日撸 Java 三百行（01-10天，基本语法）
目录
总述
01-10天，基本语法
11-20天，线性数据结构
21-30天，树与二叉树
31-40天，图
41-50天，查找与排序
51-60天，kNN 与 NB
61-70天，决策树与集成学习
71-80天，BP 神经网络
81-90天，CNN 卷积神经网络
第1天: 环境搭建
1.1 完成 Eclipse 的安装. 第一天嘛，把环境弄对就行了.
1.2 学习 package, import 和 println 语句. 其中, package 要与所建的包名（即文件夹名）一致.
1.3 编写HelloWorld.java. 一定要注意变量的写法.
警告: 成事起头难. 环境安装、第一个程序的运行如果有问题, 请及时与老手联系, 不要自己折腾!
如果身边没有老手, 可以看下 唐金玉的安装方案.
package basic;
/**
* This is the first code. Names and comments should follow my style strictly.
* @author Fan Min [email protected].
*/
public class HelloWorld {
public static void main(String args[]) {
System.out.println("Hello, world!");
}//Of main
}//Of class HelloWorld
第 2 天: 基本算术操作
2.1 加、减、乘、除、整除、取余.
2.2 熟悉 println 的中阶用法.
package basic;
/**
* This is the second code. Names and comments should follow my style strictly.
* @author Fan Min [email protected].
*/
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);
}//Of main
}//Of class BasicOperations
第 3 天: 基本if 语句
3.1 if then else.
3.2 方法（函数）调用: 增加代码的复用性.
3.3 方法（函数）头部规范的注释, 是后期生成文档的基础.
package basic;
/**
* The usage of the if statement.
*
* @author Fan Min [email protected].
*/
public class IfStatement {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
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));
}// Of main
/**
*********************
* The absolute value of the given parameter.
*
* @param paraValue The given value.
*********************
*/
public static int abs(int paraValue) {
if (paraValue >= 0) {
return paraValue;
} else {
return -paraValue;
} // Of if
}// Of abs
}// Of class IfStatement
第 4 天: 闰年的计算
4.1 if 语句的嵌套.
4.2 基本规律自行百度.
4.3 布尔类型.
package basic;
/**
* The complex usage of the if statement.
*
* @author Fan Min [email protected].
*/
public class LeapYear {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
// Test isLeapYear
int tempYear = 2021;
System.out.print("" + tempYear + " is ");
if (!isLeapYear(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
tempYear = 2000;
System.out.print("" + tempYear + " is ");
if (!isLeapYear(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
tempYear = 2100;
System.out.print("" + tempYear + " is ");
if (!isLeapYear(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
tempYear = 2004;
System.out.print("" + tempYear + " is ");
if (!isLeapYear(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
// Test isLeapYearV2
System.out.println("Now use the second version.");
tempYear = 2021;
System.out.print("" + tempYear + " is ");
if (!isLeapYearV2(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
tempYear = 2000;
System.out.print("" + tempYear + " is ");
if (!isLeapYearV2(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
tempYear = 2100;
System.out.print("" + tempYear + " is ");
if (!isLeapYearV2(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
tempYear = 2004;
System.out.print("" + tempYear + " is ");
if (!isLeapYearV2(tempYear)) {
System.out.print("NOT ");
} // Of if
System.out.println("a leap year.");
}// Of main
/**
*********************
* Is the given year leap?
*
* @param paraYear The given year.
*********************
*/
public static boolean isLeapYear(int paraYear) {
if ((paraYear % 4 == 0) && (paraYear % 100 != 0) || (paraYear % 400 == 0)) {
return true;
} else {
return false;
} // Of if
}// Of isLeapYear
/**
*********************
* Is the given year leap? Replace the complex condition with a number of if.
*
* @param paraYear The given year.
*********************
*/
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;
} // Of if
}// Of isLeapYearV2
}// Of class LeapYear
第 5 天: 基本switch 语句
5.1 Switch, case, break, default 的用法.
5.2 单元测试单独使用一个方法, main 方法里面的代码越少越好.
package basic;
/**
* This is the fifth code. Names and comments should follow my style strictly.
*
* @author Fan Min [email protected].
*/
public class SwitchStatement {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
scoreToLevelTest();
}// Of main
/**
*********************
* Score to level.
*
* @param paraScore From 0 to 100.
* @return The level from A to F.
*********************
*/
public static char scoreToLevel(int paraScore) {
// E stands for error, and F stands for fail.
char resultLevel = 'E';
// Divide by 10, the result ranges from 0 to 10
int tempDigitalLevel = paraScore / 10;
// The use of break is important.
switch (tempDigitalLevel) {
case 10:
case 9:
resultLevel = 'A';
break;
case 8:
resultLevel = 'B';
break;
case 7:
resultLevel = 'C';
break;
case 6:
resultLevel = 'D';
break;
case 5:
case 4:
case 3:
case 2:
case 1:
case 0:
resultLevel = 'F';
break;
default:
resultLevel = 'E';
}// Of switch
return resultLevel;
}// of scoreToLevel
/**
*********************
* Method unit test.
*********************
*/
public static void scoreToLevelTest() {
int tempScore = 100;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 91;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 82;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 75;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 66;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 52;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 8;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
tempScore = 120;
System.out.println("Score " + tempScore + " to level is: " + scoreToLevel(tempScore));
}// Of scoreToLevelTest
}// Of class SwitchStatement
第 6 天: 基本for 语句
6.1 循环语句是程序的核心.
6.2 算法的时间复杂度一般根据循环语句来计算.
package basic;
/**
* This is the sixth code. Names and comments should follow my style strictly.
*
* @author Fan Min [email protected].
*/
public class ForStatement {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
forStatementTest();
}// Of main
/**
*********************
* Method unit test.
*********************
*/
public static void forStatementTest() {
int tempN = 10;
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));
}// Of forStatementTest
/**
*********************
* 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;
} // Of for i
return resultSum;
}// Of addToN
/**
*********************
* Add from 1 to N with a step length.
*
* @param paraN The given upper bound.
* @param 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;
} // Of for i
return resultSum;
}// Of addToNWithStepLength
}// Of class ForStatement
第 7 天: 矩阵元素相加
7.1 矩阵的赋值.
7.2 二重循环.
package basic;
import java.util.Arrays;
/**
* This is the seventh code. Names and comments should follow my style strictly.
*
* @author Fan Min [email protected].
*/
public class MatrixAddition {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
matrixElementSumTest();
matrixAdditionTest();
}// Of main
/**
*********************
* Sum the elements of a matrix.
*
* @param paraMatrix The given matrix.
* @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];
} // Of for j
} // Of for i
return resultSum;
}// Of matrixElementSum
/**
*********************
* 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;
} // Of for j
} // Of for i
System.out.println("The matrix is: \r\n" + Arrays.deepToString(tempMatrix));
System.out.println("The matrix element sum is: " + matrixElementSum(tempMatrix) + "\r\n");
}// Of matrixElementSumTest
/**
*********************
* 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];
} // Of for j
} // Of for i
return resultMatrix;
}// Of matrixAddition
/**
*********************
* 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;
} // Of for j
} // Of for i
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));
}// Of matrixAdditionTest
}// Of class MatrixAddition
第 8 天: 矩阵相乘
8.1 三重循环是多数程序的极限.
8.2 非法输入检查是程序正常运行的基本保障. 如果检查所有的非法输入, 会导致大量代码行, 这在商业代码中是必须的.
package basic;
import java.util.Arrays;
/**
* This is the eighth code. Names and comments should follow my style strictly.
*
* @author Fan Min [email protected].
*/
public class MatrixMultiplication {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
matrixMultiplicationTest();
}// Of main
/**
*********************
* 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) {
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;
} // Of if
// Step 2. The loop.
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];
} // Of for k
} // Of for j
} // Of for i
return resultMatrix;
}// Of multiplication
/**
*********************
* Unit test for respective method.
*********************
*/
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;
} // Of for j
} // Of for i
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;
} // Of for j
} // Of for i
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));
}// Of matrixMultiplicationTest
}// Of class MatrixMultiplication
第 9 天: while 语句
9.1 while 语句本质上比 for 更基础, 因此可以替代后者. 但 for 在很多时候更方便.
9.2 break 语句又出现了, 上次是在 switch 语句里. 都是表示跳出当前代码块.
package basic;
/**
* This is the ninth code. Names and comments should follow my style strictly.
*
* @author Fan Min [email protected].
*/
public class WhileStatement {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
whileStatementTest();
}// Of main
/**
*********************
* 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);
} // Of while
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;
} // Of if
} // Of while
tempSum -= tempValue;
System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);
}// Of whileStatementTest
}// Of class WhileStatement
第 10天: 综合任务 1
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.
10.1 实际代码中，for 和 if 是最常见的, switch 和 while 使用少得多.
10.2 使用了 continue, 它是指继续跳过本次循环后面的代码，直接进入下一次循环. 而 break 是跳出整个循环体.
10.3 为了随机数，迫不得已提前使用了 new 语句生成对象.
10.4 通过数据测试找出程序中的 bug.
package basic;
import java.util.Arrays;
import java.util.Random;
/**
* This is the tenth code, also the first task.
*
* @author Fan Min [email protected].
*/
public class Task1 {
/**
*********************
* The entrance of the program.
*
* @param args Not used now.
*********************
*/
public static void main(String args[]) {
task1();
}// Of main
/**
*********************
* Method unit test.
*********************
*/
public static void task1() {
// Step 1. Generate the data with n students and m courses.
// Set these values by yourself.
int n = 10;
int m = 3;
int lowerBound = 50;
int upperBound = 65; // Should be 100. I use this value for testing.
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);
} // Of for j
} // Of for i
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;
} // Of if
totalScores[i] += data[i][j];
} // Of for j
} // Of for i
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;
} // Of if
if (tempBestScore < totalScores[i]) {
tempBestScore = totalScores[i];
tempBestIndex = i;
} // Of if
// 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;
} // Of if
} // Of for 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]));
} // Of if
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]));
} // Of if
}// Of task1
}// Of class Task1
小结
本帖讨论了分支语句 (if, switch case), 循环语句 (for, while), 流程控制语句 (continue, break), 函数的使用. 这些内容和基本 C 语言的差不多. 数组的空间分配比 C 语言简单.
特别注意: 经常在 Eclipse 中使用鼠标右键 source -> format, 自动调整程序的格式.