14.JAVA data types

Java data types - integer data 整数数据

Integers (to non-mathematicians) are whole numbers, numbers with no fractional part, no decimal point.
以下四种:

1.byte

8 bits -128 to 127 (0 to 255)
Bytes. e.g., individual bytes of image data; in 32 bit images there is 1 byte each for red, green, blue and alpha (transparency)
字节数。例如，图像数据的单个字节；
在32位图像中，红、绿、蓝和alpha(透明度）各有一个字节

2.short

16 bits -32,768 to 32,767
To save memory – instead of int. However, in most JVMs they are 32-bit anyway, so useless.
要保存内存-而不是int。然而，在大多数jvm中，它们都是32位的，非常没用。

3.int (default)

32 bits -2^31 to 2^31 - 1
Everything except very big/very negative values. This is the default.
除了非常大的/非常负的值。这是默认值。

4.long

64 bits -2^63 to 2^63-1
long bigNumber = 300000000000000000L;
You will probably use
int 95%+ of the time, long sometimes, and byte occasionally.
实用度 int>long>byte

Java data types - floating point非整数

1.float

32 bits
+-10^38 approx.
7 significant digits (decimal) float f = 1.375f;

2.double

64 bits +-10^38 approx. 15 significant digits (decimal) Default
e.g.
int a = 1000000000;
long b = 1000000000000000000l;
double c = 31.32;
float d = 31.01f;

Java data types - Strings 字符串

As introduced last week, can also use Strings

Strings

String are objects, and have many methods
○ At basic level, can store text data
○ Note, double quotes needed around string
E.g. String str = "This is a String";

Char

represents a character ('a', 'b', 'c', etc)
A different data type
includes non-Roman characters (Chinese, Cyrillic, Arabic, Greek etc)
○ It is actually a 16 bit integer, range 0 - 65,535

Java data types - Boolean

Boolean

● We can also store True/False information
● Boolean (means true or false)
Discussed later in the module
Cannot store anything except this
Very useful for checking if conditions are met
Its size varies; it only uses 1 bit, but can take up to 32 bits of memory!
boolean bool = false;

15.Assigning variables

*declaration 声明 definition 定义 initialization初始化 assignment赋值

declaration 存在此变量不占用空间
definition 为变量分配空间 （通常这两个过程是一起的）
initialization初始化 定义后需要一个确定的初始化过程（从无到有）
assignment 赋值 给予一个数值（从有到有）
Instantiate 实例化 把用类创建对象的过程称为实例化。是将一个抽象到具体的过程
Assigning different types of variables We can declare and assign separately 
int intNum; // declare a variable (assigned a default value)  ① 
intNum = 22; // assign a new value  ② 
 ①+② int intNum = 22; // initialize  
double doubNum = 22.0; 
float floatNum = 12.3f; 
String strData = "Twenty two"; 
boolean boolData = true; 
char charVal = 'c';

16.Arithmetic operators 算子

Operator Description

+ Addition operator
(also used for joining Strings) (called String concatenation) 连接算符
- Subtraction operator
* Multiplication operator
/ Division operator
% Modulo (remainder) operator（取余）
++ 自增 +1 --自减 -1

17.Java Arithmetic 计算实战

①int num1 = 3; 
int num2 = 4; 
int result = num1 / num2 ; 
System.out.println(result); 
Result = 0 //Remember, integers can only use whole numbers
②double num1 = 3; 
double num2 = 4; 
double result = num1 / num2 ; 
System.out.println(result); 
Result = 0.75
③double num1 = 7; 
double num2 = 3; 
double result = num1 / num2; 
System.out.println(result); 
Result = 2.3333333333333335
④int num1 = 7; 
int num2 = 3; 
int result = num1 / num2; 
System.out.println(result);
Result = 2
/* Why? Because an integer can only store whole 
numbers 
○ Anything after the decimal point is simply ignored 
○ Can cause coding errors, but can also be useful…
浮点精度（float、double）运算不精确的原因 */
⑤int num1 = 7; 
int num2 = 3; 
int result = num1 / num2; 
System.out.println(result) //ignore the decimal point 
Result = 2
⑥int num1 = 7; 
int num2 = 4; 
int result = num1 % num2; 
System.out.println(result); //calculate the remainder
Result = 3
⑦int num1 = 7; 
int num2 = 3; 
int result = num1 / num2 + num2; 
System.out.println(result); 
Result = 5
⑧int num1 = 7; 
int num2 = 3; 
int result = num1 / (num2 + num2); 
System.out.println(result); //⑦&⑧：order different
Result = 1

*Operator Precedence

First 1.++ -- 2.() 3.* / % 4. + - Last

18.Unary Operators

++ Increment operator; increments a value by 1
-- Decrement operator; decrements a value by 1 
int a; 
a = 41; 
a++; 
System.out.println("a = " + a); 
a--; 
System.out.println("a = " + a);
Output=
a = 42 
a = 41 
// Another way of expressing it
a += 2; a = a + 2;
a -= 3; a = a - 3;
a *= 3; a = a * 3;

19.mathematical function

Such as Square root, Random numbers
有一说一，很多时候平方不如直接做乘法，因为可能会因为精度不够造成错误结果。 
double root = Math.sqrt(num)//平方
double root = Math.max(num1,num2);//取大值
double rand = Math.random()
/*will be between 0.0 and 1.0, but not including 1.0, in set symbol [0.0, 1.0).
If you want to expand the range, you can use multiply*/
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
MethodDescriptionReturn Type
acos(x)Returns the arccosine of x, in radiansdouble
asin(x)Returns the arcsine of x, in radiansdouble
atan(x)Returns the arctangent of x between -PI/2 and PI/2 radiansdouble
exp(x)Returns the value of E^xdouble
expm1(x)Returns e^x-1double
floor(x)Returns the value of x rounded down to its nearest integerdouble
hypot(x, y)Returns sqrt(x^2 +y^2) without intermediate overflow or underflowdouble
log(x)Returns the natural logarithm (base E) of xdouble
max(x, y)Returns the number with the highest valuedouble|float|int|long
min(x, y)Returns the number with the lowest valuedouble|float|int|long
pow(x, y)Returns the value of x to the power of ydouble
random()Returns a random number between 0 and 1double
round(x)Returns the value of x rounded to its nearest integerint
signum(x)Returns the sign of xdouble
sin(x)Returns the sine of x (x is in radians) double
sqrt()Returns the square root of x double
tan(x)Returns the tangent of an angle double
toDegrees(x)Converts an angle measured in radians to an approx. equivalent angle measured in degreesdouble
toRadians(x)Converts an angle measured in degrees to an approx. angle measured in radians double

20.Triangles - Hypotenuse

Pythagorean theorem
                                                                                                               
MethodDescriptionReturn Type
hypot(x, y)Returns sqrt(x^2 +y^2) without intermediate overflow or underflowdouble
pow(x, y)Returns the value of x to the power of ydouble
sqrt(x)Returns the square root of x double
 
/* pow(x,y) = x^y 
hypot(x,y) = sqrt(x^2+y^2)
Sqrt(a) =√a*/
double a = 4; 
double b = 3; 
double c = Math.sqrt(Math.pow(a,2) + Math.pow(b,2)); 
System.out.println("Value of C is " + c);
Output: Value of C is 5

21. Angles of triangle

● Can use SOH CAH TOA
● SOH = Sin – Opposite and Hypotenuse
● CAH = Cos – Adjacent and Hypotenuse
● TOA = Tan – Opposite and Adjacent
asin acos atan 反三角函数
toDegrees(x) 弧度转角度
toRadians(x) 角度转弧度
double opp = 2; 
double adj = 4; 
double val = Math.atan(opp/adj); 
double deg = Math.toDegrees(val); 
System.out.println("Angle is " + deg);

22.Initial Programming

Last class we made simple programs Everything in main method. ints, doubles, Strings System.outs Not interactive
pF3Aup8.png

23.The Scanner Object

The Scanner object is a useful way to receive the System.input character stream, i.e. accessing characters that you type. Like a String, a Scanner is an object (we will discuss this in future weeks) It contains a number of useful methods Its purpose is to receive and handle keyboard input. It is a very useful way to provide input to your program. It wraps your character stream into a Scanner object, which has methods you can use to access it.

Scanner for keyboard

We can create a new Scanner object easily:
Scanner kb = new Scanner(System.in);
We can then use it to receive and store an input
String next = kb.nextLine();
Useful to use a System.out to prompt user Input can be stored as a variable and used
System.out.println("Name is " + next ); 
Scanner kb = new Scanner(System.in); 
System.out.println("Input name"); 
String next = kb.nextLine(); 
System.out.println("Name is " + next);
No package was added when entered
add import for java Using Package pF3AwX4.png

24.Convert data type

pF3ABnJ.png
● In fact, there are classes for ALL primitive data types
● They are called ‘wrapper classes’.
String s = "999" 
int res = Integer.parseInt(s);
String s = "99.021" 
double res = Double.parseDouble(s);
boolean x = false; 
String m = Boolean.toString(x);
// 此种代码的作用就是把字符串转化为对应的数据类型 前提是转化的数据
public static int getDice() { 
double rand = (Math.random() * 6); 
int r = (int) rand; 
return r; 
}

算子int

y = (int) x // 此类算式就是正常的取整计算 结合上文的计算 Scanner Input 
public static void main(String[] args) { 
Scanner kb = new Scanner(System.in); 
System.out.println("Input a number"); 
String next = kb.nextLine(); 
int num1 = Integer.parseInt(next); 
}
● We can convert our String to an integer or double!
● Get input from Scanner
● Run a conversion, store as integer
Variant: (using two inputs for addition, output and) 
// 变式：（采用两个输入做加法，输出和）
public static void main(String[] args) { 
Scanner kb = new Scanner(System.in); //建立一个scanner Input输入
System.out.println("Input a number"); //提示输入
String next = kb.nextLine();  //下一行输入
int num1 = Integer.parseInt(next); //输入1
System.out.println("Input a number"); //提示输入
String next2 = kb.nextLine();  //下一行输入
int num2 = Integer.parseInt(next2); //输入2
int res = num1+num2;  //进行加法
System.out.println("Total is " + res); //输出结果
}

25.Good programming style

● Your programming code must be easily read by others and by yourself.
● This saves time during program maintenance.
● It also leads to less human induced error.
● In software development projects, maintenance often accounts for 80% of effort.
● Program readability is of utmost importance.
就是说一个好的格式能事半功倍，包括空白行、空格、变量的代称、说明等。
/* ... */多行注释
//单行注释
Add blank lines to Improve readability
添加空白行以提高可读性
Put a space between each term
在每个术语之间放一个空格
importHtml 规定的缩写首字母大写 importHTML
message 不要用日常的缩写 msg

26.Lab learning

Write a Java program that takes two input integers, min and max, from the user, and generate two random integers between min and max, inclusive After that, print two two random integers and report whether one is smaller, equal to, or larger than the other
Use boolean value (true or false) to report the comparison between the two random integers
Your program’s input/output interaction should look like:
pF3EJDH.png
Boolean can just output true/false.
Boolean Specifies whether it is true or false, just like if.

Math

Math.round () 四舍五入函数，
(int) Math.round() 四舍五入后转化为整数类型。
Math：这是Java标准库中的一个类，它包含了各种数学函数和常数。
round()：这是Math类中的一个方法，用于将一个浮点数值四舍五入到最接近的整数。例如，如果你有一个浮点数值 3.6，Math.round(3.6) 将返回 4。
(int)：这是一个类型转换操作符，用于将一个值从一种数据类型（在这种情况下是浮点数）转换为另一种数据类型（在这种情况下是整数）。
Math.Pi: 调用π的值
In second lab, we use the (int) Math.round() to change the double type to integer type.
Also don’t forgot how to change string to int/double.
int num1 = Integer.parseInt(next); 
double num2 = Double.parseDoule(next);
Math.pow(n,2)也就是n^2。

27.CW1&CW2 learning

A bit A byte A kilobyte A megabyte A gigabyte A terabyte From small to large
Java is a high level language which means that it looks similar to C++ and is easier for humans to read compared to low level languages such as assembly . All Java programs run inside the Java Virtual Machine. Programmers write the source code in Java and then the compiler turns this into a file containing bytecode . The Java Virtual Machine then works as a/an interpreter when it is running the program.
JAVA编程中source code和bytecode有什么区别
编译器和解释器之间有什么区别
int i =10;
The variable i is both declared and initialised.
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