Java first steps: 20 things I'd forgotten
Dev From Legacy
Posted on September 16, 2020
If you don't work with Java or you're relearning it (like me!), I'm sharing some things:
1. Java Major features: Object Oriented, Platform independent, it has many libraries and it's similar to C++.
2. Java Virtual Machine(JVM): Java source code [.java] is compiled into Java bytecode [.class], which is executed by a Java virtual machine (JVM).
3. Important Versions: 1 [1995], 5 [2004] and 8 [2014]. Current version in 2020-09: 9.
4. Java Kit Development: it includes the Java Runtime Environment (JRE), an interpreter/loader (Java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc) and other tools needed in Java development.
5. Environment Variables: unless on Windows, you will need to set environment variable.
6. Compiling/Executing Java on Command Prompt: you can compile your .java program using javac MyProgram.java
and you can run using java MyProgram
.
7. Best options IDE for Java: currently Eclipse, Netbeans and IntelliJ IDEA.
8. Java Hello World: in the Java programming language, every application must contain a main method whose signature is: public static void main(String[] args)
.
class HelloWorldProgram {
public static void main(String[] args) {
System.out.println("Hello World!"); // Display the string.
}
}
9. Java Comments: Single-line comments start with two forward slashes //
and multi-line comments start with /*
and ends with */
10. Java naming convention - CamelCase: Java follows camel-case syntax for naming the class, interface, method and variable. Method and variable should start with lowercase letter (example: ImageSprite
). Class and interface should start with uppercase letter (example: getBackground
).
*11. Java Operators: *
- Arithmetic [+, -, *, /, %, ++, --]
- Logical [&&, ||, !]
- Comparison [==, !=, >, <, >=, <=, >>=, <<=]
- Assignment [=, +=, -=, *=, /=, %=, &=, |=, ^=]
12. Concatenation by +: using operator + is one way to build a string in Java.
public class TestVariables {
public static void main(String args[]) {
int age = 30;
System.out.println("Age: " + age);
}
}
13. Floating-Point Data Types: Java has some peculiarities about floating-point, like the outputs below.
public class TestFloatingPoint {
public static void main(String args[]) {
double salary = 1250.70;
System.out.println("S: " + salary); //Output: S: 1250.7
double division = 3.14 / 2;
System.out.println(division); //Output: 1.57
int otherDivision = 5 / 2;
System.out.println(otherDivision); //Output: 2
double newDivision = 5 / 2;
System.out.println(newDivision); //Output: 2.0
double lastDivision = 5.0 / 2;
System.out.println(lastDivision); //Output: 2.5
double number1 = 0.2;
double number2 = 0.1;
System.out.println(number1 +number2);
//Output: 0.30000000000000004
}
}
14. Java Type Casting/Conversion: Java Type Casting is classified into two types: Explicit and Implicit.
public class TestConversion {
public static void main(String[] args) {
//Implicit casting
byte a = 40;
short b = a;
int c = b;
long d = c;
float e = d;
double f = e;
System.out.println("byte : "+a); //Output: 40
System.out.println("short : "+b); //Output: 40
System.out.println("int: "+c); //Output: 40
System.out.println("long: "+d); //Output: 40
System.out.println("float: "+e); //Output: 40.0
System.out.println("double: "+f); //Output: 40.0
//Explicit casting
double g = 30.0;
float h = (float) g;
long i = (long) h;
int j = (int) i;
short k = (short) j;
byte l = (byte) k;
System.out.println("double: "+g); //Output: 30.0
System.out.println("float: "+h); //Output: 30.0
System.out.println("long: "+i); //Output: 30
System.out.println("int: "+j); //Output: 30
System.out.println("short: "+k); //Output: 30
System.out.println("byte: "+l); //Output: 30
}
}
15. Characters and Strings in Java: you can see some examples bellow.
public class TestCharacters {
public static void main(String args[]) {
char letter = 'a';
System.out.println(letter); //Output: a
char letterValue = 65;
System.out.println(letterValue); //Output: A
char nextLetterValue = (char) (letterValue + 1);
System.out.println(nextLetterValue); //Output: B
String sentence = "String test";
System.out.println(sentence); //Output: String test
String newSentence = "String test" + 1;
System.out.println(newSentence); //Output: String test1
}
}
16. Conditionals in Java: you can see some examples bellow.
public class TestConditional {
public static void main(String[] args) {
int age = 22;
int numberPeople = 1;
boolean accompanied = (numberPeople > 1);
if ((age >= 21 || accompanied) && numberPeople < 20) {
System.out.println("You are allowed!");
switch (numberPeople) {
case 1:
System.out.println("Table for 1 on the right!");
break;
case 2:
System.out.println("Table for 2 on the left!");
break;
default:
System.out.println("Table for group on the patio!");
}
} else {
System.out.println("You arent't allowed!");
}
// Output: You are allowed!
// Table for 1 on the right!
}
}
17. Scope: A block of code refers to all of the code between curly braces {}.
public class TestScope {
public static void main(String[] args) {
{ // This is a block
int x = 100;
// Code here CAN use x
System.out.println(x);
} // The block ends here
// Code here CANNOT use x
}
}
18. Loops in Java: you can see some examples bellow.
public class TestLoop {
public static void main(String[] args) {
int counter = 0;
while(counter <= 1) {
System.out.println(counter);
counter++;
}
System.out.println(counter);
for(int newCounter = 0; newCounter <= 1; newCounter++) {
System.out.println(newCounter);
}
//Output: 0 1 2 0 1
}
}
19. Break/continue:
public class TestBreakContinue {
public static void main(String[] args) {
for (int i = 0; i < 4; i++) {
if (i == 2) {
break;
}
System.out.println(i);
}
// Output: 0 1
for (int i = 0; i < 4; i++) {
if (i == 2) {
continue;
}
System.out.println(i);
}
// Output: 0 1 3
}
}
20. Arrays in Java: you can see some examples bellow.
public class TestArrays {
public static void main(String[] args) {
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars[0]);
// Outputs Volvo
}
}
Posted on September 16, 2020
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