java programming language
Learn from examples
1. Program to output an integer, a string,
and a floating point number, initially just one method.
exj1.java source code
// exj1.java first example simple output integer, string, double
// note // makes rest of line a comment
import java.io.*;
public class exj1 // class name same as file name
{
public exj1() // constructor same as class name
{
int i = 7; // declare variable i with initial value 7
char ac = 'a'; // single character
String msg = "sample string"; // declare msg with string in quotes
float x = 37.95f; // declare floating point x with initial value
double y = 127.34e10; // declare double precision with initial value
System.out.println("exj1.java running"); // simple print title
System.out.println("i="+i); // for decimal
System.out.println("ac="+ac); // for character
System.out.println("msg="+msg);// for string
System.out.println("x="+x); // for simple floating point
System.out.println("y="+y); // for double with exponent
} // end constructor exj1 optional comment
public static void main (String[] args) // "main" required
{
new exj1(); // construct and execute
}
} // end class exj1 optional comment
exj1_java.out Output of execution
exj1.java running
i=7
ac=a
msg=sample string
x=37.95
y=1.2734E12
2. commands to compile and execute the source code
at a minimum, Windows, Linux, MacOSX.
Windows:
javac exj1.java
java exj1
Linux:
javac -cp . exj1.java
java -cp . exj1
MacOSX
javac -cp . exj1.java
java -cp . exj1
save output to a file:
java -cp . exj1 > exj1_java.out
3. You must be able to declare variables and list and arrays
and matrix of various types.
exj3.java source code
exj3_java.out Output of execution
// exj3.java example declare variables, list, arrays
import java.io.*; // needed for input and output
public class exj3 // class name same as file name
{
// declarations for entire class may go here
public exj3() // constructor same as class name
{
// declarations for this procedure go here
// typically first, yet may be in code, just before use
int i = 1; // optional initialization 32 bit
byte i8 = 1; // 8 bit
short i16 = 1; // 16 bit
long i64 = 0xabcdef01; // 64 bit initialized with hexadecimal
float x = 39.27F; // needs F else warning
double y = 127.5E200;
boolean me = true; // could be false
String msg = "short"; // uppercase S for type String
int vec[] = {1, 2, 4, 8}; // initialize an array, any type
double av[] = {1.5, 2.2, 9.1};
String as[] = {"abc", "xyz", "more"};
int mat[][] = {{1,2,3},{2,3,4},{3,4,5},{4,5,6}}; // 4 by 3 matrix
int big[] = new int[100]; // allocate space for 100 integers
double small[] = new double[5]; // space for 5 doubles
double matrix[][] = new double[4][3]; // matrix 4 rows of 3 doubles
System.out.println("exj3.java running");
System.out.println("i="+i);
// System.out.println("mat="+mat); must print with loops
} // end constructor exj3 optional comment
public static void main (String[] args)
{
new exj3();
}
} // end class exj3 optional comment
Execution output:
exj3.java running
i=1
4. You need to be able to have loops, iteration statements
exj4.java source code
exj4_java.out Output of execution
// exj4.java example file iteration, loops
public class exj4 // class name same as file name
{
public exj4() // constructor same as class name
{
System.out.println("exj4.java running");
int i; // declare loop variable
int mat[][] = {{1,2,3},{2,3,4},{3,4,5},{4,5,6}}; // 4 by 3 matrix
int sum = 0;
for(i=0; i<4; i++) // columns of mat
{ // the } could be at end of previous line
for(int j=0; j<3; j++) // j only exists inside loop
{
sum += mat[i][j];
} // end j optional comment
} // end i optional comment
System.out.println("sum="+sum);
for(i=2; i≤10; i=i+2)
{
System.out.println("i="+i); // even numbers
}
} // end constructor exj4 // optional comment
public static void main (String[] args)
{
new exj4();
}
} // end class exj4 optional comment
Execution output:
exj4.java running
sum=42
i=2
i=4
i=6
i=8
i=10
5. You need if then else conditional statements
exj5.java source code
exj5_java.out Output of execution
// exj5.java example file iteration, if then else
public class exj5 // class name same as file name
{
public exj5() // constructor same as class name
{
System.out.println("exj5.java running");
double x = 2.0; // declare test variable
int i = 3;
if(x < 3.0) // < > <= >= == != compare operations
{
System.out.println("compare < > <= >= == != x="+x);
}
if(x > 3.0 || i == 3 && i > 2) // || is or, && is and
{
System.out.println("logic || is or, && is and i="+i);
}
if( x > 3.0)
{
System.out.println("x > 3.0");
}
else if(i < 3) // optional
{
System.out.println("i < 3");
}
else // optional, get here if none of the above true
{
System.out.println("none of the above");
} // end if optional comment
} // end constructor exj5 optional comment
public static void main (String[] args)
{
new exj5();
}
} // end class exj5 optional comment
Execution output:
exj5.java running
compare < > <= >= == != x=2.0
logic || is or, && is and i=3
none of the above
6. You need to be able to create functions, procedures,
subroutines.
exj6.java source code
exj6_java.out Output of execution
// exj6.java example file create function or procedure and call
import java.io.*; // needed for input and output
public class exj6 // class name same as file name
{
// declarations for entire class may go here
public exj6() // constructor same as class name
{
// declarations for this procedure go here
// typically first, yet may be in code, just before use
int i, j;
int jj[] = new int[1]; // single value made into an array to get return
double arr[][]={{1.0,2.0,3.0},{4.0,5.0,6.0}};
double val;
System.out.println("exj6.java running");
// call proc1, all functions and procedures after constructor
proc1(7);
// use funct1
i = funct1(2, 3);
System.out.println("funct1(2,3) returns "+i);
val = sum(arr); // size of array found in sum
System.out.println("sum(arr)= "+val);
i = ret2(i, jj); // pass jj by address so it can be changed
System.out.println("ret2 i="+i+", jj="+jj[0]);
System.out.println("end exj6.java");
} // end constructor exj6
double sum(double A[][]) // determine size inside
{
double asum = 0.0;
int n = A.length;
int m = A[0].length;
for(int i=0; iexj7.java source code
exj7_java.out Output of execution
8. You need to be able to use a number of files combined to
build a program. This may include packages, libraries,
operating system commands, header files, etc.
exj8.java source code
exj8_java.out Output of execution
Other examples that may be of interest:
obj.java source code
obj_java.out Output of execution
matmul_thread4.java source code
matmul_thread4_java.out Output of execution
matmulb_thread4.java source code
matmulb_thread4_java.out Output of execution
Last updated 11/23/2020