Java自学笔记02—Java基础（下） - 黑马程序员技术交流社区

import java.util.*;
public class HelloJava
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.print("How much money do you need to retire? ");
double goal = in.nextDouble();
System.out.print("How much money will you contribute every year? ");
double payment = in.nextDouble();
System.out.print("Interest rate in %: ");
double interestRate = in.nextDouble();
double balance = 0;
int years = 0;
while (balance < goal)
{
balance += payment;
double interest = balance * interestRate / 100;
balance += interest;
years++;
}
System.out.println("You can retire in " + years + " years.");
}
}

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do
statement
while (condition);

import java.util.*;
public class HelloJava
{
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.print("How much money will you contribute every year? ");
double payment = in.nextDouble();
System.out.print("Interest rate in %: ");
double interestRate = in.nextDouble();
double balance = 0;
int year = 0;
String input;
do
{
balance += payment;
double interest = balance * interestRate / 100;
balance += interest;
year++;
System.out.printf("After year %d, your balance is %,.2f\n", year, balance);
System.out.print("Ready to retire? (Y/N) ");
input = in.next();
} while (input.equals("N"));
}
}

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for语句的第一部分通常用于对计数器初始化；
第二部分给出每次新一轮循环执行前要检测的循环条件；
第三部分指示如何更新计数器。

import java.util.*;
public class HelloJava
{
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();
int lotterOdds = 1;
for (int i = 1; i <= k; i++)
lotterOdds = lotterOdds * (n - i + 1) / i;
System.out.println("Your odds are 1 in " + lotterOdds + ". Good luck!");
}
}

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多重选择switch语句将从与选项值相匹配的case标签处开始执行直到遇到break语句，或者执行到switch语句的结束处为止。

如果没有相匹配的case标签，而有default子句，就执行这个子句。
中断控制流程语句break语句用于跳出语句块。
continue语句将控制转移到循环首部。
大数值BigInteger类和BigDecimal类用于处理包含任意长度数字序列的数值。
使用valueOf方法将普通数值转换为大数值。使用add和multiply方法进行加法和乘法运算。

import java.util.*;
import java.math.*;
public class HelloJava
{
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();
BigInteger lotteryOdds = BigInteger.valueOf(1);
for (int i = 1; i <= k; i++)
lotteryOdds = lotteryOdds.multiply(BigInteger.valueOf(n - i + 1).divide(BigInteger.valueOf(i)));
System.out.println("Your odds are 1 in " + lotteryOdds + ". Good luck!");
}
}

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数组
数组一旦创建，就不能再改变大小。如果需要在运行时扩展数组，应该使用ArrayList。
for each循环用来处理元素集合中的每个元素，格式为：for (variable : collection) statement
声明并初始化数组可以有两种形式：int[] a = new int[] { 0, 1, 2 }或int[] a = { 0, 1, 2 }。
打印数组可以用toString方法，拷贝数组可以用copyOf方法。
对数值型数组进行排序，可以使用sort方法。

import java.util.*;
public class HelloJava
{
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();
int[] numbers = new int[n];
for (int i = 0; i < numbers.length; i++)
numbers[i] = i + 1;
int[] result = new int[k];
for (int i = 0; i < result.length; i++)
{
int r = (int) (Math.random() * n);
result[i] = numbers[r];
numbers[r] = numbers[n - 1];
n--;
}
Arrays.sort(result);
System.out.println("Bet the following combination. It'll make you rich!");
for (int r : result)
System.out.println(r);
}
}

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可以把二维数组看成矩阵，以行列的角度索引二维数组中的元素。

public class HelloJava
{
public static void main(String[] args)
{
final double STARTRATE = 10;
final int NRATES = 6;
final int NYEARS = 10;
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];
for (int j = 0; j < balances[0].length; j++)
balances[0][j] = 10000;
for (int i = 1; i < balances.length; i++)
{
for (int j = 0; j < balances[i].length; j++)
{
double oldBalance = balances[i - 1][j];
double interest = oldBalance * interestRate[j];
balances[i][j] = oldBalance + interest;
}
}
for (int j = 0; j < interestRate.length; j++)
System.out.printf("%9.0f%%", 100 * interestRate[j]);
System.out.println();
for (double[] row : balances)
{
for (double b : row)
System.out.printf("%10.2f", b);
System.out.println();
}
}
}

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二维数组每一行的长度可以不同。

public class HelloJava
{
public static void main(String[] args)
{
final int NMAX = 10;
int[][] odds = new int[NMAX + 1][];
for (int n = 0; n <= NMAX; n++)
odds[n] = new int[n + 1];
for (int n = 0; n < odds.length; n++)
for (int k = 0; k < odds[n].length; k++)
{
int lotteryOdds = 1;
for (int i = 1; i <= k; i++)
lotteryOdds = lotteryOdds * (n - i + 1) / i;
odds[n][k] = lotteryOdds;
}
for (int[] row : odds)
{
for (int odd : row)
System.out.printf("%4d", odd);
System.out.println();
}
}
}

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