03 JAVA 数组

一、数组定义

数组是多个相同类型数据集合（Python的列表支持不同类型的元素），我们可以对这些数据统一管理

- type varName[]

声明数组时不能指定其长度（就是先不指定数组中元素的个数，因为起先我们只是在栈中创建了varName的引用变量，而还未分配空间并给他赋值）varName是一个引用类型，指向堆中数据，数组可以被看成对象，数组中的每个元素相当于该对象的成员变量，数组元素可以是基本数据类型值或者引用类型存储reference

- 初始化

1. 动态初始化，先为数组分配空间（每个元素被隐式初始化，初始规则和类成员变量初始规则一样），之后为每个元素赋值

注意：方法返回值可以是数组引用

varName = new type[size]

varName[index] = value

2. 静态初始化，定义数组的时候就分配空间

例如: Person p[] = {new Person("Amy"), new Person("Jack"), new Person("Cindy")};

二、arrayName.length获取数组的长度，我们可以使用index来获取数组元素，index的范围是[0, arrayName.length-1]，下标可以是一个变量

三、args 一组字符串参数

java 程序名字 参数1，参数2，...

public class TestArgs {
public static void main(String[] args) {
int len = args.length;
if(len < 3) {
System.out.println("ARGUMENTS Format: \"NUM1\", \"OP\", \"NUM2\"");
System.exit(-1);
}
try {
double num1 = Double.parseDouble(args[0]);
double num2 = Double.parseDouble(args[2]);
double res = 0;
if(args[1].equals("+"))	res = num1 + num2;
else if(args[1].equals("-")) res = num1 - num2;
else if(args[1].equals("*")) res =  num1*num2;
else if(args[1].equals("/")) res = num1/num2;
else {
System.out.println("Error format");
System.exit(-1);
}
System.out.println(num1+args[1]+num2+"="+res);
}
catch(NumberFormatException e) {
System.out.println(e);
}
}
}

Additional:

- 对象的比较，成员一位一位的比较

- 将数据从栈-》堆时，我们需要将基本数据包装成对象，比如可以将double类型数据包装成Double类对象，成员变量值是基本类型值

java.lang下面包装类有Double, Integer, Float等

parseDouble(参数)，parseInt(参数)等都是静态方法

四、数组实例

1、数组排序

public class NumberSort {
public static void main(String args[]) {
NumberSort ns = new NumberSort();
int len = args.length;
int[] a = new int[len];

for(int i = 0; i < len; i++){
a[i]=Integer.parseInt(args[i]);
}
ns.print(ns.bubbleSort(a));
}

void print(int[] a) {
int len = a.length;
for(int i = 0; i < len; i++) {
System.out.print(a[i]+" ");
}
}

int[] selectionSort(int[] a) {
int tmp = 0, k = 0, len = a.length;
for(int i = 0; i < len-1; i++) {
k = i;
for(int j = i; j < len; j++) {
if(a[k] > a[j]) {
k = j;
}
}
if(k != i) {
tmp = a[i];
a[i] = a[k];
a[k] = tmp;
}
}
return a;
}

int[] bubbleSort(int[] a) {
int tmp = 0, len = a.length;
for(int i = len-1; i > 0; i--) {
for(int j = 0; j < i; j++) {
if(a[j] > a[j+1]) {
tmp = a[j];
a[j] = a[j+1];
a[j+1] = tmp;
}
}
}
return a;
}
}

2、如果有500个人手拉手，并且数三退一，最后留下的那个人是排在第几位？

import java.util.*;

public class ThreeforQuit {
public static void main(String[] args) {
Scanner ns =  new Scanner(System.in);
int len = ns.nextInt(), tmp = len, count = 0, idx = 0;
boolean[] node = new boolean[len];

for(int i = 0; i < len; i++) {
node[i] = true;
}

while(tmp > 1) {
if(node[idx]) {
count++;
if(count == 3) {
count = 0;
node[idx] = false;
tmp--;
}
}
idx++;
if(idx == len) idx = 0;
}

print(node);
}

public static void print(boolean[] node) {
for(int i = 0; i < node.length; i++) {
if(node[i]) {
System.out.println(i+1);
break;
}
}
}
}

class Node1 {
int id;
Node1 previous, next;
Node1(int id) {
this.id = id;
}
}
class LinkedCircle {
int cid = 1;
Node1 first, last;

LinkedCircle(int n) {
while (cid <= n) {
Node1 tmp =new Node1(cid);
if(cid == 1) {
first = last = tmp;
}
else {
last.next = tmp;
tmp.previous = last;
tmp.next = first;
first.previous = tmp;
last = tmp;
}
cid++;
}
}

void delete(Node1 node1) {
if(cid == 1) {
first = last = null;
}
else {
node1.previous.next = node1.next;
node1.next.previous = node1.previous;
if(node1 == first)
first = first.next;
else if(node1 == last)
last = last.previous;
}
cid--;
}
}

public class ThreeforQuit3 {
public static void main(String args[]) {
LinkedCircle lc = new LinkedCircle(500);
Node1 node1 = lc.first;
int countNum = 0;
while(lc.cid > 1) {
countNum++;
if(countNum == 3) {
countNum = 0;
lc.delete(node1);
}
node1 = node1.next;
}
System.out.println(lc.first.id);
}

}

3. 二分法查找

import java.util.*;
public class BinarySearch {
public static void main(String args[]) {
int[] a = new int[args.length];
for(int i = 0; i < args.length; i++) {
a[i] = Integer.parseInt(args[i]);
}

Scanner sn = new Scanner(System.in);
int obj = sn.nextInt();
sn.close();
System.out.print(binarySearch(a, obj));
}

static int binarySearch(int[] a, int obj) {
if (a.length == 0) return -1;

int head = 0, tail = a.length - 1;
int mid = (head + tail) / 2;

while(head <= tail) {
if(a[mid] == obj) {
//2，3 2是第一位，3是第二位
return mid + 1;
}
else if(a[mid] < obj) {
head = mid + 1;
}
else
tail = mid-1;
mid = (tail + head) / 2;
}
return -1;
}
}

4. 数组的复制和二维数组

class Dot {
int id;
Dot(int id) {
this.id = id;
}
}
public class testCopy {
static void copyInt() {
int[] a = {1, 2, 3};
int[] b = new int[a.length];
int[] c = new int[a.length];
for(int i = 0; i < a.length; i++) {
b[i] = a[i];
}
System.arraycopy(a, 0, c, 0, a.length);
a[1] = 9;
System.out.println("a[1]:"+a[1]+" b[1]:"+b[1]+" c[1]:"+c[1]);
}

static void copyHeapObj() {
Dot[] a = {new Dot(1), new Dot(2), new Dot(3)};
Dot[] b = new Dot[a.length];
Dot[] c = new Dot[a.length];
Dot[] d = new Dot[a.length];
for(int i = 0; i < a.length; i++) {
b[i] = a[i];
}

for(int i = 0; i < a.length; i++) {
d[i] = new Dot(a[i].id);
}

System.arraycopy(a, 0, c, 0, a.length);
a[1].id = 9;
System.out.println("a[1]:"+a[1].id+" b[1]:"+b[1].id+" c[1]:"+c[1].id+" d[1]:"+d[1].id);
}

static void copyString() {
String[] a = {"aa", "bb", "cc"};
String[] b = new String[a.length];
String[] c = new String[a.length];
for(int i = 0; i < a.length; i++) {
b[i] = a[i];
}
System.arraycopy(a, 0, c, 0, a.length);
a[1] = "dd";
System.out.println("a[1]:"+a[1]+" b[1]:"+b[1]+" c[1]:"+c[1]);
}

public static void main(String args[]) {
copyInt();
copyString();
copyHeapObj();
}
}

结果：
a[1]:9 b[1]:2 c[1]:2

a[1]:dd b[1]:bb c[1]:bb

a[1]:9 b[1]:9 c[1]:9 d[1]:2

public class TwoDimension {
public static void main(String args[]) {

int[][] intArray = {{1,2}, {1,2,3}, {3,4}};
int[][] int1 = new int[3][];
int[][] int2 = new int[3][];
int[][] int3 = new int[3][];

for(int i = 0; i < intArray.length; i++) {
int1[i] = new int[intArray[i].length];
for(int j = 0; j < intArray[i].length; j++) {
int1[i][j] = intArray[i][j];
}
}

for(int i = 0; i < intArray.length; i++) {
int3[i] = intArray[i];
}

System.arraycopy(intArray, 0, int2, 0, intArray.length);
intArray[2][1] = 100;
System.out.println(intArray[2][1]+","+int1[2][1]+","+int2[2][1]+","+int3[2][1]);
}
}

结果：
100,4,100,100

Reference:

1、马士兵JAVA基础视频