int i = 12;
System.out.println(search(a, i));
}
public static int search(int[] a, int num) {
for(int i=0; i<a.length; i++) {
if(a[i] == num) return i;
}
return -1;
}
}
//6
4)二分法查找
数组a中12的位置
public class TestSearch {
public static void main(String[] args) {
int a[] = { 1, 3, 6, 8, 9, 10, 12, 18, 20, 34 };
int i = 12;
System.out.println(binarySearch(a, i));
}
public static int binarySearch(int[]a, int num) {
if (a.length==0) return -1;
int startPos = 0;
int endPos = a.length-1;
int m = (startPos + endPos) / 2;
while(startPos <= endPos){
if(num == a[m]) return m;
if(num > a[m]) {
startPos = m + 1;
}
if(num < a[m]) {
endPos = m -1;
}
m = (startPos + endPos) / 2;
}
return -1;
}
}
//6下标值
5.二维数组
二维数组可以看成以数组为元素的数组。例如:
int a [] [] = {{1,2},{3,4,5,6},{7,8,9}};
java中多维数组的声明和初始化应按照从高维到低维的顺序进行。
静态初始化:
int a [] [] = {{1,2},{3,4,5,6},{7,8,9}};
动态初始化:
int a[][] = new int [3] [5];
6.数组拷贝
使用java.lang.system类的静态方法
例子:
public class TestArrayCopy {
public static void main(String args[]) {
String[] s =
{"Mircosoft","IBM","Sun","Oracle","Apple"};
String[] sBak = new String[6];
System.arraycopy(s,0,sBak,0,s.length); //从数组s中的第0个元素开始到s.length个元素拷贝到sBak中第0元素到s.length个位置
for(int i=0;i<sBak.length;i++){
System.out.print(sBak[i]+" ");
}
System.out.println();
int[][] intArray = {{1,2},{1,2,3},{3,4}};
int[][] intArrayBak = new int[3][];
System.arraycopy
(intArray,0,intArrayBak,0,intArray.length);
intArrayBak[2][1] = 100;
for(int i = 0;i<intArray.length;i++){
for(int j =0;j<intArray[i].length;j++){
System.out.print(intArray[i][j]+" ");
}
System.out.println();
}
}
}
//1 2
//1 2 3
//3 100
如果源数据数目超过目标数组边界会抛出
IndaxOutofBoundsException异常