[Java Review] 1: The World of Java

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The following notes are based on the book: << Head First JAVA >> and video series: 手把手教你用Java基础教程 - 北京尚学堂 - 马士兵

- Java is a interpreted language

After .java is compiled into .class, JVM takes .class as input, one line at a time, translates a line into machine language to have it executed.

(The other kind is complied language, ex. C/C++)

- Garbage Collection

Java has its own garbage collector to automatically clean up the unused ram. (C/C++ needs coder to manunlly specify the clean-up process)

- Java’s work flow: JVM

IMG_AE01CAEAF6E4-1

- Ram segement
  • heap: new.
  • stack: local data.
  • data segment: static data, const data.
  • code segment: code.
- Terminology

JDK: java development kit (developer)

JRE: java runtime environment (user)

- Run java in shell
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$ javac HelloWorld.class //compile
$ java HelloWolrd //execute
- Rules
  • one .java can have at most only one public class
  • public class, if exists, name has to be same as filename
  • execution entry is fixed public static void main (String[] args) 
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public class MyFirstApp{
    public static void main (String[] args){
        system.out.print("I Rule!"); //statement
    } //method
} //class
- Commement
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// single line comment

/*
	mutiple
	lines
	comments
*/

/**
	* mnutiple lines
	* can be interpreted by dos
	*/
- Java keywords
abstract continue for new switch
assert*** default goto* package synchronized
boolean do if private this
break double implements protected throw
byte else import public throws
case enum** instanceof return transient
catch extends int short try
char final interface static void
class finally long strictfp** volatile
const* float native super while
- Java Variables

java data type.001

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int i = 10; // default inteiger 
long l1 = 8888888L; // 'L' is a must!
float f = 12.3f; // 'f' is a must
double d1, d2, d3 = 0.123; // only d3 is given 0.123;
String s = 'hello';

Smaller types of variables can be cast to larger types:

byte, short, char -> int -> long -> float -> double

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public class TestConvert {
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		int i1 = 123;
		int i2 = 456;
		double d1 = (i1+i2) * 1.2; // (i1+i2) ->double
		float f1 = (float) ((i1+i2) * 1.2); // force casting
		
		byte b1 = 1;
		byte b2 = 2;
		byte b3 = (byte) (b1+b2); // auto casting to int first, must force cast back 
		
		double d2 = 1e200;
		float f2 = (float) d2; // overflow
		System.out.println(f2); // become inf
		
		float f3  = 1.23f;
		long l1 = 123; //auto cast to int 
		long l2 = 300000L;
		float f = l1 + l2 + f3; // auto cast  to float first
		System.out.println(f);
		long l = (long) f; // forced to get rid of after=decimal 
		System.out.println(l);
	}
}
- Java operators
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/* binary oprators
	+ - ! 
     & -------- and
     | -------- or
     ^ -------- 异或， 同false异true
     && || ---- 短路与，短路或： 若左表达式能确定整体取值，则不计算右表达式
*/

// triple expression
int score = 80;
String type = score < 60 ? "fail!": "pass!"; 

// string +
int c = 100;
System.out.println('c = ' + c); // c will be auto cast to string
- Others
  • java uses unicode (UTF-16), 1 character always takses 2 bytes.
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