public class Test { // class block 开始 public static void main(String[] args) { // method block 开始 System.out.println("Welcome to Java!"); } // method block 结束} // class block 结束
Classes
即类
是Java中最基础最重要的结构之一;他是创建objects的template
例如：
class Student { String name; int age;}//定义了一个名为Student的类，具有name/age两种data fields//class中通常会有两类主要内容：datafield(描述对象有什么)和methods(描述对象能做什么)
Method
即方法
指的是一组statement的集合，用来完成某个操作
public static void main(String[] args) { System.out.println("Hello");}//此处的整个code就是一个名为main的method,包含一个statement
The main method: 程序开始执行的入口, Java interpreter 会通过调用main method来开始执行应用程序
Chapter 2-7
Reading input from the console
System.out.print(...)//在控制台提示用户输入Scanner input = new Scanner(System.in);//创建一个 Scanner 对象，从键盘读取输入input.nextDouble()//读取用户输入的一个 double 数值
if (boolean-expression){ statement(s-true);}else{ statement(s-false);}
Multi-way if statement
if (boolean-expression){ statement(A);}else if(boolean-expression){ statement(B);}else if......else{ statement(X);}
switch statement rules:根据表达式的值，从多个case中选择一个执行
switch(switch-expression){ case value1:statement(s)1; break; case value2:statement(s)2; break; ... default:statement(s)default;}
printf statements
use the printf statement get formatting output
System.out.printf(format,items);/*%b a boolean value%c a character%d a decimal integer%f a floating-point number%e a number in standard scientific notation%s a string*/
For example:
int count = 5;double amount = 45.56;System.out.printf("the count is %d and amount is %f",count,amount);//输出：the count is 5 and amount is 45.560000
public class Demo { public static void main(String[] args) { System.out.println(add(1, 2)); // 两个int → 调用 add(int, int) System.out.println(add(1.5, 2.5)); // 两个double → 调用 add(double, double) } public static int add(int a, int b) { return a + b; } public static double add(double a, double b) { return a + b; }}
public class Test { public static void main(String[] args) { int x = 1; int[] y = {1}; change(x, y); System.out.println(x); System.out.println(y[0]); } public static void change(int a, int[] b) { a = 100; b[0] = 100; }}
Two-dimensional Arrays
二维数组的基本声明和构建方法
//declare:dataType[][] refVar//create:refVar = new dataType[][]int[][] matrix = new int[10][10]; //in one step//access:refVar[][]matrix[1][3] = 3;//shorthand notationsint[][] array = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12}};//等价于先创建再逐一赋值
对于二维数组int[][] x = new int[3][4]，x.length为3，因为x中包含三个行数组，而x[i].length为4，因为每个行数组包含4个元素
for (int i = 0; i < scores.length; i++) { double totalScore = 0; for (int j = 0; j < scores[i].length; j++) for (int k = 0; k < scores[i][j].length; k++) totalScore += scores[i][j][k];}
最终得到每个学生的最终成绩
ArrayList
可动态更改size的一种列表
//声明并创建ArrayList<String> obj = new ArrayList<String>();//添加元素obj.add("element");//删除元素obj.remove("element");//取得元素obj.get(0);//修改元素obj.set(0, "element");//获取长度obj.size();//清空obj.clear();//判断是否包含某元素obj.contains("element");
