First Example in Java

Advanced Programming

SUT • Spring 2019

Outline

  • Review

  • First program in java

  • Variables

  • Methods

  • Conditions

  • Loops

First Example

Review

  • Java is

    • Simple

    • object oriented

    • Robust

    • And popular

  • Java is platform independent.

    • Write Once, Run Anywhere!

First program in java

  • Create a file named First.java

    • Java class files have .java extension

    • Note to naming convention

  • Copy this lines to the file

    • Note: File name and class name should be the same.

public class First{
    public static void main(String[] args){
        System.out.println("Hello World!");
    }
}

First Example

  • Run javac First.java
  • Run java First
  • We don’t use any IDE now.
    • To highlight compile and run stages.
  • Lets watch it in real world!
behnam@behnam-UX430UNR:~$ mkdir test
behnam@behnam-UX430UNR:~$ cd test/
behnam@behnam-UX430UNR:~/test$ cat > First.java
public class First{
    public static void main(String[] args){
        System.out.println("Hello World!");
    }
}
behnam@behnam-UX430UNR:~/test$ ls -lah
total 12K
drwxr-xr-x  2 behnam behnam 4.0K فوریه  4 10:55 .
drwxr-xr-x 22 behnam behnam 4.0K فوریه  4 10:55 ..
-rw-r--r--  1 behnam behnam  116 فوریه  4 10:56 First.java
behnam@behnam-UX430UNR:~/test$ javac First.java 
behnam@behnam-UX430UNR:~/test$ java First 
Hello World!
behnam@behnam-UX430UNR:~/test$

Java Programs

  • A simple java program is a file

    • The file contains one class

  • The class name equal to the file name

    • The names are case sensitive

  • The class contains a main method

  • When we run the program, the main method is executed

Variables

Variables

  • What is a variable?
  • A piece of memory
  • Holds data
  • For example a number, string or Boolean
  • Java variables have a fixed size
    • Platform independence

Java Primitive Types

Primitive type Size Min Max Wrapper Type
boolean - - - Boolean
char 16 bits Unicode 0 Unicode 2^16-1 Character
byte 8 bits -128 127 Byte
short 16 bits -2^15 2^15-1 Short
int 32 bits -2^31 2^31-1 Integer
long 64 bits -2^63 2^63-1 Long
float 32 bits IEEE754 IEEE754 Float
double 64 bits IEEE754 IEEE754 Double
void - - - Void

Java Primitive Types

Operator Precedence

Equality and Relational Operators

Operator Java Sample meaning
= == x == y x is equal to y
≠ != x != y x is not equal to y
> > x > y x is greater than y
< < x < y x is less than y
≥ >= x >= y x is greater than or equal to y
≤ <= x <= y x is less than equal y

Associativity

  • When two operators with the same precedence the expression is evaluated according to its associativity.
  • x = y = z = 17
    • is treated as x = (y = (z = 17))
    • since the = operator has right-to-left associativity
  • 72 / 2 / 3
    • is treated as (72 / 2) / 3
    • since the / operator has left-to-right associativity

A simple program

public class SimpleProgram{
  public static void main(String[] args){
    int a;
    a = 12;
    a += 2;
    int b;
    b = 4;
    b++;
    b = a*b;
    System.out.println(b);
  }
}

Example

  • A method is like a machine
  • Zero or more inputs
  • Zero or one output
  •  

 

  • Other names
    • Function
    • Procedure

method

inputs

output

Example

public class Example{
  static double add(double a, double b){
    double result = a + b;
    return result;
  }

  public static void main(String args[]){
    double x = 3;
    double y = 4;
    double add = add(x,y);
    System.out.println(add);
  }
}

Parameter Passing

  • Local variables

  • Java passes the parameters by value

public class ParameterPassing{}
  public static void main(String[] args) {
  	double x = 3;
  	double y = 4;
  	double add = add(x,y);
  	System.out.println(add);
  	System.out.println(x);
  }

  static double add(double a, double b){
  	a = a + b;
  	return a;
  }
}

Conditions

public class Main{
  public static void main(String[] args){
    int x = 2;
    int y = 3;

    if(x>y){
    	System.out.println("X is greater than Y");
    } else if(x==y){
    	System.out.println("X is equal to Y");
    } else {
    	System.out.println("Y is greater than X");
    }

    boolean condition = x>y;
    if(condition){
    	System.out.println("X is greater than Y");
    }else{
    	System.out.println("Y >= X");
    }
  }
}

Loops

public class Loop{
  public static void main(String[] args){
    int counter = 0;
    while(counter < 10){
    	counter++;
    	System.out.println(counter);
    }

    counter = 0;
    do{
    	counter++;
    	System.out.println(counter);
    }while(counter < 10);

    for (int i = 1; i <= 10; i++) {
    	System.out.println(i);
    }
  }
}

For Loop vs. While Loop

for (X; Y; Z) {
	body();
}
X;
while(Y){
	body();
	Z;
}

goto

  • goto is a reserved word in java

  • But forbidden!

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