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前言

这里是JAVA自学与了解的同步笔记与记录，如有问题欢迎指正说明

I have loved the stars too fondly to be fearful of the night.

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Day01——从“Hello World”开始吧

public class HelloWould {public static void main(String[] args) {// TODO Auto-generated method stubSystem.out.println("Hello Would!");}// Of main
}// Of class HelloWould

Day02——基本算术操作

练习：四则运算

public class math {public static void main(String[] args) {// TODO Auto-generated method stubint tempFirstInt = 15,tempSecondInt = 4,tempResultInt;double tempFirstDouble= 1.2,tempSecondDouble = 3.5,tempResultDouble;//  +tempResultInt = tempFirstInt + tempSecondInt;tempResultDouble = tempFirstDouble + tempSecondDouble;System.out.println(tempFirstInt + " + " + tempSecondInt + " = " + tempResultInt);System.out.println(tempFirstDouble + " + " + tempSecondDouble + " = " + tempResultDouble);//  -tempResultInt = tempFirstInt - tempSecondInt;tempResultDouble = tempFirstDouble - tempSecondDouble;System.out.println(tempFirstInt + " - " + tempSecondInt + " = " + tempResultInt);System.out.println(tempFirstDouble + " - " + tempSecondDouble + " = " + tempResultDouble);//  *（乘）tempResultInt = tempFirstInt * tempSecondInt;tempResultDouble = tempFirstDouble * tempSecondDouble;System.out.println(tempFirstInt + " * " + tempSecondInt + " = " + tempResultInt);System.out.println(tempFirstDouble + " * " + tempSecondDouble + " = " + tempResultDouble);//  /（除）tempResultInt = tempFirstInt / tempSecondInt;tempResultDouble = tempFirstDouble / tempSecondDouble;System.out.println(tempFirstInt + " / " + tempSecondInt + " = " + tempResultInt);System.out.println(tempFirstDouble + " / " + tempSecondDouble + " = " + tempResultDouble);//  %（取余）tempResultInt = tempFirstInt % tempSecondInt;System.out.println(tempFirstInt + " % " + tempSecondInt + " = " + tempResultInt);}//Of main }//Of class math

运行结果

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Day03——基本if语句

练习：求绝对值

public class HelloWould {public static void main(String[] args) {// TODO Auto-generated method stubint tempNumber1 = 6;System.out.println("The absolute value of " + tempNumber1 + " is " + abs(tempNumber1));int tempNumber2 = -8;System.out.println("The absolute value of " + tempNumber2 + " is " + abs(tempNumber2));}// Of mainpublic static int abs(int paraValue) {if (paraValue >= 0) {return paraValue;} else {return -paraValue;} // Of if}// Of abs
}

运行结果

小结

if-else语句在Java编程中较为常用，是常见的多分支语句。上段程序用于求一个整型数的绝对值，取绝对值的部分单独作为一个静态方法，通过引用方法传入实参，可让方法返回一个值，这个值即为所给数的绝对值。

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Day04——if语句嵌套

练习：闰年的计算

   //闰年的判断条件：可被4整除但不能同时被100整除，或者能被400整除public class LeapYear {public static void main(String args[]) {// Test isLeapYearint tempYear = 2021;System.out.print("" + tempYear + " is ");if (!isLeapYear(tempYear)) {System.out.print("NOT ");} // Of ifSystem.out.println("a leap year.");// Test isLeapYearV2System.out.println("Now use the second version.");System.out.print("" + tempYear + " is ");if (!isLeapYearV2(tempYear)) {System.out.print("NOT ");} // Of ifSystem.out.println("a leap year.");//下面是包含双分支if—-else的方法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//下面是包含多分支if-else的方法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
}

小结

1.在一个分支中又完整的嵌套了另一个完整的分支结构，里面的分支结构称为内层分支，外面的分支结构称为外层分支。
2.嵌套最好不要超过3层，否则可读性差

Day05——基本switch语句

练习 : 判断分数的对应等级

package basic;/*** This is the fifth code. Names and comments should follow my style strictly.*/
public class SwitchStatement {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 10int 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 switchreturn 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

运行结果

小结

1.switch(表达式)中，表达式的数据类型应该和case后的常量类型一致，或者是可以自动转换成可以相互比较的类型，如常量为int，而输入是字符。
2.switch(表达式)中，表达式的返回值必须是：（byte，short，int，char,String,enum（枚举））
3.case子句中的值必须是常量，而不能是变量
4.可以没有default
5.break语句执行后跳出该switch。如果没有break，则程序会顺序执行到switch结尾

Day06——基本for语句

练习 ：1累加到N

public class e_for {public static void main(String[] args) {// TODO Auto-generated method stubforStatementTest();}// of mainpublic 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 forStatementTestpublic static int addToN(int paraN) {int resultSum = 0;for (int i = 1; i <= paraN; i++) {resultSum += i;} // of for ireturn resultSum;}// of addToNpublic static int addToNWithStepLength(int paraN, int paraStepLength) {int resultSum = 0;for (int i = 1; i <= paraN; i += paraStepLength) {resultSum += i;} // of for ireturn resultSum;}// of addToNWithStepLength}// of class

运行结果

· 补充：增强型For循环

1.形式如下

for(声明语句 : 表达式)
{//代码句子
}

2.声明语句：声明新的局部变量，该变量的类型必须和数组元素的类型匹配（即与表达式中的数据结构集合中的每个子项的类型匹配）。其作用域限定在循环语句块，其值与此时数组元素的值相等。
表达式：表达式是要访问的数组名，或者是返回值为数组的方法。

3.具体用法

public class Test {public static void main(String[] args) {int[] numbers = { 10, 20, 30, 40, 50 }; // Create a int arrayfor (int x : numbers) { // Get the elements in the array in orderSystem.out.print(x);System.out.print(",");}System.out.print("\n");String[] names = { "James", "Larry", "Tom", "Lacy" }; // Create a string arrayfor (String name : names) {System.out.print(name);System.out.print(",");}}
}

4.运行结果

5.需注意：
for(val : array)的遍历之下的每个val值是仅仅可读的，但是不可写，val只能用来获取而无法通过修改val来修改array对于中的数据。即读取的val不是这个array中那个对应数的地址，因此修改val无法改动到array。

小结

1.尝试使用主方法与一般方法分离的模式编程，main函数只负责简单的驱动作用。优点：能使主方法代码规模更小，看起来更简洁，主方法与一般方法的引用和被引用关系更明确，利于提高工作效率。
2.循环条件是能返回一个布尔值的表达式。
3.for(; 循环条件 ;)中的初始化和变量迭代可以写在其他地方，但是两边的分号不能省略。
4. 循环的初始化可以有多条初始化语句，但要求数据类型一致，并且中间用逗号隔开。

Day07——矩阵：运动的描述

1.动态创建：

//一维数组
dataType[] arrayRefVar = new dataType[arraySize];
//1.使用dataType[arraySize]创建了一个数组
//2.把创建的数组的引用赋值给变量 arrayRefVar//二维数组
dataType[][] arrayRefVar = new dataType[arrayHeight][arrayWidth];

2.静态创建：

dataType[10] arrayRefVar1;
dataType[10][20] arrayRefVar1;
double[] arrayRefVar2 = {1.2, 2.3, 3.4};

练习 ：矩阵求和

import java.util.Arrays;public class e_array {//主方法负责驱动函数public static void main(String args[]) {matrixElementSumTest();matrixAdditionTest();}// Of main//函数功能：将传入的二维数组内的每一个元素求和，并返回这个和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 ireturn resultSum;}// Of matrixElementSum/************************ Unit test for respective method.**********************/// 通过两个for循环嵌套使用，给矩阵tempMatrix赋初值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 iSystem.out.println("The matrix is: \r\n" + Arrays.deepToString(tempMatrix));System.out.println("The matrix sum is: " + matrixElementSum(tempMatrix) + "\r\n");}// Of matrixElementSumTest//两个for循环嵌套，实现两个矩阵的相加，即为其对应元素的相加public static int[][] matrixAddition(int[][] paraMatrix1, int[][] paraMatrix2) {if (paraMatrix1.length != paraMatrix2.length || paraMatrix1[0].length != paraMatrix2[0].length) {System.out.println("Error! Two matrixs must have same height and width! ");}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 ireturn resultMatrix;}// Of matroxAdditionpublic 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 iSystem.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 matrixAddtionTest}// Of class MatrixAddtion

运行结果

注意：
1.paraMatrix.length 表示矩阵的宽，即行数
2.paraMatrix[0].length 表示矩阵第一行的长度
3.具体遍历的话，基本所有语言的二维存储都是采用行优先的存储，而普遍的遍历都是先行后列的遍历思想

·补充说明array类

1.排序数组的方法
public static void sort(Object[] a)
对指定对象数组根据其元素的自然顺序进行升序排列。同样的方法适用于所有的其他基本数据类型（Byte，short，Int等）。
Arrays.sort(tempArrays); //正序排tempArrays
Arrays.sort(tempArrays,Collections.reverseOrder()); //逆序排tempArrays
Arrays.sort(tempArrays, i, j);//正序对tempArrays的i到j位置进行排序
2.填充操作
public static void fill(int[] a, int val)
将指定的 int 值分配给指定 int 型数组指定范围中的每个元素。
3.快速遍历
Arrays.toString(Object[] array)
功能：返回数组的字符串形式

        int[] nums = {2,5,0,4,1,-10};System.out.println(Arrays.toString(nums));/** 结果:[2, 5, 0, 4, 1, -10]*/

Arrays.deepToString(Object[][] arrays)
功能：返回多维数组的字符串形式

int[][] nums = {{1,2},{3,4}};System.out.println(Arrays.deepToString(nums));/** 结果:[[1, 2], [3, 4]]*/

Day08——矩阵的乘法

练习：矩阵相乘

目标：1.可以判断两个矩阵是否能相乘，即是否符合第一个矩阵的列数等于第二个矩阵的行数
2.创建矩阵后用for循环给矩阵赋初值
3.三个for循环嵌套使用，第一层拿到A矩阵的逐行，第二层拿到B矩阵的逐列，最内层实现
求和

package basic;import java.util.Arrays;public class MatrixMultiplication {public static void main(String args[]) {matrixMultiplicationTest();}// Of main//矩阵相乘实现方法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 ireturn resultMatrix;}// Of multiplication/************************ Unit test for respective method.**********************///初始化矩阵1public 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 iSystem.out.println("The first matrix is: \r\n" + Arrays.deepToString(tempFirstMatrix));//初始化矩阵2int[][] 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 + j;} // Of for j} // Of for iSystem.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 matrixMultiplication

运行结果

小结

1.\r\n相当于硬回车，会另起一行。\n\r会在中间空出一行后在另起一行

Day09——whlie语句

练习：

package good_better;import java.util.PrimitiveIterator.OfDouble;/*** The usage of the sth.** @author Wanxiang Luo 2858442831@qq.com*/public class e_while {/*** ********************** The entrance of the program.** @param args**********************/public static void main(String[] args) {// TODO Auto-generated method stubwhileStatementTest();}// of mainpublic static void whileStatementTest() {int tempMax  = 100;int tempValue = 0;int tempSum = 0;// Approach 1while (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 2System.out.println("\r\n second approach.");tempValue = 0;tempSum = 0;while (true) {tempValue++;tempSum += tempValue;System.out.println("tempValue = " + tempValue + ",tempSum = " + tempSum);if (tempMax < tempSum) {break;}//of if }// of whiletempSum -= tempValue;System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);}// of whlieStatementTest}

运行结果

小结

1.方法1每次循环判断（）内的循环条件是否为真，若为true，则执行一次循环体。方法2的循环条件一直为真，循环体内设置if语句进行判断，一旦满足条件则即刻跳出当前while循环。
2.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.

Day10——综合任务

1.任务要求

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：1.进行学生成绩的随机生成, 区间为 [50, 100].
2.找出成绩最好、最差的同学。但有挂科的同学不参加评比.

2.对任务的分析

1.每个学生的科目都是三科，故可将数据放入一个m*3的矩阵。
2.通过一个数组来统计每个学生的成绩总和，得到最低成绩和最高成绩
3.运用Java提供的Random库解随机数生成的问题：

java Random.nextInt()方法
public int nextInt(int n)
该方法的作用是生成一个随机的int值，该值介于[0,n)的区间，也就是0到n之间的随机int值，包含0而不包含n

对于本次任务还需注意：
1.随机数生成区间前闭后开，故无法生成n
2.指定[a,b)区间的生成：a + temRandom.nextInt(b - a);
3.数据是整型不是浮点型

3.代码实现

package good_better;import java.util.Arrays;
import java.util.Random;/***The usage of the sth.** @author Wanxiang Luo 2858442831@qq.com*/public class randomData {/*** **********************The entrance of the program.** @param args**********************/public static void main(String[] args) {// TODO Auto-generated method stubtask1();}public static void task1() {// Step 1. Generate the date with n student and m course.// Set these value by yourself.int n = 10;int m = 3;int lowerBound = 50;int upperBound = 101;int threshold = 60;// Here we have to use an object to generate random numbersRandom temRandom = 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 + temRandom.nextInt(upperBound - lowerBound);//生成50到101间的随机数} // Of for j} // Of for iSystem.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;//有挂科的同学不参与统计，以0代替} // Of iftotalScores[i] += data[i][j];} // Of for j} // Of for iSystem.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 ifif (tempBestScore < totalScores[i]) {tempBestScore = totalScores[i];tempBestIndex = i;} // Of if// Attention: This if Statememt 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 student have failed.");} else {System.out.println("The best Student is No." + tempBestIndex + " with scores: "+ Arrays.toString(data[tempBestIndex]));} // Of ifif (tempWorstIndex == -1) {System.out.println("Cannot find worst student. All student have failed.");} else {System.out.println("The worst Student is No." + tempWorstIndex + " with scores: "+ Arrays.toString(data[tempWorstIndex]));} // Of if}// Of task1}

运行结果

测试了两组不同的结果

小结

1.step 3中不能用if-else结构囊括求极大值和求极小值方案，因为学生的成绩可能同时是最高分和最低分。
2.step 3中使用了编程中常见得不能再常见的求极值方案——通过给预求极值设定一个反方向的最极限值（说人话就是给最大值设定一个能取的最小值为初始值，给最小值设定一个能取的最大值为初始值）
3.前十天进行了基本语法的学习，可以看出每日的难度和代码长度都在逐步递增。下一步的学习任务，是对数据结构的复习和加深认识。每一次的学习都是点亮一个又一个的知识点，但不要忘了还需将这些知识点串联起来，才能形成更高效有序的’网’。以后的学习，我会拆分较难理解的方面成一个个更小的知识点，逐步理解拾级而上。