[Java] Basic types and instruction notes

Data type

--Integer --byte: ± about 128 --short: ± about 32,000 --ʻInt: ± about 2.1 billion --long: ± 900 K --Decimal --float: Ambiguous and good decimals --double: Strict decimal --Authenticity value - boolean：trueorfalse`

• letter -- char: 1 character --String --String: String

Type conversion concept

Follows the magnitude relationship of data types. Small can be put in large, but the opposite is not possible. byte < short < int < long < float < double

public class Main {
  public static void main(String[] args){
	//When calculating between int types-> 2
	System.out.println("When calculating between int types");
	int a=5;
	int b=2;
	System.out.println(a/b);

	//When calculating between int types-> 2.5
	System.out.println("When calculating between double types");
	double c=5;
	double d=2;
	System.out.println(c/d);

	//One is int type,When the other is calculated in double type-> 2.5
	System.out.println("One is int type,When the other is calculated in double type");
	int e=5;
	double f=2;
	System.out.println(e/f);

	//One is double type,When the other is calculated as an int type(verification of accounts) -> 2.5
	System.out.println("One is double type,When the other is calculated as an int type(verification of accounts)");
	double g=5;
	int h=2;
	System.out.println(g/h);

	//When the calculation between int types is assigned to the double type variable x-> 2.0
	System.out.println("When the calculation between int types is assigned to the double type variable x");
	int i=5;
	int j=2;
	double k=i/j;
	System.out.println(k);
  }
}

Basic statement

--Screen output --System.out.print (argument 1); --System.out.println (argument 1); --Comparison of numerical values --Math.max (argument 1, argument 2); --String type numeric type conversion --ʻInteger.parseInt (argument 1); --Random number generation --new java.util.Random (). nextInt (argument 1); --Accepting input from the keyboard --new java.util.Scanner (System.in) .nextLine (argument 1); --new java.util.Scanner (System.in) .nextInt (argument 1);`

public class test02 {
  public static void main(String[] args){
    //Screen output
    System.out.print("System.out.print();");
    String a="My name is";
    String b="sample program";
    System.out.print(a+b);
    System.out.println(a+b);

    //Comparison of numerical values;
    System.out.println("Math.max();");
    int m=5;
    int n=2;
    int l=Math.max(m,n);
    System.out.println("Comparative experiment:"+m+"When"+n+"And the bigger one"+l);
    System.out.println("--");

    //String type numeric type conversion;
    System.out.println("Integer.parseInt();");
    String age="31";
    int i=Integer.parseInt(age);
    System.out.println("You next year"+(i+1)+"I'm old");
    System.out.println("--");

    //Random number generation;
    System.out.println("new java.util.Random().nextInt();");
    int r=new java.util.Random().nextInt(2); //Put an argument in the direction of nextInt.
    System.out.println("Random number generator:"+r);
    System.out.println("--");

    //Accepting input from the keyboard
    System.out.println("new java.util.Scanner(System.in).next~system");
    //Receive name and age.
    System.out.println("Please tell me your name.");
    String x = new java.util.Scanner(System.in).nextLine();
    System.out.println("Please enter your age.");
    int y = new java.util.Scanner(System.in).nextInt();
    //output
    System.out.println("Welcome"+x+"Mr. ("+y+"age).");
  }
}

Postscript 2019/05/13

new java.util.Scanner (System.in) .nextline (), but It creates an instance and uses methods in one sentence.

--Instance: java.util.Scanner --Method: nextLine ()

But this causes Eclipse to return an error.

<unassingned Closeable value>Will never be closed

It seems that you should use the Scanner.close () method to end the input reception. So it's a good idea to split the instance creation and method usage into two statements. [(reference)] (https://ja.stackoverflow.com/questions/41408/java-%E3%83%AA%E3%82%BD%E3%83%BC%E3%82%B9-%E3%83%AA%E3%83%BC%E3%82%AFunassigned-closeable-value%E3%81%8C%E9%96%89%E3%81%98%E3%82%89%E3%82%8C%E3%82%8B%E3%81%93%E3%81%A8%E3%81%AF%E3%81%82%E3%82%8A%E3%81%BE%E3%81%9B%E3%82%93)

Scanner scn = new java.util.Scanner(System.in);
String str = scn.nextLine();
scn.close(); //Close here.

Or

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String str = br.readLine();

There is also such a way of writing ↑.

• instance:
  • InputStreamReader
  • BufferedReader --Method:
  • readLine()

In addition, it seems that no error will occur if you hit javac and java directly at the command prompt.

Reference book

[Introduction to Java for a refreshing second edition] (https://www.amazon.co.jp/dp/B00MIM1KFC/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1) Pp.075-091

[Easy Java 7th Edition] (https://www.amazon.co.jp/dp/4815600848/ref=cm_sw_r_tw_dp_U_x_P7o2CbMQG50SB) Pp.61-67