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