日撸 Java 三百行day1-10
文章目录
-
- 说明
- day1 环境搭建
-
- 1.1 开发环境
- 1.2 package import 和 println
- 1.3 编写HelloWorld.java
- day2 基本算术操作
-
- 2.1 加、减、乘、除、整除、取余.
- day3 基本if 语句
-
- 3.1 if条件分支语句
- 3.2 代码
- day4 闰年的计算
-
- 4.1 思路整理：何为闰年？
- 4.2 核心代码
- day5 基本switch 语句
-
- 5.1 switch也属于条件分支语句
- 5.2 思考
- day6 基本for 语句
-
- 6.1 for语句中表达式的执行顺序
- 6.2 代码
- day7 矩阵元素相加
-
- 7.1 题目解读
- 7.2 二维数组中
- day8 矩阵相乘
-
- 8.1 题目解读
- 8.2代码
- day9 while 语句
-
- 代码
- day10 综合任务 1
-
- 10.1 题目解读
- 10.2 代码
- 自我小结
-
- 三大结构
说明
闵老师的文章链接： 日撸 Java 三百行（总述）_minfanphd的博客-CSDN博客
自己也把手敲的代码放在了github上维护：https://github.com/fulisha-ok/sampledata
day1 环境搭建
1.1 开发环境
在搭建java环境时，从官网下载了java安装包安装成功后，一定要正确配置环境变量
1.2 package import 和 println
（1）package包：就好似我们日常生活中的”收纳盒“，不同的”收纳盒“装不同的”物品“，方便我们查找和定位。在大”收纳盒“中有包含各种小”收纳盒“，这也体现了包是以树形结构存储的。
package com //一个主包
package com.project //主包下建一个项目工程包
package com.project.util //一个工具包，util工具包在com目录下的project目录中
（2）import：导入包成员，在写一个java类时，我们需用到其他包中的成员，此时就需要通过import导入相应的包，则就好似c语言需要导入头文件，才能用一些库里的函数
import java.util.ArrayList;; //导入Java util包下的ArrayList 那则可以用这个类中一些方法，变量
import java.util.*; //导入util包下所有的东西，那我们使用的范围就比上面这个更多
（3）println：打印输出语句，且在输出后会自动换行，若不换行则是print
1.3 编写HelloWorld.java
main函数是程序入口
package basic;
public class HellowWorld {
public static void main(String[] args) {
System.out.println("Hello, World");
}
}
day2 基本算术操作
2.1 加、减、乘、除、整除、取余.
package basic;
public class BasicOperations {
public static void main(String args[]) {
int tempFirstInt, tempSecondInt, tempResultInt;
double tempFirstDouble, tempSecondDouble, tempResultDouble;
tempFirstInt = 15;
tempSecondInt = 4;
tempFirstDouble = 1.2;
tempSecondDouble = 3.5;
//Addition
tempResultInt = tempFirstInt + tempSecondInt;
tempResultDouble = tempFirstDouble + tempSecondDouble;
System.out.println("" + tempFirstInt + " + " + tempSecondInt + " = " + tempResultInt);
System.out.println("" + tempFirstDouble + " + " + tempSecondDouble + " = " + tempResultDouble);
//Subtraction
tempResultInt = tempFirstInt - tempSecondInt;
tempResultDouble = tempFirstDouble - tempSecondDouble;
System.out.println("" + tempFirstInt + " - " + tempSecondInt + " = " + tempResultInt);
System.out.println("" + tempFirstDouble + " - " + tempSecondDouble + " = " + tempResultDouble);
//Multiplication
tempResultInt = tempFirstInt * tempSecondInt;
tempResultDouble = tempFirstDouble * tempSecondDouble;
System.out.println("" + tempFirstInt + " * " + tempSecondInt + " = " + tempResultInt);
System.out.println("" + tempFirstDouble + " * " + tempSecondDouble + " = " + tempResultDouble);
//Division
tempResultInt = tempFirstInt / tempSecondInt;
tempResultDouble = tempFirstDouble / tempSecondDouble;
System.out.println("" + tempFirstInt + " / " + tempSecondInt + " = " + tempResultInt);
System.out.println("" + tempFirstDouble + " / " + tempSecondDouble + " = " + tempResultDouble);
//Modulus
tempResultInt = tempFirstInt % tempSecondInt;
System.out.println("" + tempFirstInt + " % " + tempSecondInt + " = " + tempResultInt);
}
}
day3 基本if 语句
3.1 if条件分支语句
其中，if中的表达式应该为布尔表达式。这里会存在三种不同选择。（假设在if中会有数据的处理）
第一，只使用if语句，这相当于我只过滤我想要的数据；
第二：if…else 语句，(不入if就进else， 非真即假)
第三：if…else if…else 语句,这就是多条件分支判断。对不同条件进行判断
3.2 代码
package basic;
public class IfStatement {
/**
* The entrance of the program
* @param args
*/
public static void main(String args[]) {
int tempNumber1, tempNumber2;
// Try a positive value
tempNumber1 = 5;
if (tempNumber1 >= 0) {
tempNumber2 = tempNumber1;
} else {
tempNumber2 = -tempNumber1;
} // Of if
System.out.println("The absolute value of " + tempNumber1 + " is " + tempNumber2);
// Try a negative value
// Lines 27 through 33 are the same as Lines 15 through 19
tempNumber1 = -3;
if (tempNumber1 >= 0) {
tempNumber2 = tempNumber1;
} else {
tempNumber2 = -tempNumber1;
} // Of if
System.out.println("The absolute value of " + tempNumber1 + " is " + tempNumber2);
// Now we use a method/function for this purpose.
tempNumber1 = 6;
System.out.println("The absolute value of " + tempNumber1 + " is " + abs(tempNumber1));
tempNumber1 = -8;
System.out.println("The absolute value of " + tempNumber1 + " is " + abs(tempNumber1));
}
/**
* @param paraValue The given value.
* @return The absolute value of the given parameter.
*/
public static int abs(int paraValue) {
if (paraValue >= 0) {
return paraValue;
} else {
return -paraValue;
}
}
}
day4 闰年的计算
4.1 思路整理：何为闰年？
（1）非世纪年（不能被100整除的年份）能被4整除且不能被100整除的年份为闰年
（2）世纪年 能被400整除的才是闰年
文中给出两种计算闰年的方法，结合day3的if语句，第一种把所有逻辑判断放在一个if中完成，会使用一些与或非逻辑运算，如果逻辑判断条件过多，如果放在一个if判断中则会增加代码的可维护性，但第二种方法是一个条件一个条件判断，如果其中有一个不满足就退出，这样是根据if-else if顺序执行，来判断只有有不符合条件就退出、
4.2 核心代码
/**
* @param paraYear
* @return Is the given year leap? true or false;
*/
public static boolean isLeapYear(int paraYear) {
if ((paraYear % 4 == 0) && (paraYear % 100 != 0) || (paraYear % 400 == 0)) {
return true;
} else {
return false;
}
}
/**
* @param paraYear
* @return Is the given year leap? Replace the complex condition with a number of if. return true or false
*/
public static boolean isLeapYearV2(int paraYear) {
if (paraYear % 4 != 0) {
return false;
} else if (paraYear % 400 == 0) {
return true;
} else if (paraYear % 100 == 0) {
return false;
} else {
return true;
}
}
day5 基本switch 语句
5.1 switch也属于条件分支语句
switch中表达式的值去和case后的值做匹配，若匹配正确则执行其后需要执行代码，遇到break结束执行。若没有case匹配，则最后就会执行default，default 分支不需要 break 语句
5.2 思考
（1）每一个case后都要跟break吗？
答案是否定的。不加break的话将会跳转到相应的case去执行且其以下的所有语句。
（2）switch和if条件语句有什么区别呢？
最明显差异是表示执行的结构，if中的表达式结果只能是boolean类型，而switch恰恰相反，他表示式结果可以是int，char等。我在实际使用过程中if语句用的比较多，但涉及到判断的if分支较多时，我会考率使用switch,这样效率会高一点
day6 基本for 语句
6.1 for语句中表达式的执行顺序
for(a;b;c)其中a,b,c为表达式，执行顺序：先执行a表达式,一般为初始化语句，再执行b表达式，一般式判断表达式，若为ture去执行循环体，执行完再执行c表达式，若不满足b表达式，则跳出循环。
6.2 代码
package basic;
public class ForStatement {
/**
* The entrance of the program.
* @param args
*/
public static void main(String[] args) {
forStatementTest();
}
/**
* Method unit test.
*/
public static void forStatementTest(){
int tempN = 0;
System.out.println("1 add to " + tempN + " is: " + addToN(tempN));
tempN = 0;
System.out.println("1 add to " + tempN + " is: " + addToN(tempN));
int tempStepLength = 1;
tempN = 10;
System.out.println("1 add to " + tempN + " with step length " + tempStepLength + " is: "
+ addToNWithStepLength(tempN, tempStepLength));
tempStepLength = 2;
System.out.println("1 add to " + tempN + " with step length " + tempStepLength + " is: "
+ addToNWithStepLength(tempN, tempStepLength));
}
/**
* Add from 1 to N.
* @param paraN The given upper bound.
* @return The sum.
*/
public static int addToN(int paraN) {
int resultSum = 0;
for (int i = 1; i <= paraN; i++) {
resultSum += i;
}
return resultSum;
}
/**
* Add from 1 to N with a step length.
* @param paraN The given upper bound.
* @param paraStepLength paraStepLength The given step length.
* @return The sum.
*/
public static int addToNWithStepLength(int paraN, int paraStepLength) {
int resultSum = 0;
for (int i = 1; i <= paraN; i += paraStepLength) {
resultSum += i;
}
return resultSum;
}
}
day7 矩阵元素相加
7.1 题目解读
矩阵用二维数组存储，计算二维数组的和，计算两个二维数组对应行列相加组成一个新的二维数组，都需要用到for循环遍历(行优先)；对矩阵的赋值也需要循环遍历赋初值，在有循环时要避免死循环，确保循环是有限性的。
7.2 二维数组中
int[][] tempMatrix = new int[3][4];
tempMatrix.length; //代表行的长度
tempMatrix[0].length； //代表列的长度
代码：
package basic;
import java.util.Arrays;
public class MatrixAddition {
public static void main(String[] args) {
matrixElementSumTest();
matrixAdditionTest();
}
/**
* Sum the elements of a matrix.
* @param paraMatrix
* @return The sum of all its elements.
*/
public static int matrixElementSum(int[][] paraMatrix) {
int resultSum = 0;
for (int i = 0; i < paraMatrix.length; i++) {
for (int j = 0; j < paraMatrix[0].length; j++) {
resultSum += paraMatrix[i][j];
}
}
return resultSum;
}
/**
* Unit test for respective method
*/
public static void matrixElementSumTest() {
int[][] tempMatrix = new int[3][4];
for (int i = 0; i < tempMatrix.length; i++) {
for (int j = 0; j < tempMatrix[0].length; j++) {
tempMatrix[i][j] = i * 10 + j;
}
}
System.out.println("The matrix is: \r\n" + Arrays.deepToString(tempMatrix));
System.out.println("The matrix element sum is: " + matrixElementSum(tempMatrix) + "\r\n");
}
/**
* Add two matrices. Attention: NO error check is provided at this moment.
* @param paraMatrix1 The first matrix.
* @param paraMatrix2 The second matrix. It should have the same size as the first one's
* @return The addition of these matrices.
*/
public static int[][] matrixAddition(int[][] paraMatrix1, int[][] paraMatrix2) {
int[][] resultMatrix = new int[paraMatrix1.length][paraMatrix1[0].length];
for (int i = 0; i < paraMatrix1.length; i++) {
for (int j = 0; j < paraMatrix1[0].length; j++) {
resultMatrix[i][j] = paraMatrix1[i][j] + paraMatrix2[i][j];
}
}
return resultMatrix;
}
/**
* Unit test for respective method.
*/
public static void matrixAdditionTest() {
int[][] tempMatrix = new int[3][4];
for (int i = 0; i < tempMatrix.length; i++) {
for (int j = 0; j < tempMatrix[0].length; j++) {
tempMatrix[i][j] = i * 10 + j;
}
}
System.out.println("The matrix is: \r\n" + Arrays.deepToString(tempMatrix));
int[][] tempNewMatrix = matrixAddition(tempMatrix, tempMatrix);
System.out.println("The new matrix is: \r\n" + Arrays.deepToString(tempNewMatrix));
}
}
day8 矩阵相乘
8.1 题目解读
矩阵相乘（只有第一个矩阵的列和第二个矩阵的行相等）: a矩阵(mn)，b矩阵(np)则能相乘，且相乘后的矩阵为m*p。故在矩阵相乘是在一定条件下才能进行，需要用到if判断。
8.2代码
package basic;
import java.util.Arrays;
public class MatrixMultiplication {
public static void main(String[] args) {
matrixMultiplicationTest();
}
/**
* Matrix multiplication. The columns of the first matrix should be equal to the rows of the second one.
* @param paraFirstMatrix The first matrix.
* @param paraSecondMatrix The second matrix
* @return The result matrix.
*/
public static int[][] multiplication(int[][] paraFirstMatrix, int[][] paraSecondMatrix){
//m*n n*p == m*p
int m = paraFirstMatrix.length;
int n = paraFirstMatrix[0].length;
int p = paraSecondMatrix[0].length;
// Step 1. Dimension check.
if (paraSecondMatrix.length != n) {
System.out.println("The two matrices cannot be multiplied.");
return null;
}
// Step 2. The loop. m*n n*p == m*p
int[][] resultMatrix = new int[m][p];
for (int i = 0; i < m; i++) {
for (int j = 0; j < p; j++) {
for (int k = 0; k < n; k++) {
resultMatrix[i][j] += paraFirstMatrix[i][k] * paraSecondMatrix[k][j];
}
}
}
return resultMatrix;
}
public static void matrixMultiplicationTest(){
int[][] tempFirstMatrix = new int[2][3];
for (int i = 0; i < tempFirstMatrix.length; i++) {
for (int j = 0; j < tempFirstMatrix[0].length; j++) {
tempFirstMatrix[i][j] = i + j;
}
}
System.out.println("The first matrix is: \r\n" + Arrays.deepToString(tempFirstMatrix));
int[][] tempSecondMatrix = new int[3][2];
for (int i = 0; i < tempSecondMatrix.length; i++) {
for (int j = 0; j < tempSecondMatrix[0].length; j++) {
tempSecondMatrix[i][j] = i * 10 + j;
}
}
System.out.println("The second matrix is: \r\n" + Arrays.deepToString(tempSecondMatrix));
int[][] tempThirdMatrix = multiplication(tempFirstMatrix, tempSecondMatrix);
System.out.println("The third matrix is: \r\n" + Arrays.deepToString(tempThirdMatrix));
System.out.println("Trying to multiply the first matrix with itself.\r\n");
tempThirdMatrix = multiplication(tempFirstMatrix, tempFirstMatrix);
System.out.println("The result matrix is: \r\n" + Arrays.deepToString(tempThirdMatrix));
}
}
day9 while 语句
代码
还有一种循环是 do…while,其循环至少要执行一次循环体，而for和while循环需要先判断条件是否成立 在决定是否执行循环语句
package basic;
public class WhileStatement {
public static void main(String[] args) {
whileStatementTest();
}
/**
* The sum not exceeding a given value.
*/
public static void whileStatementTest() {
int tempMax = 100;
int tempValue = 0;
int tempSum = 0;
// Approach 1.
while (tempSum <= tempMax) {
tempValue++;
tempSum += tempValue;
System.out.println("tempValue = " + tempValue + ", tempSum = " + tempSum);
}
tempSum -= tempValue;
System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);
// Approach 2.
System.out.println("\r\nAlternative approach.");
tempValue = 0;
tempSum = 0;
while (true) {
tempValue++;
tempSum += tempValue;
System.out.println("tempValue = " + tempValue + ", tempSum = " + tempSum);
if (tempMax < tempSum) {
break;
}
}
tempSum -= tempValue;
System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);
}
}
day10 综合任务 1
10.1 题目解读
学生的成绩存放于一个矩阵，其中行表示学生，列表示科目。如：第 0 行表示第 0 个学生的数学、语文、英语成绩。要求：进行学生成绩的随机生成, 区间为 [50, 100]；找出成绩最好、最差的同学。但有挂科的同学不参加评比.
- 1.初始化学生成绩（会涉及到随机生成数据的区间范围：Random）
- 2.通过for循环来计算学生总成绩并排除挂科同学（借助：break,continue关键字）
- 3.for循环+if判断来找出成绩最好和最差的学生
10.2 代码
package basic;
import java.util.Arrays;
import java.util.Random;
public class Task1 {
public static void main(String[] args) {
task1();
}
public static void task1(){
//step1:Generate the data with n students and m courses.
int n = 10;
int m = 3;
int lowerBound = 50;
int upperBound = 100;
int threshold = 60;
// Here we have to use an object to generate random numbers.
Random tempRandom = new Random();
int[][] data = new int[n][m];
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
data[i][j] = lowerBound + tempRandom.nextInt(upperBound - lowerBound);
}
}
System.out.println("The data is:\r\n" + Arrays.deepToString(data));
// Step 2. Compute the total score of each student.
int[] totalScores = new int[n];
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
if (data[i][j] < threshold) {
totalScores[i] = 0;
break;
}
totalScores[i] += data[i][j];
}
}
System.out.println("The total scores are:\r\n" + Arrays.toString(totalScores));
// Step 3. Find the best and worst student.
// Typical initialization for index: invalid value.
int tempBestIndex = -1;
int tempWorstIndex = -1;
// Typical initialization for best and worst values.
// They must be replaced by valid values.
int tempBestScore = 0;
int tempWorstScore = m * upperBound + 1;
for (int i = 0; i < n; i++) {
// Do not consider failed students.
if (totalScores[i] == 0) {
continue;
}
if (tempBestScore < totalScores[i]) {
tempBestScore = totalScores[i];
tempBestIndex = i;
}
// Attention: This if statement cannot be combined with the last one using "else if", because a student can be both the best and the
// worst. I found this bug while setting upperBound = 65.
if (tempWorstScore > totalScores[i]) {
tempWorstScore = totalScores[i];
tempWorstIndex = i;
}
}
// Step 4. Output the student number and score.
if (tempBestIndex == -1) {
System.out.println("Cannot find best student. All students have failed.");
} else {
System.out.println("The best student is No." + tempBestIndex + " with scores: "
+ Arrays.toString(data[tempBestIndex]));
}
if (tempWorstIndex == -1) {
System.out.println("Cannot find worst student. All students have failed.");
} else {
System.out.println("The worst student is No." + tempWorstIndex + " with scores: "
+ Arrays.toString(data[tempWorstIndex]));
}
}
}
自我小结
三大结构
不管在c或者java，都会涉及到三大结构：顺序结构，选择结构，分支结构；通过对这三大结构的组合使用就可以解决一些很复杂的问题。
(1) 顺序结构：即按照代码的书写顺序依次执行，不存在跳转或者判断
- 要避免代码冗余，可以通过封装相应的方法或类来避免；例如day4中我们可以专门封装一个方法来判断是否为闰年，而避免每次都去重复写来判断
- 需要注意变量的使用范围等。例如day4中main方法中变量tempYear,正因为代码会顺序执行，所以可以对tempYear多次赋值从而覆盖前面的值。
(2)选择结构：会根据指定的判定条件去执行不同的代码块内容，如在day3，day4:if结构；if-else结构；if-else if结构；day5:switch结构
- 在选择结构中，要保证我们的判定条件是正确的
(3)循环结构：会根据指定的条件重复执行一段代码块内容，直到条件不符合跳出循环，如在day6-9中 for循环，while循环，除此之外还有do…while循环
- 循环条件要保证条件是能正确跳出循环条件的，否则会导致程序进入死循环