闵老板帖子: 日撸 Java 三百行（01-10天，基本语法）

0. 前言

为了提升编程能力, 重拾"java"技术, 在此开贴学习.

1. 第一个程序

搭建好所需要的编程环境, new java project, new package, and new class, then start to code our first program.

package basic;public class hello {/*** ********************** The entrance of the program.** @param args Not used now.**********************/public static void main(String[] args) {System.out.println("hello java!");}// Of main
}// Of class hello

2. if...else 条件语句

语法如下:

if(布尔表达式)
{//如果布尔表达式为true将执行的语句
}

2.1 例: 比较7与输入数字的大小

package basic;import java.util.Scanner;public class test_if {/*** ********************** The entrance of the program.** @param args Not used now.**********************/public static void main(String[] args) {int input_number;System.out.println("please input your number:");Scanner scan = new Scanner(System.in);input_number = scan.nextInt();if (input_number > 7) {System.out.println("input_number is bigger than 7");} else {System.out.println("input_number is not bigger than 7");}// Of if}// Of main
}// Of class test_if

输出:

please input your number:
5
input_number is not bigger than 7

3. switch case 语句

语法如下:

switch(expression){case value ://语句break; //可选case value ://语句break; //可选//你可以有任意数量的case语句default : //可选//语句
}

3.1 例: 创建一个可以加减乘除的计算器

package basic;import java.util.Scanner;public class operation {/*** ********************** The entrance of the program.** @param args Not used now.**********************/public static void main(String args[]) {double firstnumber, secondnumber, result;char operation;System.out.println("please input your firstnumber :");Scanner scan = new Scanner(System.in);firstnumber = scan.nextDouble();System.out.println("please input your secondnumber :");Scanner scan1 = new Scanner(System.in);secondnumber = scan1.nextDouble();System.out.println("please choose select the operation:(+ or - or * or /)");Scanner scan2 = new Scanner(System.in);operation = scan.next().charAt(0);switch (operation) {// Additioncase '+': {result = firstnumber + secondnumber;System.out.println("firstnumber + secondnumber = " + result);break;}// Subtractioncase '-': {result = firstnumber - secondnumber;System.out.println("firstnumber - secondnumber = " + result);break;}// Multiplicationcase '*': {result = firstnumber * secondnumber;System.out.println("firstnumber * secondnumuber = " + result);break;}// Divisioncase '/': {result = firstnumber / secondnumber;System.out.println("firstnumber / secondnumuber = " + result);break;}default:System.out.println("default");}// Of switch}// Of main
}// Of class operation

输出:

please input your firstnumber :
5
please input your secondnumber :
2
please choose select the operation:(+ or - or * or /)
/
firstnumber / secondnumber = 2.5

4. 循环结构

java中主要有三种循环结构:

while( 布尔表达式 ) {//循环内容
}

do {//代码语句
}while(布尔表达式);

for(初始化; 布尔表达式; 更新) {//代码语句
}

4.1 例: 矩阵加法

package basic;import java.util.Arrays;public class matrix_add {/*** ********************** Create a matrix of size 3 * 4.* * @param None.* @return A matrix of size 3 * 4.**********************/public static int[][] create() {int[][] matrix = new int[3][4];for (int i = 0; i < matrix.length; i++) {for (int j = 0; j < matrix[0].length; j++) {matrix[i][j] = i * 5 + j;} // Of for j} // Of for ireturn matrix;}// Of create/*** ********************** Add two matrices.** @param matrix1 The first matrix.* @param matrix2 The second matrix. And it must have the same size as the first*                one's.* @return The Addition of matrices.**********************/public static int[][] add(int[][] matrix1, int[][] matrix2) {int[][] addmatrix = new int[3][4];for (int i = 0; i < matrix1.length; i++) {for (int j = 0; j < matrix1[0].length; j++) {addmatrix[i][j] = matrix1[i][j] + matrix2[i][j];} // Of for j} // Of for ireturn addmatrix;}// Of add/*** ********************** The entrance of the program.** @param args Not used now.**********************/public static void main(String[] args) {int[][] matrix1 = new int[3][4];int[][] matrix2 = new int[3][4];int[][] addmatrix = new int[3][4];matrix1 = create();matrix2 = create();System.out.println("The two matrices are: \n" + Arrays.deepToString(matrix1));addmatrix = add(matrix1, matrix2);System.out.println("After adding, the new matrix is: \n" + Arrays.deepToString(addmatrix));}// Of main
}// Of class matrix_add

输出:

The matrix is:
[[0, 1, 2, 3], [5, 6, 7, 8], [10, 11, 12, 13]]
After adding, the new matrix is:
[[0, 2, 4, 6], [10, 12, 14, 16], [20, 22, 24, 26]]

4.2 例:矩阵乘法

package basic;import java.util.Arrays;public class matrix_multiply {/*** ********************** Create a matrix of size row*column.** @param paraRow    The row of the matrix.* @param paraColumn The column of the matrix.* @return A matrix of size paraRow * paraColumn.**********************/public static int[][] create(int paraRow, int paraColumn) {int[][] matrix = new int[paraRow][paraColumn];for (int i = 0; i < matrix.length; i++) {for (int j = 0; j < matrix[0].length; j++) {matrix[i][j] = i * 5 + j;} // Of for j} // Of for ireturn matrix;}// Of create/*** ********************** Matrix multiplication. Must satisfy the matrix multiplication.** @param matrix1 The first matrix.* @param matrix2 The second matrix.* @return The result matrix.**********************/public static int[][] multiply(int[][] matrix1, int[][] matrix2) {int row1 = matrix1.length;int row2 = matrix2.length;int column1 = matrix1[0].length;int column2 = matrix2[0].length;int[][] matrix = new int[row1][column2];if (row2 != column1) {System.out.println("The two matrices do not satisfy the matrix multiplication.");return null;} // Of iffor (int i = 0; i < row1; i++) {for (int j = 0; j < column2; j++) {for (int k = 0; k < row2; k++) {matrix[i][j] += matrix1[i][k] * matrix2[k][j];} // Of for k} // Of for j} // Of for ireturn matrix;}// Of multiply/*** ********************** The entrance of the program.** @param args Not used now.**********************/public static void main(String[] args) {int[][] matrix1, matrix2, matrix;matrix1 = create(3, 4);matrix2 = create(4, 3);System.out.println("The first matrix is :\n" + Arrays.deepToString(matrix1));System.out.println("The first matrix is :\n" + Arrays.deepToString(matrix2));matrix = multiply(matrix1, matrix2);System.out.println("After multiplying, matrix is :" + Arrays.deepToString(matrix));}// Of main
}// Of class matrix_multiply

输出:

The first matrix is :
[[0, 1, 2, 3], [5, 6, 7, 8], [10, 11, 12, 13]]
The second matrix is :
[[0, 1, 2], [5, 6, 7], [10, 11, 12], [15, 16, 17]]
After multiplying, matrix is :[[70, 76, 82], [220, 246, 272], [370, 416, 462]]

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Java学习(01-10天, 基本语法)

0. 前言

1. 第一个程序

2. if...else 条件语句

2.1 例: 比较7与输入数字的大小

3. switch case 语句

3.1 例: 创建一个可以加减乘除的计算器

4. 循环结构

4.1 例: 矩阵加法

4.2 例:矩阵乘法

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