How do I use the if-then-else statement?

By Wayan in Basic, Core API 0 Comment

The if-then-else control flow statement adds a secondary path to the if statement when the expression evaluates to false. When it evaluates to false the else block will be executed.

Below is the program that takes input of user test score and evaluates the score to get the corresponding grade.

package org.kodejava.basic;

import java.io.Console;

public class IfThenElseDemo {
    public static void main(String[] args) {
        System.out.print("Please enter your score: ");

        try {
            // Get an instance of system console for taking user
            // input.
            Console console = System.console();

            // Take user score input and convert the input value into
            // number.
            int score = Integer.parseInt(console.readLine());

            String grade;
            if (score >= 90) {
                grade = "A";
            } else if (score >= 80) {
                grade = "B";
            } else if (score >= 60) {
                grade = "C";
            } else if (score >= 50) {
                grade = "D";
            } else {
                grade = "F";
            }

            System.out.println("Grade = " + grade);
        } catch (NumberFormatException e) {
            e.printStackTrace();
        }
    }
}

When the program executed you’ll need to input the test score and the program will give you the grade.

Please enter your score: 75
Grade = C

How do I use the if-then statement?

By Wayan in Basic, Core API 0 Comment

The if-then is the most basic control flow statement. It will execute the block of statement only if the test expression evaluated equals to true. Let’s see the example below:

package org.kodejava.basic;

public class IfThenDemo {
    public static void main(String[] args) {
        int a = 10;
        int b = 5;

        // If the evaluation of a > b is true, the if block will be
        // executed. In the block below we just print that the value
        // of "a" is bigger than "b".
        if (a > b) {
            System.out.printf("A(%d) is bigger than B(%d)%n", a, b);
        }

        // When we have only a single command inside a block we can
        // remove the opening and closing braces for the block ({..}).
        // But it is a good practice to always enclose the block with
        // braces, so that we know the block of code to be executed.
        if (b < a)
            System.out.printf("B(%d) is smaller that A(%d)%n", b, a);
    }
}

The result of the above program is:

A(10) is bigger than B(5)
B(5) is smaller that A(10)

How do I count the occurrences of a number in an array?

By Wayan in Basic, Core API 4 Comments 
package org.kodejava.basic;

import java.util.HashMap;
import java.util.Map;

public class NumberOccurrenceInArray {
    public static void main(String[] args) {
        int[] numbers = new int[]{1, 8, 3, 4, 3, 2, 5, 7, 3, 1, 4, 5, 6, 4, 3};

        Map<Integer, Integer> map = new HashMap<>();
        for (int key : numbers) {
            if (map.containsKey(key)) {
                int occurrence = map.get(key);
                occurrence++;
                map.put(key, occurrence);
            } else {
                map.put(key, 1);
            }
        }

        for (Integer key : map.keySet()) {
            int occurrence = map.get(key);
            System.out.println(key + " occur " + occurrence + " time(s).");
        }
    }
}

The result are:

1 occur 2 time(s).
2 occur 1 time(s).
3 occur 4 time(s).
4 occur 3 time(s).
5 occur 2 time(s).
6 occur 1 time(s).
7 occur 1 time(s).
8 occur 1 time(s).

How do I use the double brace initialization?

By Wayan in Basic, Core API 0 Comment

The double brace initialization {{ ... }} is another way for initializing a collection objects in Java. It is offer a simple syntax for initializing a collection object.

package org.kodejava.lang;

import java.util.ArrayList;
import java.util.List;

public class DoubleBraceInitialization {
    public static void main(String[] args) {
        // Creates a list of colors and add three colors of
        // Red, Green and Blue.
        List<String> colors1 = new ArrayList<>();
        colors1.add("Red");
        colors1.add("Green");
        colors1.add("Blue");

        for (String color : colors1) {
            System.out.println("Color = " + color);
        }

        // Creates another list of colors and add three colors
        // using the double brace initialization.
        List<String> colors2 = new ArrayList<>() {{
            add("Red");
            add("Green");
            add("Blue");
        }};

        for (String color : colors2) {
            System.out.println("Color = " + color);
        }
    }
}

What’s actually happened is: the first brace creates an anonymous inner class and the second brace is an initializer block. Due to the need for creating an inner class the use of double brace initialization is considered to be slower.

Because of this performance issue it’s better not to use this technique for your production code, but using it in your unit testing can make your test looks simpler.

How do I get name of enum constant?

By Wayan in Basic, Core API 0 Comment

This example demonstrate how to user enum‘s name() method to get enum constant name exactly as declared in the enum declaration.

package org.kodejava.basic;

enum ProcessStatus {
    IDLE, RUNNING, FAILED, DONE;

    @Override
    public String toString() {
        return "Process Status: " + this.name();
    }
}

public class EnumNameDemo {
    public static void main(String[] args) {
        for (ProcessStatus processStatus : ProcessStatus.values()) {
            // Gets the name of this enum constant, exactly as
            // declared in its enum declaration.
            System.out.println(processStatus.name());

            // Here we call to our implementation of the toString
            // method to get a more friendly message of the
            // enum constant name.
            System.out.println(processStatus);
        }
    }
}

Our program result:

IDLE
Process Status: IDLE
RUNNING
Process Status: RUNNING
FAILED
Process Status: FAILED
DONE
Process Status: DONE