Java learning notes

Keywords: Java

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Variables and calculation

System.out.println("Hello World!");
Scanner in = new Scanner(System.in);
System.out.println(in.nextLine());

1. First Java program

output

package hello;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("Hello World!");
	}
}

  1. System.out.println()
  2. Statement ends with
  3. Use Alt + / autocomplete
  4. Package name convention starts with uppercase

input

package hello;

import java.util.Scanner;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("Hello World!");
		Scanner in = new Scanner(System.in);
		System.out.println(in.nextLine());
	}
}
  1. Scanner in = new Scanner(System.in)
package hello;

import java.util.Scanner;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int price = 0;
		int amount = 100;
		Scanner in = new Scanner(System.in);
		amount = in.nextInt();
		price = in.nextInt();
		System.out.println(amount + " - " + price + " = " + (amount-price));
	}
}

Here, two numbers are read in consecutively. The following two input methods are OK:

String connection:+

package hello;

import java.util.Scanner;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		System.out.println("10 + 20 = " + (10+20));
	}
}
  1. Parentheses mean that the expression inside is evaluated first
  2. Attention and distinction
System.out.println("10 + 20 = " + 10+20);

2. Variables

package hello;

import java.util.Scanner;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int price = 0;
		Scanner in = new Scanner(System.in);
		price = in.nextInt();
		System.out.println("100 - " + price + " = " + (100-price));	
	}
}
  1. Java is a strongly typed language. When defining variables, you should specify the type
  2. Java does not allow using variables without initialization (it needs to be assigned before the first use)
  3. Clear nouns: identifiers, keywords
  4. The rules for identifier construction of Java variables are the same as those of C
  5. Note the following statement:
int price, sum = 100;

This statement only assigns an initial value to sum, but price does not. It should be written as follows:

int price = 50, sum = 100;

From the perspective of software engineering, it is better to define only one variable per line

3. Constant

Modifier final

package hello;

import java.util.Scanner;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int price = 0;
		Scanner in = new Scanner(System.in);
		price = in.nextInt();
		final int amount = 100;
		System.out.println(amount + " - " + price + " = " + (amount-price));
		
	}
}

4. Integer and floating point

Related operation rules (int/int), operator priority, etc. are similar to C

  1. Monocular operators have the highest priority
  2. Most of the binding relationships (with the same priority to determine the direction of operation) are left to right, while the assignment is right to left
result = a = b = 3 + c

5. Cast

Similar to C, such as: (int)

package hello;

import java.util.Scanner;

public class hello {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int foot = 0;
		int inch = 0;
		Scanner in = new Scanner(System.in);
		foot = in.nextInt();
		inch = in.nextInt();
		System.out.println((foot + inch/12.0)*0.3048);
		System.out.println((int)(foot + inch/12.0)*0.3048*100);
		System.out.println((int)((foot + inch/12.0)*0.3048*100));
	}
}

Note that the last two lines of output are different:

The reason is that (int), as a monocular operator, has the highest priority and is higher than *, so we first convert (foot + inch/12.0) to type (5.583 - > 5), and then do multiplication.

Posted by expertis on Thu, 18 Jun 2020 23:10:51 -0700