继 Java基础知识的全面巩固_note1(附各种demo code)
拜读《核心技术卷》,笔记之。提纲
1.1 for each循环
1.2 数组初始化以及匿名数组
1.3 数组拷贝(以及Arrays.copyOf())
1.4 命令行参数
1.5 数组排序
1.6 多维数组
1.7 不规则数组
1.数组
声明:
以及:
1.1 for each循环
for(variable:collection) statement
例如:
for(int element : a)
System.out.println(element);
打印数组a的每一个元素,一个元素占一行。
另外:
System.out.println(Arrays.toString(a));
1.2 数组初始化以及匿名数组
!!注意使用这种语句时,不需要调用new。
匿名数组:
在Java中,允许数组长度为0。可用于方法的返回结果为空数组的情况:
1.3 数组拷贝
1)
如图所示显示了拷贝的结果:
2)
如果希望将一个数组的所有值拷贝到一个新的数组中去,就要使用Arrays类的copyOf方法:
4)
下面上一个Demo:
package Test;
//import java.lang.Math;
import java.util.Arrays;
//import java.util.Date;
//import java.util.Scanner;
public class Havaatry {
public static void main(String[] args)
{
int[] a = {3,2,5,6,9,8,4,0};
int[] b = Arrays.copyOf(a, 16);
int[] c = Arrays.copyOf(a, 3);
boolean[] A = {true,true,true,true,true};
boolean[] B = Arrays.copyOf(A, 16);
boolean[] C = Arrays.copyOf(A, 3);
System.out.println(Arrays.toString(a));
System.out.println(Arrays.toString(b));
System.out.println(Arrays.toString(c));
System.out.println(Arrays.toString(A));
System.out.println(Arrays.toString(B));
System.out.println(Arrays.toString(C));
}
}
运行结果:
[3, 2, 5, 6, 9, 8, 4, 0]
[3, 2, 5, 6, 9, 8, 4, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[3, 2, 5]
[true, true, true, true, true]
[true, true, true, true, true, false, false, false, false, false, false, false, false, false, false, false]
[true, true, true]
1.4 命令行参数
1.5 数组排序
下面上一个Demo,它产生一个抽奖游戏中的随机数值组合。假如抽奖是从49个数值中抽取6个,那么程序可能的输出结果为:
现在,就可以开始抽取k个数值了。Math.random方法将返回一个
[0,1)
之间的随机浮点数。用n乘以这个浮点数,就可以得到 [0,n)
之间的一个随机数,用(int)
强制转换类型,则是一个 [0,n-1]
的随机数。Demo code:
package Test;
//import java.lang.Math;
import java.util.Arrays;
//import java.util.Date;
import java.util.Scanner;
public class Havaatry {
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.print("How many numbers do you need to draw? ");
int k = in.nextInt();
System.out.print("What is the highest number you can draw? ");
int n = in.nextInt();
// fill an array with numbers 1 2 3 . . . n
int[] numbers = new int[n];
for (int i = 0; i < numbers.length; i++)
numbers[i] = i + 1;
// draw k numbers and put them into a second array
int[] result = new int[k];
for (int i = 0; i < result.length; i++)
{
// make a random index between 0 and n - 1
int r = (int) (Math.random() * n);
// pick the element at the random location
result[i] = numbers[r];
// move the last element into the random location
numbers[r] = numbers[n - 1];
n--;
}
// print the sorted array
Arrays.sort(result);
System.out.println("Bet the following combination. It'll make you rich!");
for (int r : result)
System.out.println(r);
}
}
结果:
How many numbers do you need to draw? 5
What is the highest number you can draw? 12
Bet the following combination. It'll make you rich!
2
6
7
8
9
关键API:
1.6 多维数组
下面上完整代码:
package Test;
//import java.lang.Math;
import java.util.Arrays;
//import java.util.Date;
import java.util.Scanner;
public class Havaatry {
public static void main(String[] args)
{
final double STARTRATE = 10;
final int NRATES = 6;
final int NYEARS = 10;
// set interest rates to 10 . . . 15%
double[] interestRate = new double[NRATES];
for (int j = 0; j < interestRate.length; j++)
interestRate[j] = (STARTRATE + j) / 100.0;
double[][] balances = new double[NYEARS][NRATES];
// set initial balances to 10000
for (int j = 0; j < balances[0].length; j++)
balances[0][j] = 10000;
// compute interest for future years
for (int i = 1; i < balances.length; i++)
{
for (int j = 0; j < balances[i].length; j++)
{
// get last year's balances from previous row
double oldBalance = balances[i - 1][j];
// compute interest
double interest = oldBalance * interestRate[j];
// compute this year's balances
balances[i][j] = oldBalance + interest;
}
}
// print one row of interest rates
for (int j = 0; j < interestRate.length; j++)
System.out.printf("%9.0f%%", 100 * interestRate[j]);
System.out.println();
// print balance table
for (double[] row : balances)
{
// print table row
for (double b : row)
System.out.printf("%10.2f", b);
System.out.println();
}
}
}
运行结果:
另外:关于for each 以及 deepToString()
1.7 不规则数组
1)
Java实际上没有多维数组,只有一位数组:
例如,在前面的实例中,balances数组实际上是一个包含10个元素的数组,而每个元素又是一个由6个浮点数组成的数组。如图:
2)
3)由于可以单独地存取数组的某一行,所以可以让两行交换:
4)
也即i中取j的组合算法,这个三角形也即杨辉三角。
编程思路:
代码:
package Test;
//import java.lang.Math;
import java.util.Arrays;
//import java.util.Date;
import java.util.Scanner;
public class Havaatry {
public static void main(String[] args)
{
final int NMAX = 10;
// allocate triangular array
int[][] odds = new int[NMAX + 1][];
for (int n = 0; n <= NMAX; n++)
odds[n] = new int[n + 1];
// fill triangular array
for (int n = 0; n < odds.length; n++)
for (int k = 0; k < odds[n].length; k++)
{
/*
* compute binomial coefficient n*(n-1)*(n-2)*...*(n-k+1)/(1*2*3*...*k)
*/
int lotteryOdds = 1;
for (int i = 1; i <= k; i++)
lotteryOdds = lotteryOdds * (n - i + 1) / i;
odds[n][k] = lotteryOdds;
}
// print triangular array
for (int[] row : odds)
{
for (int odd : row)
System.out.printf("%4d", odd);
System.out.println();
}
}
}
运行结果:
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1
1 6 15 20 15 6 1
1 7 21 35 35 21 7 1
1 8 28 56 70 56 28 8 1
1 9 36 84 126 126 84 36 9 1
1 10 45 120 210 252 210 120 45 10 1
杨辉三角组合律:
另外:
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